Temperature Archives

Cannabis Heat Stress: How to Spot, Prevent, and Treat It

Most of you know how important it is to control the temperature in your weed garden. You also understand that when it’s too hot for too long, the plants will begin to suffer and show you exactly what’s wrong with them. The trick is knowing where to look. In this post, we’ll talk about how the symptoms of cannabis heat stress look like so that you don’t mix’em up with other issues and take proper action right away.

you can notice symptoms of cannabis heat stress on both leaves and buds
leaves start to cup (curl up) and later become brown and burned
in flower, buds may start to foxtail
control the distance from the light and set up proper ventilation
maintain the right temperature in the root zone
A young plant suffering from heat stress. © Growdiaries © Sandero

Experienced cannabis growers recognize signs of heat stress right away. Heat stress may affect leaves as well as buds.

Symptoms of Heat Stress on Leaves

It all starts when those sharp points on the leaf edges begin to curl up. Thereby, the plant tells you that it has become too hot.

Serrated edges on the leaves start to point up. © Growdiaries © misterb

And if you don’t resolve the heat issue soon, the leaves curl up even more and start to look like tacos.

Cupping leaves and stunted growth due to heat. © Growdiaries © misterb

It’s hard to say why it happens. Maybe that’s how the plant tries to reduce the surface that receives light.

In an indoor setup, leaf cupping often happens on the parts of the plant that are closest to the bulb. It’s because they receive too much radiant heat from the light. And farther down the stem, leaves may remain normal.

And sometimes, only SOME of the upper leaves show these symptoms — where the light creates hot spots.

Of course, during a heatwave, your whole grow room can get too hot. Then you may notice signs of heat stress on both upper and lower leaves.

Let the problem persist, and you’ll see the situation get much worse. The leaves may get yellow and then brown. The discoloration starts at the tips and along the edges. Later, the heat burn spreads to the whole leaf, and eventually, the leaf becomes dry and crispy.

Here, the heat-related issues got out of hand. © Growdiaries © sndwich

Don’t Confuse Cannabis Heat Stress and Calcium Deficiency

It’s easy to confuse the signs of cannabis heat burn and the symptoms of calcium deficiency. Calcium-deficient leaves also have brown spots and may become dry and brittle at the tips and along the edges, but they also twist in every direction.

Brow, dead, distorted leaves are signs of calcium deficiency. © Growdiaries © sndwich

Of course, there are situations when plants have both these issues at the same time. Calcium deficiency makes the plants more vulnerable to heat stress, and heat stress makes the calcium-deficient leaves dry out faster.

Droopy Leaves

Droopy leaves aren’t a sign of heat stress. Rather, they indicate that the plant is thirsty. So, if you see that the leaves on your cannabis hang lifeless, maybe it’s time to water your garden. And don’t forget to water more often when the temperature in your grow room rises.

These plants probably get less water than they need for transpiration. © Growdiaries © stumay111

Btw, droopy leaves aren’t a very specific symptom. So, it’s easy for inexperienced growers to confuse underwatered wilting and overwatered “claws”. Besides, leaves may slightly droop every evening (before lights-out), and it’s perfectly normal.

Light Stress

Both light stress and heat burn often stem from the same problem — the light being too close to the plant tops. Though these two may manifest at the same time, they are different issues. With good ventilation, you may have an ideal air temperature in the grow tent (and no heat stress), but your plant tops will still suffer from too much light.

Light stress starts with the yellowing of the leaves. Please note that usually, it’s not the youngest leaves on the very top that start to yellow, but the ones just below them. It’s because they have been exposed to light a day or two longer and so get affected earlier.

Light stress is a very common issue in seedlings. Growers often think it’s potassium deficiency and try to treat it as such, but all they need to do is raise the light.

The scorching Californian sun isn’t kind to these seedlings. © Growdiaries © misterb

Symptoms of Heat Stress on Flowering Buds

Extreme temperature can damage flowering weed plants in several ways. It can interfere with the production of THC and terpenes, making your smoke less potent, aromatic, and flavorful. It can also lead to lower yields because heat prevents buds from filling out and they remain airy and spongy. In a worst-case scenario, buds start foxtailing, especially if you grow one of the strains with a tendency to foxtail.

This Critical Purple Auto grown on a rooftop in NorCal displays both the foxtailing and the taco-ing of the leaves. © Growdiaries © misterb

If the heat is extreme—even for a short period of time—the pistils may change their color from white to brown prematurely. Pistils are the female hairs on cannabis buds. It’s normal for them to become amber or brown at the end of the life cycle when the buds are already big and dense. But if the flowers have only begun to form, they should have white pistils.

How Much Heat is Too Much?

Just like humans, marijuana plants can tolerate very high temperatures for a short time. The problems begin when the temperatures are consistently above optimal levels. You may ask: what are those optimal levels?

Well, we’ve covered that in another article where we cite available research on the topic. In a nutshell, aim for 77-86°F (25-30°C) during the day and 10°F (8°C) lower at night. When the temp levels creep above the allowed maximum, photosynthesis slows down and you begin to notice stunted growth and heat stress symptoms.

And there’s one more thing we didn’t mention earlier: cannabis pests LOVE hot and humid environments and multiply in great numbers.

A lot of tiny spots on the leaves point to some pest infestation. © Growdiaries © sndwich

How to Take Care of Heat Indoors

First of all, don’t plan any indoor growing for the summer months. Otherwise, your grow tent will turn into an oven when a heatwave comes. Second, try to use lights that generate less heat. This means LED quantum boards instead of HPS bulbs.

A very clever trick is to run your lights during night hours—when it’s generally cooler—and turn them off for the day. The only issue could be light leaks. They don’t matter if you grow autoflowers, but with photoperiod strains, make sure there are no light leaks at the seams. Otherwise, they can disrupt flowering.

You can also try and raise the levels of CO₂ to help cannabis deal with heat stress. For example, you can place CO₂ bags in your grow tent. Please bear in mind that this method is controversial. Available research in other crops draws a very complicated picture. Sometimes, more CO₂ is helpful in dealing with high temps, and sometimes it hurts plants even more.

Clearly, the most important thing is ventilation. Your extractor fan should refresh the air inside your grow space at least once every 3-5 minutes. And if you think problems with heat are possible, make it every minute. The same goes for small grow tents. The smaller your tent is, the easier it is to overheat.

So, calculate the volume of the grow tent in cubic feet, write down this number, and buy a fan with the same CFM (the cubic-feet-per-minute capacity).

Also, install an oscillating fan that will blow just above the canopy level. It will help protect your plant tops from heat burn.

The Temperature in the Root Zone

Hydroponic growers know very well the importance of keeping the roots nice and cool. If the temperature of the nutrient solution gets way above the recommended 65°F (18°C), problems begin. The levels of dissolved oxygen go down, and the pathogens thrive.

The temperature in other mediums, such as soil or coco, is just as important, but soil growers often forget about it and learn how important that is the hard way.

So, when growing weed under powerful bulbs indoors, make sure to shade your pots from the light. Especially if the pots are black and thus absorb the radiant heat easily. For the same reason, it’s safer to use terracotta containers with thick massive walls than thin plastic ones. And when watering your plants, be sure to use water that is not too warm.

Important! Even if the temperature of the air is way off the mark but the temperature in the root zone is within the ideal range, the plants will be perfectly healthy and show no signs of cannabis heat stress.

Measures Against Cannabis Heat Stress Outdoors

Outdoors, you can help your weed garden deal with heat in three ways:

Keep the roots cool.
Shade the plants from the sun during the hottest part of the day.
Help them survive a heatwave with regular watering.

If you grow cannabis outdoors in pots (rather than on beds), make sure the containers themselves don’t receive any direct light. And when the plants grow in the ground, use a lot of mulch to cover the ground and thus insulate it from heat.

In a hot and sunny climate, plant your cannabis in a spot where trees or walls shade them for a couple of hours in the afternoon. However, a more efficient way is to plant your garden in an open spot and spread shade cloth over it when necessary. And if you grow weed in pots, you can move them around, chasing either the sun or the shade as needed.

Watering your garden generously each morning will help the plants deal with the heat. And if the water is cool, it will not only provide moisture but also lower the temperature in the root zone.

Can You Revive a Heat-Stressed Plant?

In most cases, all you need to do is adjust the temperature and simply wait for the plant to get better. However, some growers propose more hands-on approaches.

Silica (Silicon Dioxide)

Many nutrient manufacturers offer supplements that contain silica. They claim that silica makes cell walls tougher, and tougher cell walls—among other things—make sure leaves lose less water during extreme heat and drought. There are also other mechanisms of silicon’s effect on heat resistance.

The good news is that research on crops other than cannabis, such as rice and tomatoes, fully supports these claims. Silica really makes higher plants more tolerant of heat.

And this is a list of cannabis growing supplements that contain silica:

Advanced Nutrients Rhino Skin,
True Plant Science Silica Gold,
Botanicare Silica Blast,
B.A.C. Silica Power,
NPK Industries RAW Silica,
Cyco Platinum Series Silica,
Advanced Hydroponics of Holland Advanced Silica.

Humic Acid

There is ample evidence that humic acid enhances heat tolerance in plants. Researchers have observed these effects in Arabidopsis, tomatoes, and oats. The first of these studies even suggested the exact mechanisms of how humic acid makes plants more heat tolerant. Namely, it regulates several relevant genes, including Heat-Shock Protein (HSP).

