Temperature and humidity for cannabis are the two big players you don’t want to mess with if you want to have a successful grow. Too much humidity could lead to mold, bud rot or slow growth, while too little can leave them high and dry. And if it gets too hot or too cold, your plants will refuse to grow big and strong. So, give your plants the right conditions they need, get your grow room humidity and temperature under control, and they’ll make it rain buds and leaves like a botanical jackpot!
This article will walk you through the ideal humidity and temperature to grow weed at its different cannabis growth stages, how to set up and maintain a comfortable environment for your plants to thrive, and we will provide a grow room temp and humidity chart to help you keep track of your progress.
Understanding how humidity and temperature affects cannabis growth
Before diving in head first to learn how to optimize the ideal temperature and humidity for cannabis, it is necessary to have good foundation knowledge on how temperature and humidity influence the growth process of cannabis plants.
As you already knew, your plants undergo photosynthesis to convert cannabis nutrients and sugars into energy for growth. And if you remember anything from school, photosynthesis absorbs carbon dioxide from the air, and release water and oxygen via microscopic pores on their leaves back to the surrounding environment. So the amount of water already in the air has a significant impact on this process. Because:
What happens if grow room humidity is too high?
If the humidity level in the air is too high, your plants cannot get rid of water, this disrupts the photosynthesis process and the speed at which the plants process the energy. In addition, with water now still in the leaves, fresh water and nutrients cannot cannot reach them and are retained in the roots which could cause root rot (2). High humidity levels can also cause the buds to become less dense and more susceptible to pests and diseases.
What happens if grow room humidity is too low?
If the humidity level is too low, water vapors from the leaves a lot quicker, the plants will need to extract more water from their roots, leading to an increased intake of nutrients, which could result in nutrient burn. It can also cause the plant to become dehydrated and stressed, which can negatively affect its growth and reduce the potency of the buds.
Humidity levels in the air, however, is also impacted by temperature. Assuming the moisture in the air remains the same in the below scenarios, as per physics law (3) the below will happen:
- If the temperature is increased, the air molecules expands allowing them to hold more water molecules. As such, the air will feel a lot drier, and the plants will loose water faster through transpiration.
- When temperatures takes a dip, its ability to hold water also takes a trip, the air contracts and will become wetter, and your plants can’t sweat it out as much.
Overall, temperature does a lot more for your plants than affecting humidity. It regulates the rate and efficiency of various processes from germination, transpiration, photosynthesis , respiration, flowering, all the way to harvesting. In growers’ term, this means it impacts the internodal growth, water absorption rate, and pH level of the cannabis growing medium. A temperature change can also be used to simulate nighttime conditions for your plants too.
What happens if the grow room temperature is too high?
If the temperature is too high, it reduces photosynthesis rate and causes water loss and heat stress in your plants, leading to stunted growth, stretching, wilting, and reduced potency, (as terpenes are heat sensitive).
What happens if the grow room temperature is too low?
If the temperature is too low, it can cause slower growth (as the chemical processes within the plant slow down in cooler temperatures), nutrient deficiencies, and pests and disease.
For outdoor growers, 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.
Here’s an interesting fact: over 90% of your cannabis plants dry matter comes from the photosynthesis process (1). So if you want your plants to bring their A-game in terms of yield, you need to make sure their photosynthetic rate is up to speed by given them the optimal humidity, temperature and light.
What is the ideal temperature and humidity for cannabis plants?
The ideal humidity and temperature to grow weed depend on its particular growth stage. We recommend proper monitoring tools, i.e. a digital hygrometer and thermometer with a memory function, that can display the previous maximum and minimum readings. It is always a good idea to stick with a grow room temp and humidity chart.
Now let’s get down to the nitty-gritty of controlling the humidity and temperature levels like a boss to make our green babies happy.
What is the best temperature and humidity for cannabis during seedling stage?
