What Is the Ideal Temperature for Growing Weed?
Indoor growers, no matter what they’re planting, often make one big mistake.
They assume that — since plants grow outdoors in all kinds of weather — they’re hardy enough to grow indoors in just about any conditions.
That couldn’t be further from the truth.
Plants adapt to their natural environments. That means indoor weed plants will prosper when the weather is just like the conditions they’d experience outdoors: cooler in the spring and fall, hotter with lots of daylight in the summer.
Cannabis plants have four stages of growth, and the stages coincide with those natural climatic changes. If you try to grow the plants indoors without replicating the same conditions they’d enjoy outdoors, you may be in for a disappointing harvest.
Most beginning growers know they have to carefully control light exposure in their grow room. They often don’t pay as much attention to cannabis temperatures — but they should.
Why Is Temperature So Important?
When we get cold, our bodies can create heat. Even our pets can do that if we have the temperature set too low.
However, plants don’t have that ability.
If they’re indoors, they rely on growers to take care of the temperature for them. And if it gets too cold, they could have trouble absorbing the nutrients they need. That will stunt their growth and reduce their bud production. Very hot temperatures won’t kill cannabis plants but will impact their growth and decrease their potency.
There’s one other important consideration.
As we’ve mentioned, marijuana plants have developed to cope with the temperatures they expect during each stage of their growth. For example, they naturally flower during late summer and early fall; that’s when temperatures fall and nights become cool.
If those temperature changes don’t occur in a grow room, you can expect a smaller harvest. Higher temperatures during the flowering stage are also likely to create more humidity, leading to a greater chance of mold developing on the plants.
Growers aren’t just being cute when they refer to their plants as their “babies.” They’re also being accurate. Weed plants need almost as much tender loving care in order to prosper.
What Temperatures Do Cannabis Plants Like?
Generally speaking, weed plants like the same temperatures that we do: moderate and comfortable. But they need variety, depending on how close they are to producing a harvestable crop.
Germination
If you’re growing your plants from seed, 77° is the ideal temperature. That’s not a hard and fast rule, though, as cannabis seeds will germinate in temperatures between 70° and 90°.
Seedlings and Clones
The ideal temperature for newly-sprouted weed plants and young clones is between 74 and 78 degrees.
The amount of heat isn’t the real issue, though.
Very young plants are still developing their root system, which means they aren’t yet able to absorb much water. They depend on humid air for moisture until their roots develop, and high temperatures are more likely to create a humid environment.
The seedling stage only lasts for 2-3 weeks, at which point the plants will be self-sufficient and less dependent on humidity.
Vegetative Stage
Weed plants normally experience most of their growth during the summer months when temperatures are warmest, so indoor growers should try to replicate those conditions. That generally means keeping temperatures in the range of 70°-80°, and definitely no higher than 85°.
It usually gets cooler at night, so it makes sense to lower the grow room temperature by about five to ten degrees when the lights go out. Don’t reduce the temperature below 60°, though, because that will hamper the plants’ growth.
Some experienced growers supplement the grow room environment with carbon dioxide, which can boost growth and harvestable yields. In that case, plants flourish in temperatures as high as 85°-90°. Don’t try this without doing a lot of research first.
(We can hear you asking how outdoor plants can weather occasionally higher or lower temperatures on hot or cool summer days, and the answer is simple: outdoor plants are hardier and better able to withstand occasional fluctuations in temperature.)
Flowering Stage
When cannabis plants have matured and are ready to begin producing buds, they enter the flowering stage — and heat becomes the enemy. (Humidity is an even bigger enemy, but we’ll discuss that briefly in a bit.)
The plants produce their trichomes during this period, and trichomes — which contain weed’s cannabinoids and terpenes — are extremely sensitive to heat. Once the temperature gets too high, those essential compounds can evaporate, and they take much of the flower’s potency with them.
Aim for grow room temperatures between 65° and 80°, with temperatures in the low end of that range best for the second half of the flowering stage. Lowering temperatures at night, again by 5-10 degrees or so, will not only encourage bud development but will create greater terpene content and a more potent flower.
Cannabis Temperature and Humidity
Weed plants are even more sensitive to humidity than they are to temperature.
As we’ve mentioned, seedlings need high humidity because their roots can’t fully absorb moisture. Once plants reach the vegetative stage, humidity levels should be lowered little by little until they’re ready to be harvested. Ballpark numbers? Start around 60% at the start of the vegetative stage, and wind up around 40% (or even lower) at the end of the flowering stage.
What does that have to do with temperature? Quite a bit.
You probably know that humidity measures how much water vapor is in the air. But the air can only hold so much water. When it’s saturated, that’s when you start to see condensation on your plants.
And warm air can hold a lot more water vapor than cold air. As temperature increases, so does the humidity. Here’s what that means: when you’re trying to keep humidity low, temperatures that are too high can sabotage your efforts.
There are all sorts of formulas that experienced growers use to calculate the perfect temperature for their indoor crop. For most home growers, though, sticking to the temperatures we’ve suggested for each stage of their plants’ development should do the trick.
Cannabis Temperatures: FAQ
Q: I don’t have a separate heating system for my grow room, and my grow lights make the temperature higher than you recommend. Is there an easy solution?
A: There are several ways to attack the problem. An exhaust fan that removes that hot air is a crucial element in a grow room, as are fans that will “even out” pockets of hot and cold air; an evaporative cooler or a small air conditioning unit can do the trick; or you can switch to cooler grow lights like LEDs. Air conditioners are particularly good choices because they also lower humidity levels.
Q: Aren’t there some strains that are better able to deal with temperature variations?
A: Generally speaking, indicas will be a better choice than sativas. Indicas come from cooler regions so their genetics allow them to handle changes in temperatures and humidity, while sativas are tropical plants and have a more difficult time thriving in cooler weather. There are also strains that are considered “heat-resistant.” Some of the ones to consider for unpredictable growing environments are Afghani Kush, Sour Diesel, Northern Lights, and Super Skunk.
References
- Chiang, C., Bånkestad, D., & Hoch, G. (2020). Reaching natural growth: The significance of light and temperature fluctuations in plant performance in indoor growth facilities. Plants, 9(10), 1312. [1]
- Nievola, C. C., Carvalho, C. P., Carvalho, V., & Rodrigues, E. (2017). Rapid responses of plants to temperature changes. Temperature, 4(4), 371-405. [2]
- Eichhorn Bilodeau, S., Wu, B. S., Rufyikiri, A. S., MacPherson, S., & Lefsrud, M. (2019). An update on plant photobiology and implications for cannabis production. Frontiers in Plant Science, 10, 296. [3]
