These come in many shapes and sizes.
Glass panes and clips are simple and can be dismantled for storage, but the glass is easily broken. Rigid plastic sheeting may be used instead but must be handled carefully, as it can scratch, which will eventually reduce light transmission.
Polythene sheet and wire hoops (tunnel cloches) are the most useful for covering long stretches. The materials are inexpensive but the polythene soon deteriorates and should be renewed annually.
Plastic cloches are obtainable in a variety of shapes and sizes based on a rigid curved or square section frame and rigid or flexible horticultural-grade PVC. Many have tops which can be slid open without having to remove the whole structure to gain access to the crops growing underneath. Make sure that these have adequate anchorages – most of them have prongs on the bottom which can be pushed firmly into the ground — otherwise they can blow away in a strong wind.
1. To warm the soil in spring before making early sowings.
2. To give protection to early-sown crops.
3. To protect overwintering crops from conditions of extreme wet and cold.
(Note: If you only want the cloches for warming the soil, you can achieve just as good results by covering the dug and broken-down soil with sheets of polythene weighted down with bricks for a week or two before sowing. A refinement of this is to cover the ground after sowing with slitted plastic film, through which the young plants grow. The film warms the soil and acts as a mulch to the crops.)