Cannabis growers widely use humic acid which is a part of the following products:

Green Buzz Liquids Humic Acid Plus,
NPK Industries RAW Humic Acid,
Earth Juice Rich Humic,
Guanokalong Black,
Humboldt Nutrients Hum-Bolt and FlavorFul.

Seaweed Kelp Extract

Seaweed contains plant growth regulators, aka plant hormones. As their name suggests, these molecules can regulate many important aspects of plant growth. So, growers use seaweed to boost germination and rooting and to increase yields. Some say seaweed kelp extracts help cannabis recover from environmental stresses, including heat and drought.

However, in her review of the available scientific literature, Dr. Linda Chalker-Scott notes a huge disproportion between aggressive marketing of this “wonder supplement” and its middling benefits. In other words, you’ll end up overpaying for something that in fact doesn’t do much.

With that said, here are a few seaweed kelp products that weed growers use:

Kelpak Seaweed Extract ~ Vegetable Fertiliser,
Technaflora Soluble Seaweed Extract,
Gaia Green Soluble Seaweed Extract,
Platinium Hydroponics Engrais Seaweed Extract,
Greenleaf Nutrients Kelp Extract,
Maxicrop Liquid Seaweed / Liquid Seaweed Plus Iron,
Plant Magic Catalyst.

Well, this wraps it up. Now you know how to diagnose, prevent, and treat heat stress in cannabis. And if you live in a hot climate or your grow setup has permanent temperature problems, try cannabis strains that are resistant to heat.

Foxtailing Strains 2022. With Pictures and Statistics

Article Highlights

The tendency to foxtail is most common among tropical sativas
Some varieties, like the Trainwreck family, are notorious foxtailers
Some of the “purple” or “blue” genetics may tend to foxtail
Even if a strain is prone to foxtailing, not every phenotype will foxtail on you
Even “normal” strains may foxtail as a response to heat/light stress
The 11/13 light cycle increases the expression of foxtailing genes, too

This Devil Cream Auto is an example of extreme foxtailing.

In a separate article, we talked about the reasons your buds may foxtail. The usual suspects are heat and light stress. It means that either your plant tops are too close to the lights or there’s been a heatwave that raised the temperature in your growing space.

In yet another post, we also mentioned that keeping your photoperiod strains under 11/13 in flower may result in foxtailing. It’s because foxtailing is a sativa trait and the 11/13 light cycle leads to the expression of sativa traits.

And sometimes you have absolutely no control over foxtailing because it’s genetic. So, if you hate to see your buds foxtail, avoid the strains in the list below. And if you want to grow foxtailing weed, prepare to be patient. When growing cannabis from seeds, you’ll see several phenotypes in the same strain and not all of them will foxtail.

I have personally seen foxtailing in my grow room on a few occasions, but for this post, I needed a bigger sample. So I did a thorough search through the Growdiaries community to find marijuana strains that foxtail more often than others.

Foxtailing in Sativa Strains

You have bigger odds of seeing foxtailing if you grow strains that are pure sativas or have a very high percentage of sativa genes in them. A good example is any strain with “Haze” in its name.

Mature flowers of Dr Grinspoon looking like strings of pearls

A close up of a Super Lemon Haze bud with a spire on top and bullet-shaped calyces on the sides

A view of a grow tent with huge foxtailing colas and a closeup of a cola with many spires

Two Acapulco Gold buds: an indoor and outdoor one, both showing mild foxtailing

Mature swollen buds with signs of foxtailing

Closeups of cannabis buds with huge swollen calyces

A gloved hand holding a huge, mildly foxtailing marijuana cola

Very mild foxtailing on fat and dense marijuana buds

Dr. Grinspoon (Barney's Farm) © Growdiaries

Super Lemon Haze (Green House Seeds) © Growdiaries

G13 Haze (Green House Seeds) © Growdiaries

Hawaiian Snow (Green House Seeds) © Growdiaries

Acapulco Gold (Barney's Farm) © Growdiaries

Laughing Buddha (Barney's Farm) © Growdiaries

A.M.S. (Green House Seeds) © Growdiaries

Tangerine Dream (Barney's Farm) © Growdiaries

Neville's Haze (Green House Seeds) © Growdiaries

Swipe to see all images in the gallery.

Frankly, it’s not always clear if sativa buds are foxtailing or not. Sativas often have thin, spear-like colas. Or the buds may explode in every direction, become very fat but remain spongy at the same time. So, there were quite a few borderline cases, but I tried to do my best. Here are the results.

Strain, Sativa/Indica Breeder	Foxtailing
Strain, Sativa/Indica Breeder	None	Mild	Strong	Extreme
Dr. Grinspoon, 100/0 Barney’s Farm	1	6	9	2
Super Lemon Haze, 70/30 Green House Seeds	16	6	4	–
G13 Haze, 80/20 Barney’s Farm	8	4	1	–
Hawaiian Snow, 80/20 Green House Seeds	3	–	1	–
Acapulco Gold, 70/30 Barney’s Farm	13	3	–	–
Laughing Buddha, 80/20 Barney’s Farm	19	3	–	–
Super Silver Haze, 70/30 Green House Seeds	4	2	–	–
A.M.S., 40/60 Green House Seeds	1	2	–	–
Tangerine Dream, 60/40 Barney’s Farm	20	2	–	–
Neville’s Haze, 90/10 Green House Seeds	–	1	–	–
Pineapple Haze, 30/70 Barney’s Farm	2	1	–	–
Arjan’s Haze #3, 70/30 Green House Seeds	4	1	–	–

The Trainwreck Line

Whenever people start to discuss foxtailing genetics, the name “Trainwreck” pops up every time. Indeed, I managed to find a couple of examples where Trainwreck buds foxtail in a big way.

Extremely long spires on Train Wreck autoflower

Long spear-like marijuana bud with many white pistils

Train Wreck Auto (King Crop Seeds) © Growdiaries

K-Train (Green House Seeds) © Growdiaries

Dark Ghost Train (Rare Dankness) © Growdiaries

Swipe to see all images in the gallery.

Unfortunately, I only found a few grows, so the sample is very small. Anyway, these are the results:

Strain, Sativa/Indica Breeder	Foxtailing
Strain, Sativa/Indica Breeder	None	Mild	Strong	Extreme
Train Wreck Auto, 60/30/10 King Crop Seeds	2	1	1	–
K-Train Green House Seeds	1	–	1	–
Dark Ghost Train Rare Dankness	1	1	–	–
Trainwreck Auto, 55/45 FastBuds	8	1	–	–

Purple or Blue Genetics

When I was searching for foxtailing strains on Growdiaries and on different forums, I was surprised to see foxtailing in such varieties as Blue Cheese, Critical Purple Auto, and Devil Cream Auto. All these strains don’t have much sativa in their DNA, but they’re all purple.

Then I asked myself what comes to mind when I think of purple buds. Well, I picture them either bloated and loose, or with huge dense calyces that shoot in every direction and almost resemble bullets. And sometimes purple buds look like a bunch of grapes or even a string of pearls.

Buds looking like a bunch of frosty pearls

A purple weed bud with many fat tapering offshoots

A weed bud untrimmed and displaying very strong foxtailing and the same bud trimmed

Two different foxtailing colas: one with several side spikes, the other with a spear-like top

Big and lumpy purple buds before harvest

Devil Cream Auto (Sweet Seeds) © Growdiaries

Dark Devil Auto (Sweet Seeds) © Growdiaries

Black Cream Auto (Sweet Seeds) © Growdiaries

Blue Cheese (Royal Queen Seeds) © Growdiaries

Purple Punch (Barney's Farm) © Growdiaries

Swipe to see all images in the gallery.

Well, it’s just a theory of mine that many purple genetics tend to foxtail. You can check it yourself, and below, you can find a few examples. Please note that Sweet Seeds Devil Cream Auto turned out to be the second most foxtailing strain in our review. And both of its parents—Dark Devil Auto and Black Cream Auto—have this tendency, too.

Strain, Sativa/Indica Breeder	Foxtailing
Strain, Sativa/Indica Breeder	None	Mild	Strong	Extreme
Devil Cream Auto, 20/80 Sweet Seeds	10	5	2	3
Dark Devil Auto, 60/40 Sweet Seeds	9	7	1	–
Black Cream Auto, 5/95 Sweet Seeds	11	5	1	–
Blue Cheese, 40/60 Royal Queen Seeds	5	4	1	–
Critical Purple Auto, 0/100 Growers Choice Seeds	14	2	–	–
Purple Punch, 10/90 Barney’s Farm	29	4	–	–

That’s it for now. Later, I may add other genetics to this list. And if you have encountered foxtailing strains in your grows, please share in the comments.

Foxtailing Buds: Good or Bad? Causes, Fixes, and More

Foxtailing buds have their fair share of fans. It’s evidenced by the fact that some growers look specifically for foxtailing strains when they shop for cannabis seeds. However, this is a niche demand, and most of you prefer their buds to be compact and dense, and not elongated, wispy, and airy. Unfortunately, this is exactly what foxtailing in weed leads to.

Foxtailing buds can be due to either genetics or stress, usually heat and light stress. While you can’t fight genetic foxtailing, you can and should do your best to reduce environmental stresses. It’s not just about the bag appeal of your buds, but about their weight, potency, and aroma as well.