The best temperature and humidity for cannabis during seedling stage are:
Humidity (RH): 65-70%
Temperature with light on: 20-25°C (68-77°F)
Temperature with light off: drop 4°C lower, so 16-21°C (60-70°F)
The roots of little seedlings are not fully developed and are very fragile, as such the plants rely on their leaves for hydration. Seedlings should be watered with a mist, having a high humidity level allows them to absorb water through their leaves. It also reduces the risk of damage while watering and avoids drying out too quickly.
What is the best temperature and humidity for cannabis clones?
The best temperature and humidity for cannabis clones are:
Humidity (RH): 60-80%
Temperature with light on: 20-30°C (70-86°F)
Temperature with light off: drop 4°C lower, so 16-26°C (60-79 °F)
Cannabis clones don’t have roots yet and rely on their leaves to get all the water they need until their roots develop. As such, clones thrive in humid conditions with ample moisture in the air and they tend to root and grow most efficiently in slightly warmer temperatures.
What is the best temperature and humidity for cannabis during vegetative stage?
The best temperature and humidity for cannabis during vegetative stage are:
Humidity (RH): 40-70% (drop the humidity level by 5% each week)
Temperature with light on: 22-28°C (72-84°F)
Temperature with light off: 18-24°C (64-75°F)
During the vegetative stage, you need to replicate the conditions of summertime, and the plants should be getting at least 18 hours of light each day. As your marijuana plants mature, the risk of mold growth increases, so 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.
What is the best temperature and humidity for cannabis during flowering stage?
The best temperature and humidity for cannabis during flowering stage are:
Humidity (RH): 40-50%
Temperature with light on: 20-26 °C (68-79 °F)
Temperature with light off: 20-26 °C (68-79 °F) (no change)
In the flowering stage you need to replicate the end of summer by decreasing the light cycle to trigger the plants to begin to produce flowers. Lower your grow room humidity to reduce the risk of mold or pests. The heat should also be slightly reduced to increase trichome production, density & potency of buds.
What is the best temperature and humidity for cannabis before harvest (late flowering)?
The best temperature and humidity for cannabis before harvest (late flowering) are:
Humidity: (RH) 30-40%
Temperature with light on: 18-24 °C (65-75 °F)
Temperature with light off: 16-20 °C (60-68 °F)
As your plants come to the end of the flowering stage, about 1-2 weeks before harvest you can lower the humidity for cannabis grow room further.This serves as a signal to the plant that the colder months are approaching and as a final attempt they’ll put all their energy into the buds, giving you bigger yields and a flavor explosion in those nugs.
What is the best temperature and humidity for drying cannabis?
The best temperature and humidity for drying cannabis are:
Humidity: (RH) 50%
Temperature: 21°C (70°F)
What is the best temperature and humidity for curing cannabis?
The best temperature and humidity for curing cannabis are:
Humidity: (RH) 60-65%
Temperature: 21°C (70°F)
What does relative humidity mean? How does it interact with temperature?
Relative humidity (RH) is the most popular method to measure humidity level in the air. It compares the amount of water molecules/ vapor present in the air against the maximum amount that the air could contain at the same temperature. RH tells you how close the air is to saturation.
As we already knew, when temperature increases, its ability to hold water molecules increases, as such if the amount of water vapor in the air remains unchanged, then relative humidity will decrease. Put simply, if temperature goes up, relative humidity goes down. And vice versa.
For example, if the relative humidity is 70%, it means the air is already holding onto 70% of the water vapor it can handle at that temperature. In other words, the air is feeling quite sticky and doesn’t have much room left to take on any more water vapor. But if you turn the heater up, without fanning out or doing anything else, relative humidity will go down.
Grow room temp and humidity chart
Below are our visual charts to help you keep on top of the temperature, humidity and light schedule for your cannabis plants, from seedling stage, vegetative stage all the way to flowering and harvesting. Feel free to print them out, stick to your wall and start tracking your progress.
