A greenhouse is designed to increase the air temperature to allow plants to grow when outside temperatures are too low for the plants. Greenhouses can range from a simple cold frame to the large wrought iron and glass greenhouses popular in the 19th century. Greenhouses can be as small as a single garden bed or cover thousands of square feet. Select the greenhouse that will most closely suit your gardening and budgetary needs.
A cold frame is an unheated, protected plant bed. A cold frame can be as tall as the plants growing in the bed. Cold frames are plastic sheeting that helps prevent heat loss from the bed. Although the temperature inside the cold frame may only be 5 to 10 degrees warmer than the outside temperature, this is often enough to allow either early planting or late harvest of crops. A cold frame with a heater or heating element is called a hot bed. Hot beds and cold frames should have a Southern exposure to increase sun exposure. Cold frames can be removable and made from wood and plastic sheeting or can be more permanent with wood sides and a glass or hard plastic top.
Temporary greenhouses may be left in place in many locations. Temporary greenhouses have a plastic or metal frame covered in clear vinyl or some other clear plastic. Some temporary greenhouses have heaters; others are similar to cold frames in their reliance on sun to raise the interior temperature. Portable greenhouses are sometimes called hoop houses. Portable greenhouses and hoop houses are much less expensive than permanently installed structures and are also mobile in cases when you might need a greenhouse in different locations.
Permanent greenhouses are made of wood, metal, other supports. They are sometimes heated and sometimes unheated. Lean-to type greenhouses can use heat coming from the walls of a heated home or other heated structure. An unusual but effective form of a permanent greenhouse is a pit greenhouse. In a pit greenhouse, two or three walls of the greenhouse are underground, while the third is exposed to allow an access door. The glass or clear plastic roof of a pit greenhouse allows sunshine. Pit greenhouses use the natural warmth of the soil combined with the sun through the roof to increase interior temperatures. Other permanent greenhouses include A-frames, slat walled and gabled roofs.