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temperature and humidity for cannabis seeds

Autoflower Temp and Humidity: With Real-Life Examples

We give recommendations about the optimal temperature and humidity conditions in your autoflower garden

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 CO 2 .

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.

Table of Contents

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 CO 2 -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 CO 2 , such as CO 2 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:

  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.

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

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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.

Ideal Temp And Humidity For Grow Tents

It often makes it sound like you need to do everything perfectly or your plants will die.

The truth is: cannabis is very easy to grow. You don’t really have to do anything apart from give it light and water.

But if you want to maximize your harvests, you’ll need to provide the ideal conditions.

That’s what all the grow guides are teaching you.

And the environmental conditions are some of the most important.

So what are the ideal temp and humidity for grow tent marijuana grows? Keep reading to find out.

Ideal Temp And Humidity For Grow Tent

The ideal temperature and humidity for a grow tent depends on the growth stage of the plants inside. There is no optimal condition that works for all stages of the plant’s growth.

If you want to be lazy and you don’t care about maximizing your harvest, you could keep your temperature at about 80° F constantly. But you’d still need to lower the grow tent humidity at the budding stage.

  • Seedling Stage: 75° – 85° Fahrenheit / about 70% humidity
  • Vegetative Stage: 70° – 85° Fahrenheit / about 40% humidity (do not exceed 55%)
  • Flowering Stage: 65° – 80° Fahrenheit / 40% humidity (do not exceed 50%)

Because the cannabis plant’s needs are different at each stage (technically it is a different plant as a seedling than it is as a mature flowering adult) the humidity and temperature need to change with them.

That is why most professional growers have multiple rooms (or several huge grow tents) with each room holding plants in a specific growth stage.

This usually means a flowering and a vegging room, and often a third one for seedlings. This allows easy control of the environment and allows for twice as many harvests per year.

We’ll go a bit deeper into the ideal conditions for each growth stage below. First, let’s take a look at the two tools you’ll need to ensure you keep the ideal temperature and humidity for a grow tent.

Tools Needed To Maintain Ideal Temp And Humidity

To keep your grow tent in the ideal condition for healthy plants you are going to need some important tools to measure the environment.

Thermometer

I’m sure you’re already familiar with thermometers, but if not, it is a simple tool that has been around for over a hundred years that tells you the temperature of your environment.

Thermometers vary in effectiveness and fanciness. In general, a good digital thermometer will do the job best. A wall mount model is good, but keep in mind it will only measure the temperature in the area where it is mounted.

You should also have at least one small portable thermometer you can easily carry around your tent to get accurate readings at various spots, especially near all your plants.

You will find as time goes on that certain areas are naturally hotter or colder than others, and this data can help you place your plants in the ideal areas.

This does not matter of course if you have a tiny grow tent with only a few pots in it.

One of the best reasons to have a portable thermometer is to ‘fact check’ your equipment. If you have an air conditioner (which will have a thermostat) you can use your portable thermometer to check if the readings match.

Sometimes with A/C units or other big machinery the temperature reading ability can be off significantly. You need to make your decisions based on good data, especially in a commercial or semi commercial operation.

Hygrometer

This tool measures the amount of water vapor in the air. It is another simple tool and you can find wall mount models and portable models, much like thermometers.

Ideally you will own both a mounted hygrometer and a portable one, for the same reasons mentioned above with the thermometers. You can also get a combination unit that measures both temperature and humidity.

A hygrometer may be the most important tool you own. Excess humidity can destroy your entire garden, and it can sneak up on you without much warning.

Keeping humidity from getting too high can literally save your entire crop. If your buds get the bud rot, it is game over (cue Ice-T on the car shield commercial).

Having an accurate hygrometer (or a few) is critical to measure the humidity in your garden. You’ll know as soon as it starts creeping up too high and can intervene in time to save your plants’ lives!

The way a hygrometer works is actually quite interesting (to me at least), you can check it out in detail here.

Ideal Conditions For Seedling Stage

As mentioned above, the temperature should be between 75° and 85° F with 70% relative humidity. Seeds need to germinate, and germination requires humidity and heat (just not too much).

When these conditions are met and enough time passes, the seeds will sprout, and the little baby plants will peek out and start shooting up.

There is not too much change needed between the seedling and the early leafy growing stage, but the little that is needed is a game changer if it is not done correctly.

Perfect Conditions For The Vegetative Stage

Ideally, you want the temperature for this phase to be around 80° or so, with the RH being 40%. As you may have noticed the temperature does not change at all between the seeding and growth stages. The humidity requirement, however, is a massive change.

Seedlings love that moist air, but too much moisture saturating those leaves can lead to a grow tent apocalypse, and I am sure you do not want that! The reason for the dramatic decrease in required humidity between seeding and vegging is simply that leaves like drier air.

If the air gets too wet, the leaves become saturated and are not able to shed as much oxygen. This leads to them becoming unhealthy and absorbing less light and fewer nutrients, weakening the critical photosynthetic process.

Ideal Temp And Humidity For Bloom

As the plants come into the final stage, they like the air to be a bit cooler and the humidity to come down a little. Technically the humidity window gets narrower in this phase versus the previous one.