What is Foxtailing in Plants?

We say that buds are foxtailing when they begin to elongate uncontrollably at the tip or grow a lot of tapering side shoots. The cola assumes a strange frilly shape that is not to everyone’s liking.

This huge and high-yielding Green Crack Auto has very pointy buds © Growdiaries © Hawkbo

But let’s start with how cannabis buds normally develop. Flower clusters in cannabis consist of many calyces. A calyx is where the seed develops if the buds catch some male pollen. Calyces usually stack up very close to each other, forming those compact fat colas that we love so much.

However, if you happen to grow some naturally foxtailing Sativa or it’s too hot in your garden, calyces may grow far apart, with noticeable spaces in between. Then the cola may indeed remind you of a fox’s tail. (Interestingly, cola means ‘tail’ in Spanish.)

In the most severe cases, each calyx may grow on its own separate ‘stick’, but mostly a string of calyces just grow on top of each other, forming thin tapering spires.

This Zkittlez Auto was flowering during a heatwave, hence the foxtailing. © Growdiaries © Treighfunky

Quite often, you will also see a lot of small, roundish, deep-green, and not very sugary-looking ‘sugar leaves’ growing between the calyces of a foxtailing plant. They look like the plant has second thoughts of reverting back to veg. In short, foxtailing weed is something you can’t miss.

‘Sugar leaves’ on foxtailing buds have a strange shape and color. © Growdiaries © Treighfunky

What Causes Foxtailing Buds?

There are two main foxtailing causes — the plant’s genetic makeup and its specific reaction to environmental conditions. We’d say these two are closely related, and we’ll shortly explain why.

Let’s start with the second one. Why does foxtailing happen if nothing in its genes makes it particularly prone to this issue? Most marijuana growers agree that what causes foxtailing in weed is usually heat stress. Frankly, we couldn’t find any research linking bud foxtailing to heat stress in cannabis, but the issue was studied in other species.

There is the example of heads of cabbage not forming in the tropics. Tropical heat turns a cabbage plant into a tall stick with every leaf on it growing separately. At least, that’s what they told us in school. The same thing happens when people try to grow temperate-zone pines in the tropics. Individual shoots may each become several feet long without any side branching.

In cannabis, this reaction to heat may be a defense mechanism. In the tropics, heat goes hand in hand with high humidity which may spoil the buds if they are too solid. That’s why Sativa buds are generally airier than Indica buds. And foxtailing is one way to achieve that. This would explain why tropical Sativas have more predisposition toward foxtailing.

Is Foxtailing on Weed Bad?

The answer depends on genetics. If your strain of weed has genes that tell it to foxtail, you may not like the looks of the buds, but they may turn out to be outstanding in every other respect. The potency, the character of the high, as well as the aroma and the flavor may be like nothing you’ve ever experienced.

In this case, foxtailing isn’t good or bad per se. It’s just the strain’s feature, and you’ll have to accept it. Or choose another strain for the next time and move on.

The Dr Grinspoon strain is a perfect example of genetic foxtailing. © Growdiaries © DreamCatcher

But if foxtailing is not genetic, treat it as a sign of trouble and adjust your environment accordingly. People often wonder: “Does foxtailing affect potency?” Foxtailing itself doesn’t, but its UNDERLYING cause—be it heat stress or light stress—certainly does. Excessive heat also leads to the evaporation of terpenes, making your buds less aromatic and flavorful. And yields may also suffer.

So, while foxtailing weed isn’t bad, it’s also not the best bud you can hope to grow. It’s blander, less potent, to say nothing of its reduced bag appeal.

How to Fix Foxtailing Buds?

More often than not, the cause of foxtailing on buds indoors is the wrong distance of lights from the plants. You can tell that this is the case if all of the fox tailing is on the tops closest to the light. So, to stop foxtailing, simply raise the light.

Devil Cream Auto foxtailing in a bad way in week 9 from seed. © Growdiaries © scotch_egg

For traditional HPS bulbs, try the back-of-your-hand test. Place your hand under the lights on the level of plant tops, palm down. If you feel a burning sensation on the back of your hand, the light is too close. It means that the buds receive too much heat radiation, and this indeed may be the reason why they foxtail.

LED lights can cause foxtailing on weed, too, but testing the distance with the back of your hand may not work. However, most LED lights worth using in a cannabis garden come with instructions. Follow them and watch how your plants react. If the distance is too short or the light operates at full power, you will see symptoms of light stress and heat stress. If the light is too far off or dimmed too much, you may see an unusual amount of stretch. Try to find the ‘Goldilocks zone’ and stay within it.

The Importance of Ventilation

Sometimes, heat radiating from the light affects not just the top buds but causes the temperature to increase throughout the tent. It happens a lot in summer, especially during a heatwave. In this case, you may see the foxtailing affect not only the tops but the middle and lower bud sites, too. A good extractor fan may help the matter, but using an oscillating fan is also important. By constantly moving the air inside the grow space, it will make sure there are no heat pockets.

For this Auto Kaya 47, the temperature was quite low in flower. So, the foxtailing is probably genetic. © Growdiaries © Marquise_ao_Sul

How to Stop Foxtailing Outdoors

A heatwave can wreak havoc on your outdoor garden, too. In a greenhouse, you can also use fans to lower the temps inside it, but outside, the only way you can help your weed plants deal with the heat is to shade them from the sun during the hottest hours.

If you live in the desert, your best option might be to plant your garden in a spot where trees or structures shade it from the afternoon sun. Or you may span the semi-transparent shade cloth over your plants.

Can Foxtailing be Reversed?

If you have caught foxtailing early on in flower, odds are that you can still get normal-looking buds at harvest. The plant simply has enough weeks of flowering to grow a bunch of normal flower clusters on top of those that were initially foxtailing.

But if the problem persists till late in flower, and buds keep foxtailing near harvest, there’s nothing you can do to change their looks for the better. So be proactive and don’t postpone resolving the issue till the end of flower.

When to Harvest Foxtailing Buds?

In contrast to normal flowers, foxtailing buds have this unfortunate feature: they take forever to mature. You just see more and more calyces forming, with more and more white pistils on them. And when you watch the trichomes on this new growth, they are clear, meaning not ready for harvest. This is another reason to not let your buds foxtail.

It may not look it, but this foxtailing bud has actually been 8 weeks in flower. © Growdiaries © Treighfunky

Other than that, the general rule applies. Harvest foxtailing buds when the majority of the trichomes or all of them are milky and some are starting to turn amber. And if you don’t have a magnifying glass to inspect the trichs, wait at least until all of the pistils (female hairs) wither and turn brown. In either case, the constant emergence of fresh growth on your foxtailing buds will try your patience.

The Good, the Bad, and the Ugly

The bottom line is that fox tailing weed can be a good thing if it’s a genetic feature of some exotic tropical Sativa. But it’s bad if the foxtailing is due to stress.

Stress can do so much more damage to your cannabis than simply spoiling the appearance of your buds. It can negatively affect yields, flowering time, potency, smell, and taste. So, if you see any sign of stress—whether the foxtailing or anything else—act quickly and try to create for your weed the optimal conditions it deserves.

Autoflower Temp and Humidity: With Real-Life Examples

When you grow cannabis of any variety, either photoperiod or autoflower, temp and humidity are the number two and number three most important conditions. Number one is light, number four is CO².

Autoflower temp and humidity requirements are practically the same as for cannabis plants in general. Autoflowers thrive with day temperatures around 23–28°C (73–82°F) and a drop of no more than 5–10 degrees at night. The humidity should be ideally between 40 and 60%. Ruderalis genes presumably make autoflowers more cold-resistant.

It’s important to keep temp and humidity in your autoflower garden within the acceptable range because otherwise plants slow down photosynthesis or stop it altogether. Cold stress or heat stress also shock weed and stunt its growth. So does very dry air, while the opposite—too much humidity—can cause issues with mold and bud rot.

Dealing with excess humidity is especially important during the flowering phase. When buds are getting fat and dense, mold and bud rot are very common problems. Novice growers don’t even imagine these problems exist—until they see their entire crop spoiled beyond salvation.

Mold (left) and bud rot (right) due to wrong humidity and temperature. © Growdiaries

Especially dangerous is the temperature drop when lights go off. The air quickly cools, can hold less vapor, and the excess vapor condenses as dew on every surface, including buds. Coupled with low temperatures, this creates a perfect environment for mold growth.

Btw, one way to fight mold is to defoliate your autoflower at certain moments and thus prevent stale air inside the canopy.

How to Control Temp and Humidity Indoors

Start with purchasing a thermometer and a hygrometer. Ideally with remote sensors so that you can place them inside the grow tent while keeping the monitor outside it. This will allow you to keep track of temperature and humidity without opening the tent. Both the thermometer and the hygrometer should be placed at canopy level.

Ventilation

Proper ventilation is the single most important thing that will take care of temperature and humidity, as well as a constant supply of CO²-rich fresh air. In most cases, an extractor fan alone is enough, especially if it has adjustable speeds. The more advanced extractor fans for grow rooms have temp and humidity sensors of their own. They can be programmed to keep environmental conditions at the desired level.

Air Conditioners and Heaters

Experienced growers may replace ventilation with air conditioning. In this case, a grow room needs to be hermetically sealed and requires a source of CO², such as CO² tanks. It’s doable but rather complicated for an amateur grow.