For a more comprehensive table, follow this link to see a summary of marijuana plant stages of growth and optimum conditions of each phase (including humidity, temperature, nutrients, pH light intensity and light schedule).
Tips to adjust and maintain the correct temperature and humidity for cannabis grow room
Use a reliable hygrometer/ thermometer
A thermometer measures temperature, while a hygrometer measures humidity levels in the air. They can help you monitor the grow room conditions to make prompt adjustments.
Keep track of your plants growth stages and stick to the temperature and humidity chart
At each growth stage, your cannabis plants become increasingly efficient at using water in their roots, branches, and leaves. Their requirement for the air temperature and humidity changes. So you need to keep track of what growth phase they are in and what requirements of that stage are and adjust accordingly. Print out a temperature and humidity chart to help.
Use air ventilation
Ventilation systems help remove humid warm air and replace 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. If you use a grow tent, remember to route/ point the ducting out the house or window, rather than to the room your grow tent is located within. Otherwise you will not be bringing in fresh cool air from outside.
Use air conditioning (AC)
Air conditioners 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.
Use fans
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 change the temperature, they simply move air around, so it is important to use ventilation or AC alongside them as needed.
Oscillating tower fans 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.
Dehumidifier and humidifier
A dehumidifier is used to reduce humidity levels when they are too high, while a humidifier is used to increase humidity levels when they are too low. Using both devices can help maintain a consistent and ideal humidity range for your cannabis plants.
Pay attention to grow light types. Adjust location, intensity and schedule
Some types of cannabis grow lights emit heat and therefore, when the grow lights are on, the temperature goes up, and when they’re off, it goes down. So you can place your lights closer or away from the plants to regulate the temperature and humidity and consider using cooltubes if necessary.
Another great trick to regulate grow room temperature is to have your lights on during the night-time and off during the daytime. This is because in summer temperatures are much hotter during the daytime, and much cooler night-time. 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.
Keep an eye on the weather forecast
Changes in the outdoor temperature and humidity levels can impact your grow room climate. So if the weather forecast predicts a period of rain or a heatwave heading in your direction, be prepared to adjust your settings accordingly.
Buy the cannabis strains suitable for your climate and region
Remember to do a little bit of research into what types of weed 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.
How to lower humidity for a cannabis grow room?
1. Increase ventilation. Consider adding an exhaust fan or increasing the speed of your existing fan to draw out moist air and bring in drier air.
2. Increase cool air supply by adding a ventilation intake to help bring in drier air from outside your grow room. Just be sure to install a filter to prevent pests and pollutants from entering.
3. Turn on the dehumidifier. If you can afford a decent one, get a decent one.
4. Water your plants immediately after the lights are switched on. Allow the soil to dry out between watering.
5. Place a bag filled with silica gel, moisture-absorbing crystals, or desiccants (these are substances that absorb moisture from the air) in the grow room. Remember to make some holes in the bag, and replace the substance when they loose their capacity to hold moisture.
6. If your grow lights emit heat, move the lights closer to the plants to raise the temperature around the plants (higher temperature, lower humidity).
How to increase humidity for a cannabis grow room?
1. Turn down the power on your ventilator.
2. Use a humidity dome for cannabis clones or if your plants are young.
3. Mist your cannabis plants or use a humidifier. One with a large tank it will save you time refilling.
4. Water the plants during cooler part of the day. Keep the soil moist.
5. Place a water bucket or wet towels in your grow room, the heat from your grow lights will gradually evaporate the water in your towels increasing humidity.
6. If your grow lights emit heat, move the lights higher up away from the plants (lower temperature, higher humidity)
How to lower a grow room temperature?
1. Increase ventilation. You should have a reliable exhaust system to effectively remove hot air.
2. Turn on the air conditioner.
3. Turn the lights off during the daytime and on during the night-time.
4. Adjust the lighting schedule or use LED lights, which emit less heat than other types of lighting.
5. If your grow lights emit heat, move the lights higher up away from the plants or use cool tubes.
6. Use a cool mist humidifier: A cool mist humidifier can help reduce the temperature in the grow room while also increasing humidity. This can be particularly useful during the vegetative stage when higher humidity levels are desirable.
