Marijuana grow room temperatures are one of the most important factors you can control in your grow op. Indoor grow op temperatures affect the health, growth rate, size, bud quality, and yield of your cannabis plants. They also affect how much electricity money you spend on your indoor grow op.
As with many marijuana growing factors, the ideal temperature for your grow op is variable, depending on several factors. One important factor is the difference between lights-on and lights-off temperature. In bloom phase, you want your lights-off temps to be about 4-8 degrees cooler than your lights-on temps. This promotes resin production, purpling, and bud growth—while decreasing the stretch that usually comes in the first weeks of bloom phase.
Another big factor for marijuana grow room temperatures: are you using carbon dioxide (C02), or not? If you’re using C02, your marijuana grow room temperatures can be as high as 87°F, which is 7-10 degrees higher than ideal lights-on temperature in a grow room where you’re not using C02.
Yet another factor is what type of marijuana growing system or style are you using. If you’re using pure or media-based hydroponics including deep water culture (DWC), aeroponics, NFT, rockwool, coco coir, grow rocks or coco coir, your nutrients water temperature is a factor in your marijuana grow room temperature. Marijuana root zone temperature should be at 68-69°F, and that’s why many growers use a chiller to keep their hydroponics water at the right temperature. When your hydro water temp is in that range, it offsets too-warm marijuana grow room temperatures to lower the stress your marijuana plants experience from hot grow rooms.
Yet another factor for marijuana grow room temperatures is what strains of marijuana are you growing. Sativa-heavy cannabis strains, especially those with tropical genetics such as Haze varietals, like warmer marijuana grow room temperatures in the range of 76-81°F. Indica and Afghanica dominant marijuana strains, especially those with genetics that come from the Himalayas, Northern India, Afghanistan, Canada, Holland, and other cooler climates, usually do best in 74°F grow rooms. Many purple marijuana strains prefer cooler temperatures, especially during lights-off in bloom phase. Note that these temperatures are for rooms without added C02.
Marijuana grow room temperatures that are too hot create plant stretch, plant stress, fluffy buds, and increased pest and disease problems.
However marijuana grow room temperatures that are too cold (below 72°F) may slow growth and decrease yield. The marijuana root zone shouldn’t be below 67°F because cold temperatures impair root function and harm roots.
Note that relative humidity for growing marijuana should be between 55-64%, and that non-optimized marijuana grow room temperatures combined with the high or low humidity creates many problems for marijuana plants.
Achieving the right marijuana grow room temperatures and humidity involve air conditioning, venting, air exchange, and dehumidification. You may have to switch from high intensity discharge (HID) grow lights to LED grow lights to reduce the heat load, as LEDs generate about half the high of HIDs. You might have to add air conditioning, dehumidifiers, aeration fans and/or venting capacity to treat and/or remove heated air from the room.
Note that heated air rises and gathers starting at the ceiling and going down about four feet. This can create a heat inversion layer in the top of the plant canopy, which is not where you want extra heat because it damages buds and makes them lighter.
Excessively hot indoor grow spaces often occur in grow tents, attics, and closets. If you’re growing in those kinds of spaces, be prepared to remove heat or else your plants will suffer. What’s true in all growing situations–indoors or out–is that excessive heat can damage your plants and cause other problems that you want to avoid if possible.