You can build a small hobby greenhouse quickly and economically. If you simply build a frame of 2x4s and cover it with clear corrugated fiberglass, that's all you need to help extend your growing season for frost-tender plants -- such as sweet basil, potted palms and even small potted citrus trees. But the frost protection your greenhouse provides will only go so far in giving your plants the warmth they need, especially if you live in an area where winter temperatures often dip below freezing. You can heat your greenhouse by using one or more methods that won't send your electric bill through the ceiling.
Heating a Greenhouse in Winter
Spread a thick layer of compost or other form of organic mulch around the base of your plants. It will help keep the soil warmer and at the same time will provide nutrients to your plants.
Set plants on top of purchased heating coils designed for use with plants.
Create a heat sink by filling gallon plastic jugs with water. Set them on the floor of your greenhouse at frequent intervals (several inches to one foot apart). They will heat up a bit during the day and then radiate the heat in your greenhouse at night.
Hang 300-watt incandescent lights above your plants. Although they use electricity, using them is less costly than using a space heater. Their light will also benefit your plants.
Attach some form of insulation on the inside walls of your greenhouse. Alternatives to standard insulation include egg cartons and corn stalks from last summer's vegetable garden.