The idea is to provide a greenhouse-like atmosphere for seeds and small plants in order to get an early start on the season, or to extend the fall garden as long as possible. Cloches are set out over individual plants or are made into tunnels for whole rows. They trap solar radiation and moisture evaporating from the soil and plants. More elaborate types are fiber glass tunnels, special plastic cloches or row covers with slits in them to allow for some aeration, and panes of glass connected by specially-designed hinges to form a tent. There are a variety of forms on the market now and some are easily constructed from materials around the home.
Cloches are generally lightweight, portable, and re-usable.
It is preferable to have a design which can be closed completely at night to prevent frost damage and opened or completely removed during the day for good air circulation. Cloches should be anchored or heavy enough that they don't blow away. There is an almost overwhelming selection of greenhouses on the market, and plans for building even more types are available. If you intend to purchase or build a greenhouse, it is wise to investigate the alternatives thoroughly, preferably by visiting as many operating home greenhouses as possible. List your needs and wants ahead of time and determine the uses you plan for your greenhouse.