How to heat your greenhouse is the big decision, not so much in terms of installation and operation but in the selection of the kind of heat most economical for your type of greenhouse and its particular location. Efficiency and economy are possible with any fuel—gas, oil, or electricity—if properly selected and utilized, and with either system —warm-air or hot-water. At one time, only hot water was considered a practical means of providing adequate, evenly distributed heat. Pipes were run under the benches and around the entire greenhouse. But research proved that warm-air systems, as they have been developed for today's greenhouses, are just as effective for small greenhouses.
No matter what the system or the fuel, several factors determine the amount of heat required. These are the size and location of your greenhouse, whether it is attached or detached, the minimum night temperature necessary for the plants you want to grow, and the severity of the winter in your area. Also, costs may vary somewhat from season to season, depending on the weather in a particular year.
No matter where you live, if you plan year-round operation, some means of heating is necessary, even though most or all of the heat required during the day is furnished by the sun. In southern states, small gas units or electric heaters are adequate for even a fairly large hobby greenhouse, since outside temperatures are not usually so low as in the North. Even in the Deep South, where both days and nights are warm, auxiliary heat should be provided to offset the occasional drop in temperature that brings a few chilling and possibly freezing nights.