7. Use a reflective surface to reflect heat away from the plants.
How to raise a grow room temperature?
1. Reducing the ventilation system’s airflow.
2. Adjust the heating system (using the space heater or grow room heater).
3. Insulate the room.
4. Adjust your lighting. Switch to higher wattage bulbs or use HID lights.
5. If your grow lights emit heat, move the lights closer to the plants
6. Use a heating pad underneath your cannabis plants.
Frequently asked questions
Does humidity for cannabis grow room influence the potency of the buds?
Yes, grow room humidity can influence the potency of cannabis buds. As explained in more detail above, if your grow room is too humid, your buds might get moldy and lose their oomph. And if it’s too dry, your plants will get dehydrated and stressed out, which can totally bum out your buzz.
Is 75% humidity too high for cannabis?
If we are talking about a typical cannabis plant at any growth stage, then yes 75% humidity is generally considered too high for cannabis. Ideal cannabis humidity typically range between 40% -70% relative humidity (RH) during the vegetative phase and 40% – 50% RH during the flowering phase. This drops to 30%-40% when your plants are 1-2 weeks away from harvest. Refer to the grow room temp and humidity charts above.
However, if we are talking about cannabis clones or cuttings of which roots have not developed, then 75% RH is a good humidity level as clones prefer higher humidity. This is because their small roots limit their ability to absorb water and they depend on their leaves to obtain all the water they require. So the high humidity in the grow room helps them retain water until their roots are developed.
Does cannabis grow room temperature influence the potency of the buds?
Absolutely. Temperature affects everything from the plant’s structure to its chemical composition. If you want buds loaded with terpenes, bursting with trichomes, and packed with THC and other cannabinoids, it’s crucial to maintain the ideal temperature in your indoor grow room accordingly to the plants light schedule (light on/ off) and their growth stage.
How hot is too hot for a grow room or grow tent?
Generally, the maximum grow tent temperature (or grow room) for a successful harvest is 30°C (86°F). Anything above this temperature without CO2 enrichment would cause stretching, heat stress, or buds that are airy, loose buds and less potent. It also may result in the plants’ untimely death.
However, as explained above, the temperature requirement does change depending on the growth stage your plants are in. For example, if they are in the middle of its flowering stage, then the maximum temperature is 26 C° (79 °F), so use our guidelines in the section above.
Experienced growers who grow their cannabis plants in a low humidity setting, under extremely bright grow lights, and in a CO2-enriched environment (above 1,500 PPM), can make their plants more resistant to heat, up to 35°C (95°F).
What temperature is bad for weed plants?
As a rule of thumb, anything above 30°C (86°F) or below 16C° (60°F) is bad for your weed plants. While some plants can endure the cold without displaying signs of stress, their yield may not be as high as expected considering the amount of light they are receiving. As such, do remember to monitor the development of buds as it is just as important as overall plant health. Just because your plant is looking fine and dandy, it doesn’t mean you’ll be rolling the dankest of joints.
References
(1): Chandra S, Lata H, Khan IA, Elsohly MA. Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L. Physiol Mol Biol Plants. 2011 Jul;17(3):297-303. doi: 10.1007/s12298-011-0068-4. Epub 2011 Jun 1. PMID: 23573022; PMCID: PMC3550580. Available online
(2): IOP Conference Series: Materials Science and Engineering, A critical review on the influence of humidity for plant growth forecasting, Volume 1257, 33rd Symposium of Malaysian Chemical Engineers (SOMChE 2022) 08/08/2022 – 09/08/2022. Available online
(3): Britannica, The Editors of Encyclopaedia. “Boyle’s law”. Encyclopedia Britannica, 6 Jan. 2023, https://www.britannica.com/science/Boyles-law. Accessed 1 April 2023. Available online
