To keep your cannabis plants happy you need to make sure they are kept at optimal growing temperature, if the plants are subject to heat stress or extreme colds this can have a detrimental effect on the overall yield. Read our guide on how to control the climate in your grow room to ensure your marijuana plants stay happy and healthy.

how to lower grow room humidity and heat

There are many aspects to a perfect grow room climate, the two most important factors that will influence your cannabis plants growth and how much they yield are, humidity and heat. If you can get both elements under control, you will be one step closer to having healthy cannabis plants and big yields.

ideal temperature for marijuana plants

Growing cannabis indoors is very energy intensive and depending on which type of lights you use, keeping the temperature of your tent or grow room cool can be challenging.

thermometer being using in cannabis grow room to regulate the temperature

The ideal temperature for cannabis plants is 25°C during the daytime and 17-20°C (62-68°F) at night-time. It is almost impossible to keep the temperature consistently at 25°C, so try to aim at keeping it within the range of 21°C-27°C (70–80°F), anything above 85°F this and the plant will begin to suffer heat damage.

  • Daytime temperature for marijuana plants = 21°C-27°C (70–80°F)
  • Night-time temperature for marijuana plants = 17-20°C (62-68°F)
  • Perfect temperature for marijuana plants = 25°C (77°F)

When growing cannabis, the idea is to try to ensure that plants surrounding environment replicates its natural habitat as much as possible. To do this you need to think about the two main stages of growth that a cannabis plant goes through, vegetative and flowering, and in nature what the conditions would be like during these stages.

temperature for marijuana during vegetative stage

In nature as the seasons change from summer to winter, the temperature gradually decreases and so does the amount of sunlight. When you are growing cannabis take this into consideration. During the vegetative stage, you need to replicate summertime, this means that the temperature needs to be slightly warmer at around 70-85°F (20-30°C) and the plant should be getting at least 18 hours of light each day. Read more on growing during the vegetative stage.

temperature for marijuana during flowering stage

In the flowering stage you need to replicate the end of summer by decreasing the light cycle to 12 hours of light, with photoperiods this is what will trigger the plant to begin to produce flowers. The heat should also slightly reduce during the vegetative stage to around 65-80°F (18-26°C). This helps your cannabis plant to increase trichome production, density & smell of buds during this period. Read more on growing during the flowering stage.

TIP – Want your colour to really pop out? To really make the colour of your bud’s pop,  bring down the temperature at night by around 10°F (8°C). This works particularly well with purple strains such as Grandaddy Purple or Purple Skunk Auto. This disrupts the uptake of phosphorus and causes the leaves of the plant to turn purple or red. Bear in mind though that this is only suitable for some strains and lower temperatures can decrease yields slightly. The lowest temperature you should go down to at night is 55°F (13°C).

How to lower grow room temperature

Use the right light schedule

A great trick to regulate grow room temperatures is to have your lights on during the night-time and off during the daytime. Particularly in summer during the daytime temperatures are much hotter, and at night-time naturally much cooler. By putting your lights on at night instead of in the daytime, your plants will stay warm at night, and cool during the daytime. You can keep your lights on during the day, and plenty of people do. But doing this increases the risk of overheating, especially if you live somewhere hot.

Use air ventilation

If your grow room is getting too hot use an air ventilation system to keep the air fresh and draw warm air out of the room. Ventilation systems work by sucking out old humid warm air and replacing it with fresh cool air. Not only does this help to regulate the temperature of your grow room but it also reduces humidity and gets rid of stagnant air that can lead to issues such as mold.

Having a good air ventilation system is important to having healthy plants, spending some time to do your research into a decent air ventilation system will pay off. You can get great sets of equipment from grow shops or from amazon. This complete system from amazon includes a fan, speed control, timer, ducting, carbon filter and a digital thermometer all for $145. If you are not sure what to buy this set makes it simple and pretty much includes almost everything you need to control your grow room climate.

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Use Air conditioning

AC’s are by far the most efficient way to bring down the temperature of your grow room. Although an AC may seem like an expensive piece of kit, the benefits of having one can be invaluable at reducing heat stress or damage from other things such as mold. You can now get a decent AC online for around $200-$400 that will easily reduce the temperature of a small grow room or tent. 

This Black and Decker Air Con unit from amazon is $367 and will easily manage a room of 150 – 250 square feet. if you are growing in a hot country then this is a worthwhile investment that will keep your plants cool and healthy.

Buy the right cannabis strain

Remember to do a little bit of research into what type of strain you are going to grow before you make the purchase. If you are growing in a hot humid environment then you will probably be best using cannabis strains that are sativa dominant. Sativas such as Thai or Amnesia are native to regions such as Thailand, Central & South America, India and South Africa. Because of this Sativa’s are used to warm humid climates and so issues such mold and heat stress will be less prevalent among these strains. Similarly if you are growing somewhere cold any autoflower strain such as Northern Lights Auto will be better suited to cool temperatures because of their ruderalis genetic line that originates from Siberia.