While too high temps are a constant worry for many growers—because grow lights generate a lot of heat—low temperatures are less frequent. Usually, this happens when people grow in a garage, attic, basement, and other such spaces.

It can get especially cold at ‘night’—when lights are off. However, with autoflowers, you don’t have to turn off lights at all. Most autoflowering strains can be raised from seed to harvest with a 24/0 light schedule. But if you feel like nighttime is necessary and are worried about a temperature drop, buy a thermostat-controlled electric heater.

Humidifiers and Dehumidifiers

If the air is too dry, you can use a hand mister to regularly spray some water on tent walls and other surfaces. Or even plants themselves although not during flowering, or else you’ll run the risk of mold.

You can use such a simple ‘device’ as a towel on a coat hanger with one end dipped in water. The towel will draw up water like a wick and then evaporate it. You can increase the rate of evaporation by making a fan blow on the towel.

However, a more serious approach would be buying an electric humidifier. It needn’t be very big. Even a bedside humidifier may be enough for a 3’x3’ grow tent.

If you have the opposite problem—the relative humidity in your grow room is too high—you can try and reduce evaporation by mulching or covering the surface of your pots. Also, make sure there are no open containers with water in the tent. However, the plants themselves create a lot of vapor in the process of transpiration, so you probably can’t do without an electric dehumidifier.

Autoflower Temp and Humidity Changes from Seed to Harvest

We already mentioned the importance of decreasing humidity during the flowering stage. And also explained the reason. Now we can show more specifically how autoflower temp and humidity requirements change throughout a plant’s life cycle.

The table below shows just that, along with the issues you may have if your temp and humidity readings are way off the mark.

Germination Stage
Ideal Day Temp		Ideal Humidity
22–25°C (71–77°F)		70–90%
Too Cold	Poor germination rates Stunted tap root Fungi/Mold	Too Dry	Seeds won’t germinate Yellow/dry tap root
Seedling Stage
Ideal Day Temp		Ideal Humidity
26–28°C (80–82°F)		60–70%
Too Cold	Slow growth	Too Damp	Damping off
Too Hot	Damping off (pythium) Leaves taco-ing	Too Dry	Slow growth
Vegetative Stage
Ideal Day Temp		Ideal Humidity
22–26°C (71–80°F)		45–55%
Too Cold	Slow growth	Too Damp	Slow growth
Too Hot	Leaves taco-ing	Too Dry	Slow growth
Flowering Stage
Ideal Day Temp		Ideal Humidity
20-25°C (68-77°F)		35–45%
Too Cold	Mold/Bud rot	Too Damp	Mold/Bud rot Powdery mildew
Too Hot	Dry/brittle leaves Lower Yields Less potent buds Terpene evaporation	Too Dry	Slowed down bud growth
Drying & Curing
Ideal Temp		Ideal Humidity
26-21°C (60-70°F)		55–65%
Too Cold & Damp	Mold	Too Hot & Dry	Evaporation of terpenes Smoke smells like hay

If you can’t keep your environmental values within the given ranges, don’t lose sleep over it. It’s not really an exact science. Just keep in mind two things:

Humidity should be higher at the start and gradually decrease by harvest time.
It should be warm throughout the life cycle but a bit cooler during flowering.

Orange Sherbet Auto spending her first week under a humidity dome. © Growdiaries

Again, you may note that the flowering stage requires special attention in terms of autoflower temp and humidity. We already mentioned the risk of mold and bud rot due to low temperature and high humidity.

The other concern is the concentration of THC and terpenes (aromatic substances) in buds. Too high temps during flowering cause terpenes to evaporate, making your smoke bland and flavorless. And the same thing happens when you dry and cure your buds after harvest. As for THC, the heat either suppresses its production or ‘burns out’ what’s already there. In either case, buds grown in hot conditions tend to be less potent.

When Autoflower Temperature and Humidity Are Off: Examples

Monster Zkittlez affected by bud rot (botrytis) at week 11 and the temp and humidity chart for the flowering stage

The grower kept the relative humidity too high (at 55%) in flower and discovered bud rot in week 11. Bud rot, or botrytis, is a kind of fungus that loves high humidity. As for temperature, botrytis thrives when it’s colder than normal or there is a sharp drop in temp between day and night. However, high temps coupled with high humidity may also cause bud rot.

We can’t say that in this grow the temperatures and the humidity were really that wrong. The day temps might have been too high, and the difference between day and night too great. Also, the relative humidity could have been lower for the whole of the flowering stage, especially in the last couple of weeks. The mold probably appeared because the buds were so huge and dense, and the slightly wrong temp and humidity played their part, too.

leaf edges cupping like tacos because of hot summer temperature

In this outdoor garden, there were several autoflowering and photoperiod strains, but only one plant reacted to summer heat with cupping leaves. This symptom is indeed very often the consequence of heat stress, but sometimes it is due to calcium deficiency. What is different about calcium deficiency is the presence of black spots on affected leaves.

Critical Purple Autoflower with foxtailing buds at week 12 and the day and night temperature chart for late flower

At the end of this grow, the temperatures ran out of control. Especially the night temperatures which were the same as day temps for two weeks. And that was the most likely reason for foxtailing which is the growth of new elongated shoots on maturing buds.

seedling fell over because of pythium (damping off) duw to temperature stress

The grower made a mistake of planting his outdoor garden in the middle-of-July heat. At the same time, the nights were rather cool. This temperature stress weakened the seedlings and made them susceptible to pythium, aka damping off. As a result, the stems just above the soil thinned and the seedlings fell over.

powdery mildew spots due to cool nights and 100% humidity

In this outdoor run, the autoflowers were finishing in the middle of autumn when warm days and cool nights led to the dew forming on leaves and causing powdery mildew. The grower didn’t monitor the relative humidity of the air, but it doesn’t matter. When the dew forms, it means the RH is 100%.

Temp and Humidity Requirements of Different Autoflower Genetics

Indicas and sativas have different genetic heritage, so keep it in mind when planning for your autoflower temperature and humidity.

Sativas	Indicas
Originate in hot and humid tropics Have open, well-ventilated bush structure Buds are airy or drawn out Are less susceptible to mold Don’t like cold but are tolerant of humidity	Originate in the arid mountain climate Have a dense, poorly ventilated bush structure Buds are compact and hard Are susceptible to mold and bud rot Don’t like high humidity but tolerate cold

Frankly, if you grow autoflowers, all this is of minor concern. Most autos on the market today are more or less well-balanced hybrids with the indica/sativa ratio fluctuating in the 60-40 to 40-60 percent range. When buying such seeds, it’s a toss of a coin which phenotype you’ll get — indica-or sativa-leaning.

Autoflower Outdoor Temperature and Humidity

Outdoors, autoflowers are very rewarding because you can bring them to full maturity in any 2-3 months of the growing season. Even in very cold climates, there are at least 2 months of what may be called summer weather. And if the season is long enough, you can have several consecutive harvests. Or choose the optimal period in terms of temperature and humidity.

Gorilla Glue grown outdoors. © Growdiaries

We’ve written a separate post about outdoor marijuana temperature (it has all the numbers you’ll need). Let’s stress just a couple of points here:

better start your seeds indoors and transfer them outside at 2 weeks from sprouts,
try not to expose young plants to day temps lower than 15°C (59°F) and night temps lower than 10°C (50°F),
make sure night temps never get below freezing point,
if possible, time the grow so that it’s not too hot during flowering.

Btw, transferring an autoflower outdoors can be stressful for its root system. Click the link below to learn just one hack of how to reduce transplant shock.

Greenhouse Could be a Better Alternative

In colder climates, it’s best to plant your autoflowers in a greenhouse which will protect them from cold weather, rain, and strong winds. A greenhouse can also extend your growing season by two whole months.

A greenhouse grow of Zkittlez OG Auto, week 10. © Growdiaries

The one mistake you should avoid is sealing your greenhouse completely for the night. The thing is that when the air inside the greenhouse cools down on a cold night, heavy dew forms on everything. A couple of nights like this and your crop will begin to rot, get covered by powdery mildew and what not.

So leave the greenhouse open for the night to let the excess moisture evaporate. It’s better when it’s very cold but ventilated than kind of cold and sealed off hermetically.

Final Thoughts

Although we were talking about autoflower temp and humidity, the general principles apply to any weed variety. If you want vigorous and healthy growth without any issues, try to always be in control of environmental conditions. If you manage to keep them within the given ranges, you’ll get tons of chunky heavy buds at harvest. Happy growing!

All images in this post were taken from GrowDiaries, the world’s largest weed growing community.

Outdoor Marijuana Temperature Range: Ideal, Okay, Extreme

If your climate is too cold (like mine), or too hot, and you think that the only option for you is indoor cultivation, think again. The acceptable outdoor marijuana temperature range is quite wide, and marijuana plants, at least mature ones, can be surprisingly hardy. As a result, they can take a lot of bad weather and still reward you with enough high-quality buds.

Ideal Outdoor Marijuana Temperature

Well, ideal outdoor marijuana temperature is actually the same for any type of grow. It’s because indoors we simply try to recreate artificially the natural environment that cannabis plants have best adapted to. The only difference is that, in an indoor setup, we have more control in maintaining the perfect conditions. And they are:

77-86°F (25-30°C) during the day, or when the lights are on.

