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 on harvested autoflower buds and bud rot on top of a cannabis cola due to wrong temp and humidity
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.


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
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:

  1. Humidity should be higher at the start and gradually decrease by harvest time.
  2. It should be warm throughout the life cycle but a bit cooler during flowering.
a weed seedling under a humidity dome for humidity control
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.
mold in fat and solid autoflower bud 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.

A closeup of a flowering top of a cannabis plant grown outdoors
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.

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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 closeup of a marijuana flowering top with white pistils grown in an amateur greenhouse
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.

Marijuana Germination: 3 Basic Methods Compared

Poor marijuana germination rates are the first issue that inexperienced growers face. Recently, a customer said he failed to germinate any of the seeds he had bought from me and complained. I decided to show him that my seeds were all viable and could be germinated successfully even with the crudest methods imaginable. Here’s how it went.

I took three caps from glass jars, three cotton disks, and 10 weed seeds.

marijuana seed germination temperature celsius 25C

Into one cap, I just poured a little tap water (of drinking quality) and dropped three of the seeds on the surface.

Then I soaked all the disks in the same tap water. I placed one disk into a cap without squeezing out the excess water and put three seeds on top. And then I squeezed the remaining two disks so that they were moist, but not wet through. I put the remaining four seeds there.

I placed all three caps on a window sill and covered them with a book because marijuana germination requires darkness. The temperature was about 77° Fahrenheit (25° Celcius). It’s well within the acceptable temperature range for marijuana.

weed seedling germination basic methods

After 64 hours, the results of my marijuana germination experiment were as follows:

On the surface of water – 2/3 germinated
On top of the soaked disk – 0/3 germinated
Between soaked but squeezed out disks – 4/4 germinated

Also, between the disks, the tap roots were the longest.

cannabis seeds germination rates experiment day 3

Interpreting the Results

What is the rationale behind this experiment? All too often, novice growers ‘drown’ their seeds in too much water. The thing is that embryos inside the seeds need oxygen to start growing. If you deprive them of oxygen, all processes stop. It happens when you put them too deep in the soil and water excessively. The same thing can happen when you don’t squeeze out the cotton disks.

So basically, pot seeds can be successfully germinated on the surface of water because below them there’s moisture and above them – oxygen, and there’s nothing to stop them from popping. I thought that by the same principle they would also easily germinate on the surface of a wet cotton disk, but the experiment has proven me wrong.

The method of germinating marijuana seeds between cotton disks demonstrated the best results in my experiment. No wonder this is the standard method for thousands of weed growers worldwide.

I also plan another experiment where I’ll be germinating one batch of seeds between squeezed out disks and the other one between disks that are dripping wet – just to show you how detrimental too much water is for the process of germination. Stay tuned.

Marijuana Germination in Soaked vs Moist Medium

Ha-ha! Great hypothesis! I’ve finished the experiment, and it was the exact opposite of what I had expected. As you can see on the photos below, the seeds that were germinating between wet-through disks have higher germination rates (5/5) and have on average longer tap roots (some of which are already covered by a lot of tiny ‘hairs’).

cannabis seeds germination between cotton disks or paper towels
On the left, you can see the seeds that I kept between cotton disks that were thoroughly squeezed out (they were pretty dry after 3 days). On the right, there are seeds germinating between very wet disks.

So I propose another hypothesis (hope it’s not as far off the mark as the previous one): excess moisture is okay provided that seeds have free access to breathable air. It’s what happens to plants in DWC where their roots float in water all the time, but get all the oxygen they need in the form of bubbles from air stones. So, if you use very wet cotton disks (or paper towels), don’t press them too tight against each other. Let there be a bit of space between them for embryos to breathe.


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.

Related Post  Autoflower Temp and Humidity: With Real-Life Examples

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.

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

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