The ideal temperature for growing cannabis
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Getting the temperature right in your grow room is extremely important as a grower because it affects your plant's ability to photosynthesize and grow.
Photosynthesis is comprised of a variety of processes that are either light-dependent or light-independent. Temperature affects the light-independent process of photosynthesis, either increasing or decreasing the rate of photosynthesis.
The light-independent reactions of photosynthesis take place in the stroma—fluid surrounding the grana (stacks of thylakoids) in chloroplasts. These reactions are catalyzed by specific enzymes which work best at certain temperatures. Hence, plants growing in ideal temperatures will photosynthesize faster than plants grown in temperatures that are too high/low.
In general cannabis plants grow best at temperatures between 20–30°C (70–85°F) during the day. Growers tend to agree that the ideal temperature is around 25°C (or roughly 75°F). At night, cannabis plants like slightly cooler temperatures of roughly 17-20°C (62-68°F).
WHAT HAPPENS WHEN THE TEMPERATURE IS TOO HOT?
High temperatures are usually less of a concern than cold temperatures, as cannabis plants rarely die from too much heat. However, temperatures over 30°C will lead your plants to grow more slowly as the enzymes involved in photosynthesis aren’t able to work properly.
Apart from stunting growth, heat also attracts pests and diseases. Spider mites and white powdery mildew, for example, tend to thrive in hot conditions.
Hot conditions will also cause water to evaporate from your plant’s soil more quickly. Not only will you need to water your plants more regularly, but they may also struggle to take up the nutrients in the soil. This can lead to nutrient burn, which causes damage to your plant’s roots and foliage.
Also, remember that high temperatures combined with high humidity can expose your plants to a whole variety of additional issues. Plants growing in hot, humid conditions without enough air circulation can develop bud rot or mold, which causes a lot of stress on the plant and essentially destroys the buds you’ve been working so hard to grow all season.
WHAT HAPPENS WHEN THE TEMPERATURE IS TOO COLD?
Cold temperatures can be just as detrimental to cannabis plants as high temperatures. Unlike hot temperatures, however, extremely cold temperatures can shock plants or even kill them entirely.
As a general rule of thumb, cannabis growers should avoid growing in temperatures below 15°C (59°F). At these temperatures, your cannabis plant will struggle to grow and will eventually die.
Remember, cold temperatures can reduce a plant’s rate of photosynthesis just like hot temperatures. The enzymes responsible for the light-independent reactions of this process are not as active at low temperatures, meaning your plant will grow at much slower rates.
Lower temperatures can also create ideal environments for mold. Some molds prefer cold, damp environments, and may form on your plant’s roots, leaves, or buds should temperatures drop below an acceptable range.
Again, note that most cannabis plants prefer slightly colder temperatures at night (roughly 18°C). Some plants may also develop beautiful purple or blue hues when grown at lower temperatures, which can make for great-looking buds come harvest time. The ability to develop purple or blue hues, however, heavily depends on genetics and is seen more on indica plants like those native to mountainous regions (like Hindu Kush).
FINDING THE IDEAL TEMPERATURE FOR YOUR GROW ROOM
As we mentioned earlier, the ideal temperature for growing cannabis is somewhere around 25°C. But remember, this can vary slightly from one plant to another. Whenever you’re working with a new plant, feel free to play around with the temperature settings in your grow room a bit, and see how they affect your plant. You’d be surprised just how much variety there is in the world of growing cannabis.