These are usually low structures, about 30 in. (750 mm) tall at the highest, covered with glass, horticultural plastic or polythene, but vertical ones which can be converted to tomato houses are also available. The walls can either be of transparent material, in the case of a low frame, or solid. The clear top of the frame (or front of a vertical one) is removable or openable and solid walls can be made of brick, wood, concrete, or other similar material, even straw bales or turves if you only want a temporary structure, or alternatively of glass or horticultural rigid plastic. There are many good self-assembly and ready-assembled models available, or you can make your own quite simply by obtaining an old window, complete with glass (these are often advertised in the small ads in local papers by people who have had replacement windows installed) and fitting it over a large, strong, wooden box.
Although ‘cold’ frames are so called because they are unheated, they can have soil-warming cables installed if required.
Propagating frames work on the same principle but are smaller and tighter fitting in order to provide the necessary humid atmosphere. These can be home-made and unheated, or have soil-warming cables incorporated, but there are scores of very good electrically and paraffin-heated ready-made models on the market. They are generally used to provide a warm environment in a larger glass structure, or can be placed on a light window-sill.
- To speed up germination of seeds by giving them a warm atmosphere.
- To raise early crops which would not be hardy in the open ground (e.g., salads).
- To protect slightly tender plants and newly rooted cuttings over winter.
- For striking cuttings — the added protection facilitates rooting.
- For hardening off greenhouse-raised plants before planting out in summer. In this case the young plants are removed from the greenhouse and placed in the frame, the top of which is left open during the day and replaced at night in case of late frosts. Eventually the top can be left off all the time, and when all risk of frost is past the plants can be put out in their final positions.