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

electricity. To make the most efficient use of electricity, purchase soil-heating tape or cable, with a thermostat that will automatically control the temperature. Lay the cable on the soil at the bottom of the bed or on a bed of sand or vermiculite and cover with about two inches (5 cm) of sand. You will need to provide 10-15 watts of electric heat for every square foot (30 cm 2 ) of growing area. Heat cables are also useful in greenhouses for warming seedlings, clones, or flowering plants without the cost of heating the entire structure.
    Small greenhouses can be heated relatively economically with an electric space heater, or more effectively with thermostatically controlled forced air using ducts or plastic tubing to distribute the heat. Larger units may be heated with forced air or by a coal or natural hot-water or steam system. Steam can also be used to sterilize growing beds and potting soils. Then there is the low-tech method of greenhouse warming: compost. A grower in Portland, Oregon, stacks organic matter on the sides of the greenhouse to a height of about five feet (1.5 m) inside and out. As the compost decomposes, it gives off heat keeping the structure warm at a very low cost.

This Swiss clone greenhouse was converted from a greenhouse that grew bedding flowers and vegetables.
    Evaporative cooling eliminates excess heat and adds humidity, reducing water needs. Moist air circulates through the structure while warm air is expelled through roof vents or exhaust fans. Properly installed, a cooler can reduce the interior temperature as much as 30-40°F (15-23°C) in hot, dry climates, less in wetter areas. As with fans, the size of the cooler is determined by the size of the greenhouse. A general guideline is to find a cooler equal to the total cubic space of the structure plus 50%. To provide both cooling and humidifying effects, the cooler must be installed on the outside of the greenhouse; otherwise, it simply humidifies without dropping the temperature. Turner greenhouses has a handy site ( http://www.turnergreenhouses.com/Cooling/coo_tip ) with some quick tips on selecting a cooling system for your greenhouse. Other great greenhouse sites include: http://www.igcusa.com/greenhousecooling information.htm for some helpful graphics and http://www.cpjungle.com/nuecool.htm for a detailed explanation of cooling needs and resources.
    Misting and watering are also important components of greenhouse gardening. Extended periods of growing and higher sustained temperatures make adequate water essential. Again, there are methods to suit every temperament from low-tech to automatic.
    Most companies offer watering and misting systems by component, which can be mixed and matched to suit the grower’s needs.Automatic systems will have a timer that triggers the mist or water at preset intervals. You may want a toggle switch that allows you to rotate between manual and automatic watering. For more information on specific uses and types of watering systems, go to a website such as www.cloudtops.com which covers a variety of topics pertaining to the internal greenhouse environment.
    A lower-tech method of mist and watering control consists of a series of screens that tilt downward with the weight of the water shutting off the flow then raising to restart the cycle as the screens dry. It is fully automated by the weight of the water or lack thereof. Of course, there is also hand-watering which is very effective and requires no mechanical intervention. Automatic systems, both high and low-tech, are alternatives to hand-watering that can be most helpful during a gardener’s absence.
    Heating and watering devices depend on that other cost of greenhouse keeping: how much time the grower has to spend tending plants. You can keep equipment costs to a minimum if you plan to spend a lot of time in the greenhouse. For growers who are away from the structure for long periods, automatic systems are a good investment.
    In addition to shelter, heat, water, and ventilation, plants need light. This section will offer a brief treatment of lighting, since it is covered in greater depth in Chapter Nine . Fluorescent light offers higher efficiency with low heat and is the most widely used. Incandescent light–60-500 watts–may be used to extend day-length. High-intensity discharge (HID) offers long life, and the sodium lamps emit the best light to be combined with natural sunlight. Regardless of light source selected, you may want to