Use fans

When growing cannabis in a sealed room or tent the air can become hot and humid, especially with hot grow lights and regular watering of plants. Pockets of air can get trapped in amongst the dense foliage and buds of the plants and eventually go stagnant. Using fans helps to circulate air around the grow space which reduces humidity and keeps air fresh. However, it is important to know that fans do not decrease the temperature, they simply move air around, so it is important to use venting alongside them to remove the hot air.

I have found that oscillating tower fans like this one from amazon are great because they don’t just focus the air flow on a small patch as would a standard round fan. They also rotate and have a silent mode which is useful for stealth growing. Because they are tall and thin they take up very little room, simply sticking one of these in the corner of your grow room can make all the difference.

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How adverse temperatures can impact your grow

When you are growing cannabis if you want good yields and high-quality buds it is important to ensure that your plants are happy. Besides from giving them sufficient water and light you also need to make sure their climate is suitable, and this includes regulating the temperature.

grow room temperature too cold?

If temperatures are too cold it can cause stress to your cannabis plants and growth will slow down. If the temperature gets extremely cold, it can kill a cannabis plant. Some strains will be more suitable for growing in cold conditions and will be able to survive quite harsh environments. For example, autoflowers all contain ruderalis genetics that are native to the cold mountainous regions of Siberia and Northern Asia, these strains are much better suited to cold weather and can survive through cold winters, although they will not yield nearly as much. Anything under 60°F (15°C) will cause stress to a cannabis plant. If you are growing indoors and the temperature is dipping below this you should turn on your radiators or put a heater in your grow room.

During cold weather frost can build up on your plants, when this melts your plants will be covered in water, this can increase the risk of mold developing.

grow room temperature too high?

Cannabis plants, particularly sativas, are more suited to hot temperatures and generally yield more in a warm environment. However, overheating can also cause stress to them. Any temperature that goes above 80°F (26°C) while the plant is flowering will stress the plant and begin to slow down the growth of its buds. If you do not pick up on this quickly the plant will also suffer a loss of potency and eventually could become susceptible to mold.

Although cannabis plants grow better in warm climates, heat brings with it, its own set of problems. Growing cannabis plants in hot conditions makes your cannabis plants more susceptible to:

  • Pests such as Spider Mites and Aphids
  • Mold such as powdery mildew, bud & root rot.
  • Nutrient burn
  • Wilting leaves
  • Cannabis plants can handle quite extreme temperatures, the upper limit for cannabis plants is around 95°F (35°C) providing humidity is low.

How humidity affects your grow

The results of your cannabis grow are massively impacted by the humidity level of your grow room. Humidity that is too high can be damaging to your plants, because in mild cases it can reduce the yield and quality of the buds, and in severe cases it can lead to mold or root rot which could decimate your entire crop.

You can get a good humidity meter like this one that has a built in thermometer too for as little as $15 – $20 from amazon so there is no excuse for not owning one. You should always keep a humidity meter in your grow room and check it daily for any fluctuations in the levels, you can then adjust your grow room accordingly.

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What are the ideal humidity for cannabis plants?

Seedling: 65-70%

Seedlings are delicate and should be watered with a mist, having a high humidity level allows them to absorb water through their leaves reducing the risk of damage while watering.

Vegetative: 40 – 70%

Start off with 70% and use a dehumidifier to gradually reduce humidity levels by around 5% each week until you get to 40%-50% as you enter the flowering stage.

Flowering: 40-50%

As with vegging, gradually lower the humidity by around 5% each week to get to around 40%. Keeping your grow room at around 40% humidity during the flowering stage will massively reduce the risk of mold or pests.

Late flowering: 30-40%

As your plants come to the end of the flowering stage you can lower the humidity further, this can help to increase yields and boost flavour in your buds.

Adjusting grow room humidity levels

how to lower humidity in a grow room

Dehumidifier

Excessive humidity and warm temperatures can create a breeding ground for pests and mould, so it is important you keep a humidity monitor in your grow room. Cannabis plants like humidity at different levels during the seedling, vegetative & flowering stages of growth (listed above). If your fans and ventilation system is not working well enough to bring humidity levels below these, particularly in the flowering stage then using a humidifier is the best way to lower them.

Increase ventilation power

A simple way to decrease your humidity level in your grow room is to simply turn up the power on your ventilator. More airflow through the grow space will bring down humidity.

Fans

Using fans does not have much of an effect on humidity levels, however they do increase air circulation, which prevents moisture settling on plants, this can help to reduce the risk of mold.

Water your plants as soon as the lights are switched on

This is when the grow tent will be at its coolest because the lights will have just been off for the last 6-4 hours. If you water the plants when the tent is at its coolest, less water will be evaporated, hence the humidity will not increase.

how to raise humidity in a grow room

Turn down the ventilator

Turning down the power on your ventilator will decrease the airflow and gradually increase humidity.

Keep them moist

To increase humidity, you can spray your cannabis plants with a mister spray regularly throughout the day.

Hang wet towels in your grow room

The heat from your grow lights will gradually evaporate the water in your towels increasing humidity.

Use a humidifier

Purchasing a humidifier is probably the easiest way to increase humidity. However, I have never done a cannabis grow myself where I have had to increase humidity so much that I needed a humidifier.

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