From Ed Rosenthal's Book :-
Temperature, Humidity & Air Quality
Marijuana grows well in moderate temperatures – between 70° and 85° F (21°-29° C). Both high and low temperatures slow marijuana’s rate of metabolism and growth. Plants grow fastest when the temperature during the lighted period is kept between 72° and 77° F (22°-26° C). When CO2 is being used, the plant prefers to be a few degrees warmer, between 79° and 85° F (26°-29° C). Individual marijuana varieties differ in their temperature preferences by a few degrees, so some experimentation is required to find the ideal temperatures for the strain you are growing.
Ideal temperature is tied to light conditions. As more light is available, the ideal temperature for normal plant growth increases. Strong light and low temperatures slow growth and decrease stem elongation. Conversely, when plants are given high temperatures and only moderate light, the stems elongate.
Plants growing under moderate intensity lamps (40-50 watts per sq ft, 430-540 watt per sq m) should be kept on the low side of the recommended temperature range. Plants growing under higher intensity lamps (60 watts per sq ft, 650 watts per sq m and higher) should be kept on the warmer end of the scale.
During dark periods, the temperature can be kept as much as 10˚ F (-12˚ C) cooler than the lit period without any negative effects. Wider temperature differences cause slower growth, stem elongation and delayed flower ripening.
Plants that are kept at a constant temperature are likely to grow stouter, sturdier stems and have denser bud growth. Plants that experience a large differential between day and night temperatures suffer from stretching and slowed growth rates.
At temperatures below 60° F (15° C), photosynthesis and plant metabolism slow, stopping growth as it waits for better conditions. As soon as the temperature rises, the plant resumes full functioning. When the temperature falls below 40° F (4° C), marijuana plants experience tissue damage and require about 24 hours of warmer conditions to resume growth. Young marijuana plants are somewhat tolerant of low temperatures; when outdoors, seedlings have been known to pierce snow cover without ill effect. But low temperatures during ripening, even just overnight, delay or prevent bud maturation. Some equatorial varieties stop growth after a few nights with temperatures below 40° F (4° C).
