Cannabis is a tropical plant, so temperature is super important for its growth. In general, cannabis plants prefer warmer temperatures; however, this doesn’t mean that warmer is always better. In fact, if temperatures are too high, you can actually kill your cannabis plants.
Cannabis Natural Environment
Cannabis flourishes in the steppes of central Asia. This leads to long, warm – but not hot – days during the summer, and a cold, dark winter. This is why cannabis thrives in 16+ hours of light, but requires conditions between 68-78 degrees Fahrenheit.
The long days of summer and the bright sunlight helped ancient cannabis grow quickly. The quick loss of daylight after the summer solstice encouraged cannabis to develop flowering triggered by equal amounts of light and dark. The mountainous terrain and the cold winters prevented summer from getting too hot, and left modern cannabis plants sensitive to extreme heat and cold.
Effects of Temperatures Being Too High
If your cannabis plant is too warm, you’ll feel it first. Cannabis plants actually prefer a temperature range right around what humans find comfortable. If you’re too warm, your plants are probably too warm.
If your plants are overheated, then you’ll begin to see signs of heat stress within a few days. Your plants may start to yellow, dry out, or wilt. However, the key sign Is “leaf cupping,” or leaves curling upwards at the edges. All your plants leaves should be affected, and they will start to look like little canoes or taco shells. This is what differentiates heat stress from other issues like underwatering or light burn, which have similar symptoms otherwise.
Light burn only affects the leaves closest to the lights, while underwatering results in limp leaves that don’t curl upward. Nutrient burn only results in the tips of leaves curling, not the entire leave folding. If you see leaf cupping, you know that your plants are definitely too warm.
Effects of Temperatures Being Too Low
Cannabis plants can be too cold, too. It doesn’t take freezing temperatures for this to happen, either. Even a few nights consistently under 60 degrees Fahrenheit can lead your plants to show signs of cold stress. Again, if you’re chilly, your plant is too, and you should install a heater if possible.
Cold temperatures slow down cannabis growth and seriously hinder nutrient uptake. For example, magnesium is no longer absorbed by cannabis plants once root temperatures drop below 64 degrees Fahrenheit. Phosphorus and other nutrients are affected as well. This results in cold symptoms that are similar to nutrient deficiency problems, including spotty leaves, stem color changes, and yellowing.
Cold stress isn’t usually an issue for indoor growers because their rooms contain heat and grow lights run warm. However, outdoor growers regularly face the effects of cold, especially in climates like the Pacific Northwest that are otherwise great for cannabis. Some growers go so far as to cover their cannabis field with plastic at night and place a heater underneath it, so that the plants don’t deal as much with the chill.
Having your temperature fluctuate wildly is worse than simply staying a little low or a little high all the time. This stresses your plant, and can result in leaf browning, leaves dropping, and death of the plant. If you’re not sure whether your temperatures are fluctuating, check how cold it is at night versus how warm it is at 2pm – if you see a difference of more than 15 degrees, you should try to insulate your grow tent setup better.
Cannabis likes stability. If you can keep your temperatures stable in a range that’s comfortable for you (a human), your plants will be happy too!