At least these are the figures that are the most conducive to photosynthesis and gas/water exchange processes, according to research¹. And, in case you’re wondering, the ideal level of light intensity (PPFD) would be ~1500 μmol/m2/s.

All this may guarantee the fastest rate of growth for cannabis plants. However, our goal is a bit different — we grow for the highest possible production of THC and other cannabinoids, as well as terpenes that make the smoke so aromatic and flavorful. So, with that in mind, growers have gradually come up with the following empirical figures:

Seedling and vegetative stage. Young weed plants prefer a little milder conditions than the flowering ones, with day temperature between 70 and 85°F (20-30°C).
Flowering stage. When plants start to form buds and then all the way to harvest, the temps should be a little lower — 65 to 80°F (18-26°C). The reason is that higher temperatures burn terpenes and cannabinoids making the buds less aromatic and potent.

During any stage of growth, the night temperatures should be 10°F (8°C) lower than during the day. This gives marijuana plants a chance to shift to a lower gear and have a rest. However, too radical a change between day and night temperatures can lead to even more stress, and should be avoided.

That Was Ideal, But What is Acceptable?

Outdoors, we work with what we have. Even in climates that give us the luxury of the perfect outdoor marijuana temperature range there are spells of too cold and too hot weather. So the question is: what is okay for outdoor weed and what is not?

Just like a dashing bachelor in a Jane Austin novel who chooses himself a wife, anything from 15 to 30 would do just fine. (Sorry for this potentially offensive remark, but in my experience they are the most memorable.) These figures are in Celsius btw, and in Fahrenheit it’s 59° to 86°.

At higher temps, no irreparable damage is done, although the photosynthesis slows down. And so does the THC production, while terpenes evaporate from buds at a faster rate, robbing them of flavor. The texture of buds deteriorates as well, and they become less compact and more airy.

So if you live in a really hot climate, like Arizona or Nevada, try to mitigate the effects of heat stress. Make sure that trees, walls or other structures provide some shade for your marijuana during the hottest hours in the afternoon. Or stretch some shade cloth over your garden (60% transparency should be just fine). Anyway, if higher temps may hurt weed plants, they can’t kill them. Drought can though.

The effects of cold temps are much worse. When the daytime temperatures drop below 15°C (59°F), all processes slow down to a crawl or are put on pause. And at 13°C (55°F), more delicate strains may experience a shock. It means that once the temps are back within the acceptable range, the plants will need some time to recover before they resume bud production.

The Effects of Frost and Snow

Below freezing air temperatures don’t kill marijuana overnight, but they can do a lot of damage to leaves, flowers, and even branches. They wilt and get brown and mushy in places, so if there’s a chance of snow or even a couple of frosty nights in a row, it’s best to harvest your buds before that.

These buds might be still salvageable, but the quality will be very subpar, to say the least.

I myself once harvested a couple of bushes after a heavy snowfall. They started to flower extremely late, so I had to push the limits with them. When I was cutting the branches, I had to shake them very hard to free them from snow. Only a minor percentage of buds were spoilt beyond being usable, but the rest turned out okay. I don’t recommend this to anyone, though. It’s safer to use a faster-flowering variety or an auto.

How to Protect Cannabis from Cold

1. Choose Genetics Wisely

Start with choosing a strain that is fast enough to finish flowering before the end of the growing season. The only sure thing are autoflowers because in practically any climate there are a couple of warm months in the middle of summer, and it’s all an auto needs. Besides being very quick to finish, autoflowers are more resilient to cold weather: they have Cannabis ruderalis genetics in them, and ruderalis is a subspecies that originated in places like Siberia and is very tolerant of cold.

This is a natural ruderalis plant. Looks pathetic, doesn’t it? Luckily, modern autoflowers are so much better in every respect.

If you prefer to grow a photoperiod variety, look for strains with words ‘fast’, ‘early’ or ‘quick’ in their names. They are the result of crossing a true photoperiod strain with an autoflowering one. These plants still wait for the days to become short enough before they start flowering, but this happens much earlier.

Important! When shopping for an outdoor photoperiod strain, take with a grain of salt the harvest times that a breeder promises. If, for example, they say late September, they probably mean places like Spain or Southern France, not Scotland or Denmark. The farther up north, the longer the summer days, and the later the flowering begins (and finishes). That’s a ‘perk’ of living at higher latitudes.

2. Find a Good Spot

The second most important thing is finding a spot that receives as much sunlight as possible and is sheltered from wind. It also helps to plant your cannabis near a wall (preferably a stone wall) that shelters it on the north side and is warmed by the sun. And by all means avoid planting your cannabis in a low spot. The thing is that cold air tends to ‘roll down’ from higher places into depressions in the ground, and then stays there. A south-facing slope makes for an ideal spot for your outdoor garden.

3. Using Pots Increases Mobility

Another trick that’ll help you make the best of your cold climate is growing your outdoor marijuana in pots rather than in the ground. In this case, you can move the pots around, always choosing the sunniest and warmest spot and even take the plants inside if their very survival is under threat.

4. Start Seedlings Indoors

Keep in mind that, while mature plants don’t mind some cold weather, young seedlings do. They may be shocked or killed if the temps are outside the acceptable range. So make sure that summer weather has come for good—no ground frosts in the early morning and at least 15°C (59°F) during the day—before taking the seedlings outside. Also, it’s recommended not to sow cannabis seeds directly into the ground. It’s best to grow them for 2 weeks inside, then harden them a bit by taking outside for a couple of hours every day, and finally place them permanently outdoors.

Outdoor Marijuana Temperature: Focus on the Root Zone

One thing that is overlooked by most growers is that the temperature of the air plays a far less important role than that of the medium. The air may be as cold as 5°C (41°F), but marijuana plants will thrive if the temp in the root zone is at a comfortable level². It’s like when you are neck-deep in a hot spring, you don’t mind that your head is exposed to a chilling breeze, right?

Remember all those numbers we’ve given above? They all apply to the root zone.

This really gives you a lot of freedom. The most obvious choice is to use a really thick layer of mulch to insulate the ground from the air. It works both ways, by the way: by keeping the ground cooler in hot climates, or keeping it warmer in cold ones.

Or you can go high-tech. A pump, a water tank with a heater/cooler and some clever piping laid in the ground or through the containers/grow bags will allow you to circulate water with a set temperature to warm up or cool down the medium. (And mind you we’re not talking about watering here, right? Watering is a different story. This is only about heat exchange.)

Also google ‘geothermal greenhouse’. This is basically an underground (or partially underground) greenhouse with a glass roof that is generally made sloping and facing south. A geothermal greenhouse makes use of the fact that deep below the ground surface the temperature remains stable throughout the year. It’s much cooler in summer, and much warmer in winter compared to above ground. For such a greenhouse to really work its magic the depth must be 10-12’ (~ 3 meters).

Outdoor Winter Grow

Wanna try and grow marijuana in winter? First check if your climate supports this idea. Again, the temperature, especially in the root zone, should be above 15°C (59°F) most of the time. Short periods of colder weather are unwelcome, but acceptable. Opt for cold-resistant strains, like Indica autoflowers. Also, use all the tips and tricks of protecting your plants from cold that we’ve shared above. And again don’t forget about the importance of the root zone).

Also keep in mind one more thing about marijuana winter grows: the days in winter are short, and the nights long. This will make any photoperiod variety start flowering after just a few weeks from sprouts. It’s not unlike the ‘12/12 from seed’ method. This usually results in much smaller plants, so if you want them bigger, think about auxiliary lighting to make the days longer.

If you don’t use auxilliary lights to extend short winter days, any photoperiod strain will go into the flowering mode.

Conversely, if your winter cannabis doesn’t finish before spring, or more specifically before the spring equinox (March 20), days will gradually become long enough to disrupt the flowering. So make sure you plan your winter grow accordingly.

Outdoor Marijuana Temperature Range is Quite Inclusive

Outdoor horticulture of medical marijuana is feasible even in colder climates. Just learn to make the best of the most adverse conditions and choose a strain that is quick to finish and is tolerant of low temperature. And now, after the arrival of autoflowers, almost anybody can grow top-shelf buds outdoors.

External Links

Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions, Suman Chandra et al., Physiology and Molecular Biology of Plants, October 2008
Root Zone Heating & Cooling of Cannabis (PDF), Sustainable Agricultural Technologies Ltd.

Cannabis Seedling Problems: How to Resolve Any Issues

Newbie growers tend to run into cannabis seedling problems with their plants from day one. That’s why our diagnosis tool can come handy to you. Start by finding your cannabis seedling issues in the list below (we tried to group them for more convenient navigation), and then use page jumps (or scroll down) for possible causes and solutions.

It may be a serious disease, or just a temporary issue that is easy to fix, or even a perfectly normal thing that you mistook for a sign of trouble. In the latter case, we recommend you to read our article on the anatomy, development and look of a healthy seedling. And, of course, it’s better to ensure that no cannabis seedling problems arise in the first place, so we encourage you to read another article with tips on creating perfect conditions for your sprouts and caring for them the right way.

