You can take tony's word for it or you can read what Ed has to say.
Ed Rosenthal's Grow Tips
Maintaining Optimum Grow Room Temperature Marijuana plants are very hardy and survive over a wide range of temperatures. They can withstand extremely hot weather - up to 120 degrees - as long as they have adequate supplies of water. Cannabis seedlings regularly survive light frost at the beginning of the season. Both excessively high and low temperatures can slow marijuana's rate of metabolism and growth. The plants function best in moderate temperatures - between 60 and 85 degrees. As more light is made available, the ideal temperature for normal plant growth increases. In high temperature and moderate light conditions, the plant's stems elongate. Strong light and low temperature conditions will decrease stem elongation. During periods of low light, sem elongation can be decreased by lowering the temperature.
Temperatures below 50 degrees slow growth in most varieties of marijuana. When the temperature drops below 40 degrees, the plants may experience some damage and require about 24 hours to resume growth. Low nighttime temperatures may delay or prevent bud maturation. Some equatorial varieties stop growth after a few 40 degree nights.
A sunny room or one illuminated by high wattage lamps heats up rapidly. During the winter, the heat produced may keep the room comfortable. However, the room may get too warm during the summer. Heat rises, so that room temperature is best measured at the plants' height. A room with a 10 foot ceiling may feel uncomfortably warm at head level but be fine for plants 2 feet tall.
If the room has a vent or window, an exhaust fan can be used to cool it. Totally enclosed spaces can be cooled using a water conditioner which cools the air by evaporating water. If the room is lit entirely by lamps, the day/night cycle can be reversed so that heat is generated at night, when it is cooler out.
Marijuana is low-temperature tolerant. Outdoors, seedlings sometimes pierce snow cover, and older plants can withstand short, light frosts. Statistically, more males develop in cold temperatures. Low temperatures, however, slow down the rate of plant metabolism. Cold floors can lower the temperature in containers, thereby lowering root temperature and slowing germination and plant growth. Ideally, the medium temperature should be 70 degrees.
There are several ways to warm the medium. The floor can be insulated using a thin sheet of styrofoam, foam rubber, wood or newspaper. The best way to insulate a container from a cold floor is to raise the container so that there is an air space between it and the floor. Overhead fans, which circulate the warm air downward from the top of the room also warm the medium.
When the plants' roots are kept warm, the rest of the plant can be cooler without danger. Heat cables or heat mats, which use small amounts of electricity, can be used to heat the root area. These are available at nursery supply houses.
When watering, tepid water should be used. Cultivators using systems that recirculate water can heat the water with a fish tank heater and thermostat. If the air is cool, 45-60 degrees, the water can be heated to 90 degrees. If the air is warm, over 60 degrees, a 70 degree water temperature is sufficient.
Gardens using artificial lighting can generate high air temperatures. Each 1000 watt metal halide and ballast emits just a little less energy than a 10 amp heater. Several lights can raise the temperature to an intolerable level. In this case, a heat exchanger is required. A venting fan can be used to lower temperatures.
Greenhouses can also get very hot during the summer. If the sun is producing too much heat, opaquing paint may lower the amount of light and heat entering the greenhouse. Fans and cooling mats also help. Cooling mats are fibrous plastic mats which hold moisture and which can be purchased through nursery supply houses. Fans blow air through the mats, lowering the greenhouse temperature. They are most effective in hot, dry areas.
