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In cold climates, a reliable method of greenhouse heating is usually needed to protect delicate plants from freezing temperatures. The best method of greenhouse heating generally depends on the availability of an energy source, plant type, and size of the structure. Soil warming trays and pads are often the best way to keep small seedlings warm during the winter months, but are usually not cost effective for larger structures. Propane heaters are a low cost method of greenhouse heating, but are not suitable for all plant varieties due to harmful exhaust emissions. Electric heaters are the often first choice for greenhouse heating because of their low operating cost, efficiency, and ease of use.
Soil warming trays work in the same manner as an electric blanket by utilizing a network of insulated heating wires to produce a gentle heat. These heating wires are situated beneath the soil near the bottom of the tray and typically controlled by a thermostat. Warming pads operate in a similar manner, but are placed beneath the seedling trays. Soil warming trays and pads require a nearby source of electricity, and a separate circuit may be needed to power multiple units. These devices are usually an inexpensive greenhouse heating method for small structures.
Propane is also a good method of greenhouse heating because of its moist heat and low operating cost. The moist heat produced by propane units can sometimes create condensation or mold problems, however, and a dehumidifier may also be necessary. Propane heaters sometimes allow cold spots to form in areas of poor air circulation, and a fan may be needed for even heat distribution. These units are usually not equipped with a thermostat and may cause the greenhouse to overheat at times. The gases produced by propane heaters are harmful to certain plant species and an exhaust ventilation method also may be required.
An electric heater is usually the most popular greenhouse heating method because of its even heat distribution and relatively low operating cost. This heater variant is typically fan forced, and a single unit can circulate warm air throughout a small structure in a short period of time. Electric heaters are thermostat controlled to permit operation only when the temperature falls below a certain level. These heaters produce a very dry form of heat, and a humidifier may also be needed to prevent damage to sensitive vegetation. Electric heaters must be designed specifically for greenhouse use; an ordinary household unit should never be used for this purpose.