What is the best temperature and humidity for growing cannabis? This is a debatable topic and you may get different answers depending on who you ask. In this article I'll talk about the temperature and humidity ranges that I’ve been successful using when growing cannabis indoors.
First, let's talk temperature. The ideal air temperature during the entire grow cycle is 70-85°F/21-29°C when your grow lights are on for the day. When the grow lights are off, during the plant’s night cycle, you can allow a drop in temperature to a little bit below 65°F/19°C without causing problems. A plant will technically make it anywhere between 59-91°F/15-33°C, although, a temperature range of 70-85°F/21-29°C is the most comfortable zone. Some growers will also specify the ideal temperature according to what grow light you are using. For example, if you are using high intensity discharge (HID) lighting, between 70-80°F/21-26°C is ideal. Now if you are using light-emitting diode (LED) lighting, the temperature can go up to 85°F/26°C. Do your best to avoid major temperature swings. A 10-degree temperature swing is ideal and a 20-degree temperature swing should be the maximum. If you have temperature swings larger than 20 degrees, then that could certainly stress out your plant. If your air temperature is in the right range and doesn't fluctuate like crazy, you will reap the rewards of better growth and healthier plants which are less prone to problems.
Now let's talk about something slightly less critical than air temperature but still very important - humidity! Humidity is the amount of moisture in the air and it changes relative to temperature. This is why I have grouped the two topics together. The ranges of humidity that I am about to mention are applicable to the temperature range that I just gave. Keep in mind, humidity should not be the same for all stages of plant growth. Also be warned that this is another debatable topic. The humidity ranges mentioned in this article may be different from what other growers recommended.
For clones, humidity needs to be high. A humidity range of 80-95% will best best for them while they are rooting. It’s also highly recommended that you use a humidity dome as this will help you to achieve such a high humidity rate. For plants that have sprouted from seed, aim for 50-75% for the first two weeks. The higher in that range, the better. For the remaining weeks that the plant is in the vegetative stage, aim for 40-55% humidity. You could even go higher than that but if the humidity in your grow environment exceeds 60% then you are more likely to encounter powdery mildew. So be careful! During the flowering stage, target a humidity range of 30-55%. Finally, for the last few weeks prior to harvest, I’ve had great success reducing the humidity to as low as possible. I live in a desert region and I’m able to get the humidity in my grow environment as low as 20%. This helps signal the plant that its life is coming to an end and the plant will focus its energy on fattening up those buds!
Here are a few other tips about temperature and humidity. If you are planting regular seeds, high humidity increases the number of female plants and low humidity increases the number of males. To keep track of your grow room temperature and humidity, place a thermometer/hygrometer at canopy level. Here are two that I recommend:
If your air temperature/humidity is out of the ideal range, adjusting the speed of your inline fan should help. If you are struggling to keep the humidity level up, either set up a bucket of water inside the grow room or put a humidifier in the grow room. Here are two humidifiers that I recommend:
There you have it - the basics on temperature and humidity for growing cannabis. I’m interested to know what you’ve heard in the past as the best temperature and humidity ranges for cannabis plants. Let me know by leaving a comment. Also, I'm always willing to answer questions so feel free to leave those in the comment section as well.
NEXT: How Much Yield For One Cannabis Plant?
Beginners Start Here
Grow Room & Nutrients
Seedling Stage & Cloning