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

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A simple electric heater may be necessary to warm the room after lights go out.
    Uninsulated grow rooms or grow rooms that experience significant temperature fluctuations require special consideration and care. Before growing in such a location, make sure it is the only choice. If forced to use a sun-baked attic that cools at night, make sure maximuminsulation is in place to help balance temperature instability. Enclose the room to control heating and cooling.
    When CO 2 is enriched to 0.12-0.15 percent (1200-1500 ppm), a temperature of 80°F (27°C) promotes more rapid exchange of gases. Photosynthesis and chlorophyll synthesis are able to take place at a more rapid rate causing plants to grow faster. Remember, this higher temperature increases water, nutrient, and space consumption, so be prepared! Carbon dioxide-enriched plants still need ventilation to remove stale, humid air and promote plant health.
    The temperature in the grow room tends to stay the same, top to bottom, when the air is circulated with an oscillating fan(s). In an enclosed grow room, HID lamps and ballasts keep the area warm. A remote ballast placed near the floor on a shelf or a stand also helps break up air stratification by radiating heat upward. Grow rooms in cool climates stay warm during the day when the outdoor temperature peaks, but often cool off too much at night when cold temperatures set in. To compensate, growers turn on the lamp at night to help heat the room, but leave it off during the day.
    Sometimes it is too cold for the lamp and ballast to maintain satisfactory room temperatures. Grow rooms located in homes are usually equipped with a central heating and/or air conditioning vent. The vent is usually controlled by a central thermostat that regulates the temperature of the home. By adjusting the thermostat to 72°F (22°C) and opening the door to the grow room, it can stay a cozy 72°F (22°C). However, using so much power is expensive, and could cause a security quandary. Keeping the thermostat between 60 and 65°F (15-18°C), coupled with the heat from the HID system, should be enough to sustain 75°F (24°C) temperatures. Other supplemental heat sources such as inefficient incandescent lightbulbs and electric heaters are expensive and draw extra electricity, but they provide instant heat that is easy to regulate. Propane and natural gas heaters increase temperatures and burn oxygen from the air, creating CO 2 and water vapor as by-products. This dual advantage makes using a CO 2 generator economical and practical.
    Kerosene heaters also work to generate heat and CO 2 . Look for a heater that burns its fuel efficiently and completely with no tell-tale odor of the fuel in the room. Do not use old kerosene heaters or fuel-oil heaters if they burn fuel inefficiently. A blue flame is burning all the fuel cleanly. A red flame indicates only part of the fuel is being burned. I’m not a big fan of kerosene heaters and do not recommend using them. The room must be vented regularly to avoid buildup of toxic carbon monoxide, also a by-product of combustion.
    Diesel oil is a common source of indoor heat. Many furnaces use this dirty, polluting fuel. Wood heat is not the cleanest either, but works well as a heat source. A vent fan is extremely important to exhaust polluted air and draw fresh air into a room heated by an oil furnace or woodstove.
    Insect populations and fungi are also affected by temperature. In general, the cooler it is, the slower the insects and fungi reproduce and develop. Temperature control is effectively integrated into many insect and fungus control programs. Check recommendations in Chapter 14, “Pests, Fungi and Diseases.”
Humidity
    Humidity is relative; that is, air holds different quantities of water at different temperatures. Relative humidity is the ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at the same temperature. In other words, the hotter it is, the more moisture air can hold; the cooler it is, the less moisture air can hold. When the temperature in a grow room drops, the humidity climbs, and moisture condenses. For example, an 800 cubic foot (10 × 10 × 8 feet) (21.5 m 2 ) grow room will hold about 14 ounces (414 ml) of water when the temperature is 70°F (21°C) and relative humidityis at 100 percent. When the temperature is increased to 100°F (38°C), the same room will hold 56 ounces (1.65 L) of moisture at 100 percent relative