Symptom	Possible Cause/Remedy
weed seedling won’t shed shell	see Removing the ‘Helmet Head’ by Hand
weed seedling not opening	see Removing the ‘Helmet Head’ by Hand or The Problem of Weed Seedling Damping Off and Other Root Problems
marijuana seedling too tall weak/thin stem seedling bending/falling over	see Seedling Stretching: Reasons and Solutions
cannabis seedlings keep dying seedling sprouted but not growing seedlings not growing true leaves	see The Problem of Weed Seedling Damping Off and Other Root Problems
cannabis seedling very short	see The Problem of Weed Seedling Damping Off and Other Root Problems or The Seedling Gets Too Much Light
weed seedling limp	see Water Stress (Overwatering / Underwatering) or The Problem of Weed Seedling Damping Off and Other Root Problems
leaves light green and going yellow marijuana seedling leaf tips turning brown	see Cannabis Seedling Problems with Nutrients
marijuana seedling turning yellow	see The Seedling Gets Too Much Light
leaves turning white / bleaching	see The Seedling Gets Too Much Light or Cannabis Seedling Problems with Nutrients
yellow veins or seedlings yellow in middle white / red / purple / brown stem purple leaves/veins	see Discoloration Isn’t Always a Sign of Trouble
marijuana seedling dark green	see Cannabis Seedling Problems with Nutrients
leaves pointing up seedling cupping/praying leaf edges curling up	see Heat Stress is Common Among Cannabis Seedling Problems
leaves pointing/curling down	see Water Stress (Overwatering / Underwatering) or Wind Burn
wavy leaves	see Water Stress (Overwatering / Underwatering)
deformed/twisted leaves	see Weed Seedling Mutations
cannabis seedlings eaten / uprooted / broken etc.	see Physical Damage: When a Seedling Can and Cannot be Saved?

Removing the ‘Helmet Head’ by Hand

Too often, when a weed seedling is sprouting from the medium, the cotyledons are still stuck in the shell and it won’t come off on its own.

This is day 1 for three OG Kush Auto seedlings. Only one of them has cast off its shell on its own.

Probably, most seedlings will find enough strength to eventually shed the shell, but meanwhile a lot of time will be lost and you’ll see a lot of stretching. So you have to remove the shell yourself as soon as possible. Moisture is the key here. If the seedling is still tiny and can be covered by moist medium, by all means do so. Probably it will be enough. If not, the shell will be wet enough in a couple of hours, so that you can easily remove it with your fingers.

If the seedling is already too tall to be covered with wet soil, spray it with water or put a droplet of water on the shell: this will make it wet and mushy after a while. Then remove the shell with your fingers.

We’re wetting the shells with a drop of water. It will be enough to easily remove them from cotyledons.

Sometimes the shell comes off, but a thin membrane covering the cotyledons remains. Usually, it’s the reason why weed seedling is not opening. The membrane is too dry and acts as a straightjacket. Make it wet and when it gets soft, pluck it with your fingers. It usually slips off quite easily.

We have removed shells from the seedlings on the right and in the center. The one in the center still retains the membrane.

Warning: Sometimes, the seedling hasn’t rooted too deeply and has still a very short tap root (like an inch or so). In this case, the procedure described above can result in your plucking the sprout out of the soil completely. So never apply excessive force. It’s better to make the shell wet again and wait a bit more. If you did pull the sprout out of the ground, you can still stick it back in (probably, it’s best to remove the shell first). Most seedlings can survive this without apparent shock or stress.

In your future grows, make sure that sprouts emerge from the medium already without a shell. Keep the surface of the medium moist at all times, and you probably should think about placing the seed a little deeper than you did before. In this case, the seedling will encounter more resistance on its way up and the shell will slide off.

Seedling Stretching: Reasons and Solutions

Most seedlings are tall and skinny and look vulnerable, and generally it’s not a problem. But sometimes the stem is growing too tall, weak and thin and can’t support the weight of cotyledons anymore. As a result, the long stem is not growing straight, but sideways or even upside down. The reason is simple: there’s not enough light. Either the source is too weak (e.g. you keep your seedlings on the window sill), or the light is placed too high above. In either case, seedlings stretch toward light uncontrollably and keep falling over.

We still recommend that you play it safe the first couple of days and keep the light at a distance. Let the seedling reach the height of 2-3 inches (5-7 cm) – it’s normal and healthy and much better than the other way around: when the sprout is stressed by too much light (and sometimes heat). Slowly correct the distance before you see the seeding bending, leaning or even falling over. In time, you’ll know the exact distance for your type of bulb.

But what to do if a seedling already bent and flopped over? You can always use a toothpick to prop a weed seedling not standing up. However, it is best to make the stem that is too long shorter. You can do this by adding more soil (or coco, or other medium) – then the part of the stem that you cover by the medium will grow more roots, and it’s great. Or, if the container isn’t tall enough for that, you can press the seedling to the soil horizontally (it’s easy when the stem is already bent or the seedling fell over) and cover it with a thin layer of soil. If there’s too much resistance, gently crush the stem with your finger tips, probably just above the surface – it won’t really hurt the seedling.

This seedling has clearly stretched too much and needs to be made shorter. Strart by crushing its stem with your fingers just above the surface of the soil and then do the same in several places above.

Now that the stem is soft and pliable, lay it on the soil, probably in a half-ring shape.

Now cover the bent stem with soil and don’t forget to water. Note that this procedure will slow down the growth a bit, compared to seedlings that you didn’t let stretch in the first place.

The Problem of Weed Seedling Damping Off and Other Root Problems

Stunted growth, or slow growth, is common among cannabis seedling problems. The reason can be anything: from wrong conditions to stress to shock, but here we’ll talk about problems in the root zone. Healthy root growth is the single most important factor for a plant’s well-being, and if you see that your marijuana seedling stopped growing or is growing very slow, or especially if cannabis seedlings keep dying on you time after time, the first thing to check are the roots and what’s going on with them. Are they affected by some type of fungus? Is there root rot?

Most of the time, these problems arise when the medium is constantly wet and cool at the same time. These are ideal conditions for mold and fungi, and can lead to the weed seedling damping off. Apart from the seedling not growing leaves or its slow growth in general, you will notice that the stem gets very thin just above the surface. Next thing it will flop over. And, of course, you may notice some whitish substance on the surface of the soil. When the soil feels damp and cold to the touch, it happens very often. And the problems are not just on the surface, but more importantly deep down, too.

Cannabis Seedling Problems Can be Caused by Bugs

Another reason why a seedling has sprouted but is not growing is that some kind of bug living in the soil has eaten the tap root. You wonder why the seedling won’t grow without realizing that there is in fact nothing at all below the surface, no roots to speak of. This sprout will die, and there’s nothing you can do about it. To prevent this from happening in the future, kill all the parasites in the soil with heat. Just make the soil wet and place it into the oven at around 200° F (90-100° C) for a half hour or so.

A Root-Bound Seedling is Hardly Ever a Problem

Some novice growers wonder if a cannabis seedling can become root bound. It’s the situation when there is too little room in the medium, so that the roots reach the sides of the container and start to grow along them, simply because they have nowhere else to go. It’s quite a common situation for mature plants growing (or shall we say ‘not growing’?) in too small pots, but seedlings generally haven’t enough time for their roots to use up all available space. So seedling becoming root bound is hardly ever an issue.

And, of course, if you feel that your marijuana seedling is growing slow or not growing fast enough for you or is very short, make sure that you don’t expect too much. Read about the healthy and normal seedling development here.

The Seedling Gets Too Much Light

Most marijuana growers would swear that the more light the better. However, the issue of too much light is the real thing, especially for weed seedling. We have been sent countless pictures of seedlings showing the signs of stress from too much light. To be frank, we sometimes (erroneously) diagnosed the issue as potassium (K) deficiency which is often manifested by the lower leaves turning yellow at the edges. However, if you see this symptom in sprouts, the first thing you should suspect is that you have a source of light that is too intense or too close.

You may ask why then this symptom (yellow spots along the edges) appears not on the topmost leaves, but on the lower ones? Well, it’s because the lower leaves have had more exposure to the stressful levels of light (simply because they are older and have been around longer). Leaves yellowing due to too much light are very rare if you use CFL or T5 lights, but with HID (high intensity discharge) bulbs, like HPS or MH, or with the LED technology, these problems are very common.

Too Much Light and Heat Stress Often Go Hand in Hand

If you see seedling leaves turning white, sometimes to the point of complete bleaching, it also can be caused by too much light, but also made worse by heat stress (see below) and maybe calcium deficiency (these two often come hand in hand). Remember that for plants grown in coco calcium deficiency is especially common.

Another sign that your light is simply too much for this stage is that the seedling stays very short. It not only has no reason and motivation to stretch toward the bulb, but the dense flux of photons is constantly pounding on it, causing it to keep close to the ground.

If you see any of these signs, simply raise the lights, or use weaker ones.

This seedling was exposed to the harsh light of a LED bulb for too long. That’s why its cotyledons and the first pair of true leaves look yellow, dry, and brittle.

Another severe case of too much light. Again the culprit is a LED bulb that was kept too close to the seedling.

These two plants stand next to each other under two CFLs each. There’s clearly something wrong with their lower leaves. Our guess is they have received too much light. See how compact and dense they are.

