The best temperature for cannabis plants
Proper temperature is one highly variable factor. Most books state optimum grow temperature to be 21-27° Celsius (70- 80 F), but many list extenuating circumstances that allow temperatures to go higher. Assuming genetics is not a factor, plants seem to be able to absorb more light at higher temps, perhaps up to 32° C (90 F).
High light and CO2 levels could make this go as high as 35° Celsius (95 F) for increased growth speed. An optimum of 35° Celsius is new scientific data that assumes very-high light, CO2 enrichment of 1500 ppm and good regular venting to keep humidity down. It is not clear if these temperature will reduce potency in flowers. But higher temperatures will make plants grow vegetatively much faster, by exciting the plants metabolism, assuming the required levels of CO2 and light are available, and humidity is not allowed to get too high.
With normal levels of CO2, in a well vented space, 32° C (90 F) would seem to be the absolute max, while 29° C (85 F) may be closer to optimum, even with a great deal of light available. Do not let the room temperature get over 35° C (95 F) as this hurts growth.
The optimal temperature is 27-30° C (80-86 F) if you have strong light with no CO2 enrichment, which is most likely your average indoor growing situation. Less than 21° C (70 F) is too cold for good growth.
Low temperatures at night are OK down to about 15° C (60 F) outdoors, then start to effect the growth in a big way. Mid 50s will cause mild shock and 40s will kill your plants with repeated exposure. Keep your plants warm, especially the roots. Elevate pots if you think the ground is sucking the heat out of the roots. This is an issue if you have a slab or other type of cold floor.
As temperature goes up, so does the ability of the air to hold water, thus reducing humidity, so a higher average temperature should reduce risk of fungus.
Contrary to many reports, high humidity is not good for plants except during germination and rooting. Lower humidity levels help the plant transpire CO2 and reduce risk of molds during flowering.
It may be a good idea to reduce temperatures once flowering has started, to preserve potency, even if it does reduce growth speed. Studies indicate the potency of buds goes down as the temperature goes up, so it is important to see that the plants do not get too hot during flowering cycles. When flowering the temperature should not drop below 15° C (60 F).