Humidity can be anywhere from 40 to 50% in this phase, which is a little less than the prior stage. The reason for this is that buds are fickle.

Too much heat means more capacity for the air to hold water. More dense air means less evaporation of surface moisture on the buds, and constant moisture on the buds without evaporation leads to bud rot, which is the angel of death for your plants.

Keeping all this in mind (or better yet in a notebook you carry around) can help your grow tent be the ideal place for your plants. As you have learned, the temperature is important but not as important as the overall humidity of the air.

The main reason temperature matters is because it effects the air’s ability to hold water, which comes back to humidity. This does not mean temperature is not important, because it is.

But if you slip up and it gets too cold or hot for a little while, you are not going to lose your harvest. On the other hand, if humidity gets to high and you plants get bud rot, you have a real problem. So make sure you always have sufficient fans in your grow tent.

Ideal Grow Tent Humidity And Temp: Final Thoughts

Maintaining the ideal temperature and humidity in your grow tent is vital in maximizing the yield you harvest from your marijuana plants. But the temp and humidity your plants want does not stay constant throughout the grow.

In the early stages of growth, plants need warm and humidity conditions. As they move toward the later stages, the temperature and humidity need to come down.

If you follow the guidelines above, your plants will not suffer any ill effects from a poor environment and they will reward you with massive harvests. Assuming you are doing everything else right, of course.

Growing weed is simple, but cultivating marijuana and getting the best possible harvest requires everything to be perfect.

That said, you don’t have to buy the highest quality and most expensive grow tents to successfully grow weed. Some deep budget brands work perfectly well and save you a ton of money. Check out our Quictent grow tent review for one such brand.

What’s the Best Grow Room Temperature and Humidity Level?

Ideal grow room temperature and humidity varies depending on the stage of plant life. Cloning requires higher temperature and humidity than vegetative growth and flowering plants have different ideal atmospheric conditions as well. In order to master the art of marijuana growing, dialing in the proper environment at the right time remains the most essential ingredient for success. So, what is the best grow room temperature and humidity level?

Measuring and Changing Temperature and Humidity

In order to properly measure temperature and humidity, you’ll need a thermometer and hygrometer. Best to invest in a digital one that can give you current readouts as well as highs and lows when you’re not inside the room. To raise heat, you’ll need a heater and to lower heat, you’ll need an air conditioner. These can be outside or inside the growing space depending on the size of your space and how much the temps and moisture levels fluctuate. A humidifier and a dehumidifier can be employed to raise and lower humidity rates. Larger grow rooms can benefit from a controller that uses a sensor to keep track of temps and humidity and turns on the appropriate appliance to regulate and keep them within your set parameters.

Cloning

Because cannabis cuttings root best in warm conditions with high humidity, the cheap trays with clear plastic domes work remarkably well. In cool conditions, a heat mat should be placed underneath the trays to maintain an optimum temperature of 74-78 degrees F. and relative humidity at 75-85%. No matter where and into what medium you plan to root your clones, keep warmth and high humidity on your priority list. Clones allowed to get cold or dry will perish quite quickly. Too much humidity (over 90%) can also cause mold and rot, so cut a quarter-sized hole or two in your clear plastic dome to allow some air movement and circulation.

The Vegetative Stage: Best Grow Room Temperature

The best grow room temperature during the vegetative stage of growth is 70-78 degrees F. when the lights are on during the “daytime” and no more than 10-15 degrees cooler at “night” with a relative humidity of 45-55%. With these settings, your plants will best be able to convert light into energy for growth. This is the time when the plant puts on leaves and branches and expands it’s root system throughout your growing medium. If it gets too cold or hot, growth stops and you eventually risk losing your plants altogether.

The Flowering Stage: Best Grow Room Temperature

The best grow room temperature during the flowering stage of growth is 68-75 degrees during the day and no more than 10-15 degrees cooler at night. If you’re supplementing with CO2, daytime temps can be as high as 75-82 or so. During flowering, you should lower your relative humidity to 35-45% and even lower (30%) for the last couple of weeks before harvest. This will help you avoid issues with mold, bud rot and PM (Powdery Mildew) that can arise in higher humidity.

Drying and Curing

The drying room is a place that must be carefully monitored. Keep in mind that your plants will be giving off a large amount of moisture into the room as they dry. It’s important to pull wet air out and keep air circulating in the room without actually having fans blowing right on your hanging branches, which can dry them out prematurely resulting in a harsh taste and burn. Also, growers in dry places such as Colorado struggle to extend their drying time with humidifiers, while farmers in more humid climates such as Northern California use dehumidifiers to pull water from the air in order to avoid mold growing on their buds.

The ideal temperature for a drying room is between 65 – 74 degrees Fahrenheit and humidity between 45 – 55 percent in a dark well-ventilated room. Cannabinoids, terpenes, and flavonoids can evaporate and be released at temperatures above 80 degrees, diminishing the scent, flavor and potency of your buds. Within 6 – 10 days your branches should snap instead of bending and the buds should feel popcorn dry on the outside. This is the time to cut the individual buds from the branches and put them into glass jars to begin the curing process. Cure your buds in a cool (68-72 degree F.) and dark place.