You can save your precious frost-tender plants when Jack Frost comes calling by moving them into a small greenhouse for the winter. Or perhaps you have a prized dwarf citrus tree---you can grow such plants in a planter box in your greenhouse year-round, but it will need more heat than an unheated greenhouse will provide. You can provide some warmth in your greenhouse without using space heaters and making your electric bill go sky high by choosing one or more of several methods of heating your greenhouse economically.
Set up one or more 300-watt incandescent lights above the plants in your greenhouse. This wattage puts out a fair amount of heat, and while it will affect your electric bill, it is more economical than other heating methods, such as space heaters.
Insulate your greenhouse with common materials, such as egg cartons or dried plants from last summer's garden. Insulation will block the light that normally flows into your greenhouse, so install grow lights or fluorescent shop lights to give your plants the light they need to stay healthy throughout the winter.
Spread a thick layer of compost or other mulch at the base of all your plants. You can even mulch hanging plants.
Make a passive solar heating system by filling gallon plastic jugs with water and setting them on the floor of your greenhouse. They will heat up during the day and disperse their warmth at night.
Use heating coils under your potted plants if they require extra warmth.
Place one or more small electric fans in your greenhouse to circulate the air and heat around your plants. You run the risk of introducing fungal diseases if your greenhouse has a warm, still, humid environment.