Since marijuana varieties are most often grown in semi-tropical and tropical areas, the idea that high temperatures are necessary for potent marijuana is firmly entrenched in marijuana lore. This myth, like many others, is slowly disappearing as marijuana farmers and researchers accumulate more experience and knowledge. There are only a few published papers on the effects of temperature on potency. The best study we've seen 19 grew four different varieties in a controlled environment under artificial lights on a 15-hour day-length. Two temperature regimes were used: a "warm" regime, with temperatures of about 73F during the day and 61F at night (about average for most homes); and a "hot" regime, set at 90F daytime and 73F at night. In all four varieties, the concentration of THC and of total cannabinoids was higher under the "warm" regime. For instance, a Nepalese strain was 3.4 times higher in concentration of total cannabinoids, and 4.4 times higher in THC, when grown under the "warm" regime than the same strain grown under the "hot" regime. Although we agree with the findings in principle, these figures are higher than our experience tells us.
Interpretation of the data does show one point clearly. In all four varieties, the amount of THC lost as CBN was higher under the "hot" regime (see Table 16 - currently excluded from this guide), even though the concentration of THC was higher under the "warm" regime.
Another research group in France has looked at the relationship of potency to temperature. The most recent paper 79 compared four temperature regimes, given in descending order of potencies found: 75F day, 75F night (highest potency); 72F day, 54F night; 81F day, 81F night; and 90F day, 54F night (lowest concentration of THC). In each, the day period was 16 hours and the night period eight hours.
Interestingly, this same research group in an earlier paper 20 reported that the concentration of THC was higher for male plants grown at 90-72F then for those grown at 72-54F. For the female plants, the differences in THC concentration were small. The variety used was a propyl variety (type IV) containing about half as much THCV as THC. For both the male and female plants, the concentration of THCV were high under the 90-72F regime.
The simplest interpretation of all these results is that mild temperatures seem to be optimum for potency. Temperatures over 90F or below 60F seem to decrease the concentration of THC and total cannabinoids. Also, at higher temperatures, much more THC will be lost as CBN. And last, propyl varieties may produce less THCV under a cool regime. Bear in mind that none of these papers accounted for all of the many variables that could have affected the findings. For instance, the concentration of THC was 18 times higher at 75-75F than at 90-54F. We've never seen differences of this magnitude, and sampling error undoubtedly influenced the findings.
In terms of growth rate and potency, daily temperatures of about 75F, give or take a few degrees, are roughly optimum. Normal household temperatures are in the low 70's during daytime and the low 60's at night. The heat from a light system will raise the garden's temperature a few degrees. In most gardens temperatures will be near 75F during the day. Night-time temperatures drop about 10 to 15 degrees. When night-time temperatures drop into the 50's or below, set the light cycle to turn on during the early morning, when the temperature will be lowest. In a small room, the light system will generate enough heat to warm the garden without any need for a heater. Whenever you wish to raise the temperature by, say, five or 10 degrees, it is better to add more lights than a heater. The plants will benefit from the additional light, as well as from the heat they generate. And an electric heater, watt for watt, doesn't generate much more heat than a lamp and its fixture.
