TEMPERATURE - Cannabis Growing Guide
Proper temperature is one highly variable factor. Most books state optimum
grow temperature to be 70-80 degrees, 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 90 degrees.
High light and CO2 levels could make this go as high as 95 degrees for increased
growth speed.* An optimum of 95 degrees is new 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. It
may be a good idea to reduce temperatures once flowering has started, to
preserve potency, even if it does reduce growth speed. 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, 90 degrees would seem to
be the absolute max, while 85 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. Optimal temperature is 27-30 C (80-86 F) if you have strong light
with no CO2 enrichment. Less than 21 C (70 F) is too cold for good growth.
Low temperatures at night are OK down to about 60 degrees 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.
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
* D. Gold: CO2, Temperature and Humidity, 1991 Edited by E. Rosenthal.