Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

doubles every 20°F (10°C). Root respiration increases as the roots warm up, which is why fresh air is important both day and night.
    Temperatures above 85°F (29°C) are not recommended even when using CO 2 enrichment. Under the proper conditions, which are very demanding to maintain, higher temperatures step up metabolic activity and speed growth. The warmer it is, the more water the air is able to hold. This moist air often restrains plant functions and decelerates growth rather than speeding it. Other complications and problems result from excess humidity and moisture condensation when the temperature drops at night.
    Heat buildup during warm weather can catch growers off guard and cause serious problems. Ideal grow rooms are located underground, in a basement, taking advantage of the insulating qualities of Mother Earth. With the added heat of the HID and hot, humid weather outdoors, a room can heat up rapidly. More than a few American growers have lost their crops to heat stroke during the Fourth of July weekend, since it is the first big holiday of the summer, and everybody in the city wants to get away to enjoy it. There are always some gardeners that forget or are too paranoid to maintain good ventilation in the grow room while on vacation. Temperatures can easily climb to 100°F (38°C) or more in grow rooms that are poorly insulated and vented. The hotter it is, the more ventilation and water that are necessary.
    The cold of winter is the other temperature extreme. Montreal, Quebec, Canada, growers will remember the year of the big ice storm. Electricity went out all over the city and surrounding areas. Water pipes froze, and heating systems failed. Residents were driven from their homes until electricity was restored some days later. Many growers returned to find their beautiful gardens wilted, stricken with the deepest, most disgusting green only a freeze can bring. Broken water pipes, ice everywhere! It is difficult to combat such acts of God, but if possible, always keep the grow room above 50°F (10°C) and definitely above freezing, 32°F (0°C). If thetemperature dips below this mark, the freeze will rupture plant cells, and foliage will die back or, at best, grow slowly. Growth slows/stops when the temperature dips below 55°F (13°C). Stressing plants with cold weather conditions is not recommended; it may yield a proportionately higher THC content, but will reduce plants’ overall productivity.
    A thermostat measures the temperature, then controls it by turning on or off a device that regulates heating or cooling, keeping the temperature within a predetermined range. A thermostat can be attached to an electric or combustion heater. In fact, many homes are already set up with electric baseboard heat and a thermostat in each room.
    A thermostat can be used to control cooling vent fans in all but the coldest grow rooms. When it gets too hot in a room, the thermostat turns on the vent fan, which evacuates the hot stale air. The vent fan remains on until the desired temperature is reached, then the thermostat turns off the fan. A thermostat-controlled vent fan offers adequate temperature and humidity control for many grow rooms. A refrigerated air conditioner can be installed if heat and humidity are a major problem. Just remember, air conditioners draw a lot of electricity. If excessive heat is a problem, but humidity is not a concern, use a swamp cooler. These evaporative coolers are inexpensive to operate and keep rooms cool in arid climates.
    Common thermostats include single-stage and two-stage. The single-stage thermostat controls a device that keeps the temperature the same both day and night. A two-stage thermostat is more expensive but can be set to maintain different daytime and nighttime temperatures. This convenience can save money on heating, since room temperature can drop 5-10°F (2-5°C) at night with little effect on growth.
    Many new grow room controllers have been developed in the last ten years. These controllers can operate and integrate every appliance in the grow room. More sophisticated controllers integrate the operation of CO 2 equipment, vent, and intake fans. Relatively inexpensive computerized controllers are also available for grow rooms. If temperature and humidity regulation are causing cultural problems in the grow room, consider purchasing a controller.

This air conditioner lowers grow room temperature and humidity. The hot radiator is outside, about 10 feet (3 m)