Cannabis Seedling Problems with Nutrients

When you see a yellow leaf on your seedling, it’s not necessarily the symptom of too much light (as described above). It can also be a nutrient deficiency. It is seldom the case in soil, because soil mixes come preloaded with fertilizers, organic or synthetic, and it’s enough for the first two weeks or so. But if you use some sterile medium like coco or rockwool, it’s your job to provide your seedlings with plant food. Unfortunately, it’s more than easy to make mistakes here.

This seedling looks yellow and sickly. Most probably, it suffers from many deficiencies, including N deficiency. Note that the medium here is coco. In soil, you can give your seedlings only plain water and still see no deficiencies whatsoever.

Read carefully the instructions for your nutrients formula and follow them, not forgetting to adjust the dosage for various stages. Look at the color of your seedling: its leaves should have a healthy and vibrant green color. If leaves are light green or yellowish or you see yellow cotyledons, it could be nitrogen (N) deficiency. Although sometimes, if you overwater your young plants for days and weeks, they can show the same symptom (but there will also be others, more specific; see below).

If the seedling is dark green, it looks more like N toxicity (too much nitrogen). In severe cases, seedling leaves will also be curling down.

If you see such deep, dark green color of the leaves, probably you’re feeding your plant too much nitrogen (N).

This is how nitrogen toxicity can look in a mature plant. Also check if it’s not a wind burn (if there isn’t a fan too close to the plant).

We already mentioned seedling bleaching and white spots while discussing the problem of excessive light, and mentioned that it could be caused or made worse by calcium deficiency, most common in coco.

And surely there is this widespread symptom of leaf tips turning brown which is a sign of nutrient burn. It simply means that the dosage of fertilizers that you give your plants when watering (or that is present in the soil mix) is way too high, and you should lower it when you see your seedling burn like this.

Discoloration Isn’t Always a Sign of Trouble

When you suspect that the color of your seedling is not quite normal, maybe you’re wrong. Of course, yellow hues should always raise a flag. The exception is when you look at a plant the first thing in the morning and see yellow in the middle of uppermost young leaves or yellow veins. It could simply mean that this is the new tissue that has grown in the dark and simply hasn’t had a chance yet to be filled with green chlorophyll. Expose it to light, and it’ll become green in no time.

Stem color can vary from plant to plant. It can be white, red, purple, and sometimes even brown. If there are no other worrying signs, it’s probably natural and caused by the specific genetics. Purple stem, veins, or whole leaves can also be due to colder night temperatures (in genetically predisposed strains). So when your seedling is turning purple, see if it doesn’t get too cold at night.

Heat Stress is Common Among Cannabis Seedling Problems

If your weed seedling experiences heat stress, it affects the shape of leaves in a very specific way. You may see the leaves pointing up.

Back in the day, we used to keep CFL bulbs too close to the seedling, and, instead of shying away from it, the seedling seemed to try and hug the bulb with its upper leaves. Other growers see it differently: to them it seems as if the seedling were praying. Still others name it ‘cupping’.

The underlying cause is the same though: not able to move away from the light, the seedling tries to position its leaves in such a way that the light falls on them only obliquely. This helps them absorb less heat energy. If you continue to expose your plants to heat (be it hot air, or the energy from the light), you’ll see a more sinister symptom: leaf edges curling up. The heat often aggravates calcium deficiency and vice versa.

To combat this problem, always try to maintain the correct temperature range in your grow room and make sure the light isn’t close enough to burn. The ‘backhand test’ can help you with that. Place your hand palm down under the light at the level of plant tops and hold it there for some time. If you feel the light ‘bite’ or there’s any discomfort, it’s best to raise the light.

If leaves on your cannabis plant (at any stage) curl up like this at the edges, it means they are overheated by light or hot air.

This young plant kept under a CFL has grown so fast overnight that it actually touched the bulb. Hence the burned leaves on top.

Water Stress (Overwatering / Underwatering)

Newbies overwater marijuana all the time. Luckily, the plants have a way of showing their caregivers that they overdo the watering. The first sign of overwatering is clawing of the leaves, of their curling down. When you touch the leaves on an overwatered plant, you’ll feel that they are quite rigid. Conversely, if the leaves are pointing down, but feel limp and lifeless, the seedling is probably underwatered.

Leaves on this seedling are shriveling up and curling down. The plant has clearly been overwatered. At the same time, it shows signs of heat stress.

Leaves can even change color because of water stress. If you keep them overwatered for long periods, they’ll begin to yellow and eventually die off. The bottom line is: when you see marijuana seedlings shriveling up, correct your watering schedule.

This chronically overwatered cannabis seedling has leaves that look yellowish and lifeless along the edges, almost as if they were touched by frost.

Sometimes, weed seedlings have wavy leaves. We don’t know the exact reason why it happens, but have this theory that the issue is due to irregular watering. No one has yet disproved this theory, and we continue observations. In any case, it’s not a grave symptom. Seedlings with wavy leaves may not look fit for a beauty contest, but are otherwise healthy and vigorous.

All of these seedlings have wavy leaves: a not uncommon symptom. Probably, at some earlier point, the grower has overwatered these plants.

Wind Burn

Another reason weed leaves can become deformed is when there is strong wind blowing on them all the time. For instance, you may have placed a fan close to a seedling, and it’s blowing straight at it and not above it. After a while, it may cause wind burn. Leaves that are stressed this way are pointing down and forming ‘claws’ similar to overwatering. But in this case, they also curl at the edges in the shape of tubes. Often, they also have dark green color, like they do when there is too much nitrogen in their feed.

Weed Seedling Mutations

Deformed, twisted, shriveled leaves – these symptoms can appear in marijuana seedlings for no reason and should be considered mutations. These plants are never the best specimens in terms of growing, but, if you bring them to harvest, the smoke may surprise you. It’s up to you to choose what to do with mutants. Seeds don’t come cheap, and you may decide to grow every one of them. That said, breeders always discard seedlings with leaves twisting, as well as with other mutations. There is one interesting mutation though that we personally wouldn’t mind to keep and propagate: weed seedlings with 3 leaves. Usually, leaves grow in pairs, opposite each other on the stem. But here you have three leaves growing from every node. Delightful!

Twisted leaves point to some kind of mutation. You can still succesfully bring this plant all the way to harvest, but it’s not the genetics that you’d want to keep.

A rare and cool-looking mutation: a cannabis plant with 3 leaves.

Physical Damage: When a Seedling Can and Cannot be Saved?

A special case of seedling problems is when a grower damages it by mishandling. A seedling can be knocked over and uprooted. If you don’t disturb the root system too much, the more vigorous sprouts can survive the repotting without even slowing down their growth.

Root damage is a bigger issue. It often happens during transplant when a piece of soil falls off, tearing off a part of the root ball in the process. You just put the rest back in soil, and hope for the best. When the root broke, it’s a significant stress for a seedling, so try to make its life for the next couple of days less stressful. You can do this by creating milder conditions, like light and temperature, and, of course, refrain from any major changes (repotting, switching to 12/12, or moving outdoors) until the damaged seedling resumes its growth.

And what if the stem broke? Partially broken stem can heal, no doubt about it. Just make a splint and put a bandage where the seedling snapped, and the tissue will not only heal, but even be stronger. Even if you break the stem clean off, it arguably can be made whole again in this way.

What you can’t do is make any use of a seedling that only consists of the roots and the stem, but no growing point, in other words no leaves, as when something has eaten them. The thing is that cannabis doesn’t grow from roots. There are simply no growing points there, so you can’t salvage such seedlings.

Say Goodbye to Cannabis Seedling Problems!

Most cannabis seedling problems can be resolved, and quite easily, too. Just don’t freeze with panic and use common sense. After all, there’s nothing magical about cannabis. It’s just a plant, and one that is quite hard to kill. And if you fail to save a particular seedling, learn not to repeat the same mistake next time.

External Links:

SEED GERMINATION AND SEEDLING GROWTH, Keith Luna
Variation in germination and seedling development, Md Anisur Rahaman

Cannabis Seedling Care: Tips and Optimal Conditions

If you want great results come harvest time, start to build your success early — with the proper cannabis seedling care. This includes creating ideal conditions for your seeds when they still germinate and following our tips for the best practices during the seedling stage. It will save you time growing your sprouts and ensure that they move to the next stage (the vegetative one) full of vigour and in the best possible shape.

Medical marijuana is mostly grown indoors. And even if you keep your adult plants outside, you’d want to start them inside. The reason is that the seedling stage is the most delicate and vulnerable in the cannabis plant’s life. So most of what follows deals with indoor growing: a grow tent, a growbox, a special marijuana seedling starter kit, or even a window sill. And if you are willing to risk starting your sprouts outdoors, scroll to the bottom part of this article.

Also, some of you have probably never grown pot before. So you have no idea how a healthy young plant looks like, how fast it grows, and what are the stages of its development. We suggest that you read our article on weed seedling anatomy and growth progress (where you can also look at pictures). And if you suspect that you have run into some problems with your sprouts, read our guide to diagnosing and resolving cannabis seedling issues.

Cannabis Seedling Care: Let’s Create Ideal Conditions

Cannabis seedling care is a very easy thing. Young plants don’t require much light, water and nutrients, and there’s very little that you can do in the way of plant care. Instead, focus on creating the best possible conditions and then simply sit on your hands.

Marijuana Seedling Temp and Humidity

Temperature and humidity are of utmost importance. Moisture is what sets off marijuana germination, and the correct/ideal temperature range allows sprouts to grow vigorously from day 1. For marijuana in general, 70-90° F (20-30° C) temps during the day (when lights are on) is acceptable. But it’s better to aim at 73-82° F (23-28° C). Seedlings prefer the upper part of this range. Let’s say 80-82° F (26-28° C). The night temps should be about 5° lower.

Temperature (day)	80-82° F	26-28° C
Temperature (night)	5° lower than day temperature
Relative Humidity	60-70%

The correct humidity range for cannabis is the same as is recommended for human dwellings: 40-60%. It’s very convenient for those who grow at home. Seedlings prefer a bit more humid conditions than vegging or flowering plants. So aim at 60 to 70 percent, and you’ll be alright. If the air in your grow room is drier than recommended, you can use a humidity dome to cover your sprouts. Better still, buy a starter kit for seedlings which comes not only with a dome, but also with a heat mat. This allows for easier temperature control as well.

Marijuana Seedling Light Needs: Type, Intensity, Distance

Weed plants need light throughout their life cycle, but during the seedling stage not really so much. And if you wonder how much light is ideal, we recommend to err on the side of caution.

It’s hardly the best option to keep your cannabis seedlings under HPS grow lights. Use weaker lights, instead, like CFLs, T5s, or LED lights. This will not only allow you to save on electricity, but spare your sprouts from shock. And if you insist on using HPS or other high intensity discharge (HID) lamps, raise them high, probably as high as your grow room allows.

Still, under light of this type, seedlings can show signs of stress, slow down their growth, and demonstrate various worrying symptoms. In this case, though it may seem counterintuitive to you, you’ll see better results if you temporarily keep your weed in the window — until it’s mature enough to handle strong light.

When to Put Weed Seedling Under Light?

Some growers do it even before their sprouts emerge from the medium. Others wait first for the germination to complete. So, when the seeds break the surface, put them under light. Before that, it’s completely optional.

Distance from Light

If you follow our recommendations and use weaker types of light, like CFL, there’s a question of CFL light distance. The optimal light height is at least 4 inches (10 cm) for the first couple of days, even for such weak bulbs as 20W or so. Let the seedlings stretch toward light instead of being pounded by it. If you see that your seedling stays short and its leaves won’t grow or worse—turn yellow and dry—the light is too close.

The same goes for young plants that grow under T5 or under LED lights — keep a comfortable distance. Nobody likes stretchy plants and especially stretchy seedlings, but 2-3 inches (5-7 cm) in the first few days is healthy and desirable. Unless the stem bends over, it’s okay. So start far away and move the light closer, if your plants start to stretch too much. If you use LED and can measure PPFD, start with a conservative level of 200, although you can get away with 300-400, too.

Is Ventilation Important for Cannabis Seedling Care?

Very small plants obviously need very small amounts of light and water for photosynthesis, and, correspondingly, very little CO2. And one of the main reasons to use ventilation in your grow room is to provide plants with the fresh supply of CO2. The bottom line is you don’t really need any extractor fan for seedlings. But the other reason to use ventilation is to control temperature. If you need fans for that, by all means use them.

There is another type of fan that growers use in their grow tents — the one that blows air in the vicinity of plants and helps to mix air inside the grow space, to prevent mold issues, and to cause the plants to sway a little and thus make stems and branches stronger. You can use a fan on seedlings for this purpose, but be sure that it doesn’t blow directly on them. Otherwise, you might experience the cannabis seedling wind burn.

Marijuana Seedling Light Schedule

Even novice growers understand that cannabis needs light to transform its energy into plant matter. And the more light hours a seedling gets, the more energy it has a chance to absorb. So the question is: do marijuana seedlings need 24 hours of light?

They can certainly take it, no problem, but they hardly really need so many light hours. Many experienced growers argue that nighttime is important for plants because it gives them some rest which protects them from overstress. Besides, there is this theory that the root system grows more actively during dark and cool hours, so think about it when you decide not to turn off lights in the evening, or decide to give your plants a good watering before the lights-off. Either action won’t do your plants any good (probably).

So we suggest that you experiment with more conservative light schedules: 18/6 or 20/4. However, in our personal experience, the 24 hour light schedule works fine, too. Just don’t forget to change it to 18 hours of day and 6 hours of night when the seedling turns into a young plant, i.e. it enters the vegetative stage and starts to grow very rapidly.

Weed Seedlings Can be Grown in Various Mediums

The choice of the medium really depends on where you plan to keep your plants after the seedling stage is over and you are ready to transplant. It’s very convenient to put sprouts into rockwool for a DWC grow. When you grow in coco, it’s best to start in coco.

If your grow medium of choice is potting soil, use small pots or party cups filled with soil. As far as pot size is concerned, 8-10 fl oz (250-500 ml) containers should be enough for the first 2 weeks. Alternatively. you can grow seedlings in jiffy pellets or jiffy cups. Some growers look down upon them, but they are definitely convenient when you need to transplant without stress. Especially if you start your seedlings indoors but plan to move them outside.

Marijuana Seedling Transfer

You should never forget that repotting marijuana seedling is a risky and potentially stressful procedure that can slow down or even stop growth, and this is especially dangerous for autoflowers. If you don’t want to see your cannabis seedling drooping or wilting after transplant, you should do it as quickly as possible and give the root zone the least disturbance.

Don’t Fuss Too Much over Cannabis Seedling Care!

There’s only so much that you can do when your medical marijuana is still small. Apart from watering and maybe occasional feeding, sprouts hardly need any love and care. So let’s look at what you actually can do.

Watering (Obligatory, Duh)

This is the single most important thing of cannabis seedling care.

How much water do your seedlings need and when is best to give it to them? Well, if you keep your young plants in party cups or very small containers, it’s pretty straightforward: when the surface of the soil is dry and the cup or container feels light, it’s time to water. For a 8 fl oz cup (250 ml), 2 fl oz (50 ml) of water should be enough. For bigger pots, adjust the water amount accordingly.

It’s not recommended to put your seeds or sprouts into final size containers (where the plants will spend all their life, till harvest). This makes it very difficult to come up with the right watering schedule and calculate the ideal amount of water.

Watering Autoflower Seedlings

Autoflowers don’t like being transplanted, and it’s best to grow autos in the same container from seed to harvest. So, if you put your seedling in a big pot, you’ll probably need to water the whole container lightly once and then pour only very small amounts and only near the seedling, gradually widening this area and increasing the water amount. It’s difficult for a newbie and is best reserved for growers with some experience.

The Best Time of Day for Watering

Always try to water your plants in the morning, that is at the very start of the period when water is needed the most: for photosynthesis and perspiration. During the night, photosynthesis is nonexistent, and perspiration slows down. Besides, roots which supposedly grow most actively during the dark hours don’t need water for this, and an excess will suffocate them.

Feeding (Optional in Many Cases)

Grow your weed seedlings in soil and you’ll probably never need to feed them until they enter the vegging stage sometime after two weeks. It’s because a soil mix that you can buy in a store is pre-loaded with plant food. You can still try and boost the growth by giving your seedlings a very light dose of nutrients. Start with about a quarter of what a fertilizer manufacturer recommends. And even in this case, make your first feeding when the plant is at least 2 weeks old (1 week for risk takers and rule breakers).

Of course, in hydro or coco, there are no nutrients in the medium whatsoever. So, you’ll have to add some to your solution from the very beginning. We recommend 100 PPM for a start, with up to 400 PPM as you progress. It’s better to underfeed and increase the dosage as needed than overfeed and flush. Cannabis seedling and flushing are two things that don’t go well together

Marijuana seedlings need the same pH level as mature plants. pH is a constant throughout the plant’s life cycle. It’s 6.0-6.5 in soil and 5.5-6.0 in hydroponics.

Growing a Marijuana Seedling Outdoors

Believe it or not, cannabis does grow outdoors in the wild, but you’ve probably paid a pretty penny for seeds and aren’t eager to expose your precious seedlings to elements too early. We can only commend you for that. It’s best to start inside and put marijuana seedlings outside when they’re healthy, robust, and growing rapidly, and not younger than two weeks from sprout. At this stage, they can take pretty much any weather and not only survive, but continue to get bigger and stronger. If they are just a couple of days old, a heavy rain, let alone hail, can kill them.

However, we admit that some growers simply can’t start their seedlings inside. So when is it more or less safe to start your outdoor grow? Look at the weather forecast and choose a period when there can be no early morning frosts anymore, and the outside temperature during the day is at least 60° F (15° C). Mind you that at this temperature your seedlings will be in the survival mode. Look above for the correct and ideal temperature range.

As for sunlight—which can also be deadly for very young plants—you can provide some shade for them in the afternoon to protect them from full sun. The same principle applies when you start indoors and want to make your young plants get accustomed to the sun. We used to believe that marijuana seedlings shouldn’t be in direct sunlight for a whole day, but some experiments have shown that in spring this rule can be ignored. In summer, it’s best to expose your seedlings to direct sunlight only for a couple of hours in the morning and the evening when the air is cool enough, but never around noon.

Cannabis Seedling Care: Take Things Easy!

First-time growers tend to get nervous at the slightest sign of trouble, real or imaginary, especially during the germination and the seedling stage. But there’s no need to panic. Keep it simple, and you and your plant will be just fine!

External Links:

Factors determining yield and quality of illicit indoor cannabis (Cannabis spp.) production, Wouter Vanhove