Cold frames consist of unheated enclosures used to protect a narrow area of garden, allowing for an early start to the growing season and extending it in the fall. Semi-hardy species may be overwintered fairly safely, and the frame provides a good environment for stratifying seed, as for hardening off plants in spring and early summer. In milder climates cold frames may be used to grow plants year round.
The fanciest models are resemble miniature greenhouses, with top and sides made of glass. Commonly, however, gardeners improvise and use wood, brick or concrete for the sides; all retain heat well. Old windows may be recycled, frame and all, to form an excellent top. If wood-frame cold frames are used to grow fruits and vegetables, use untreated wood (cedar lasts longest) or use the preservative recipe presented under Raised Beds.
Typically, the top sashes are made of glass, which holds heat and transits light well, though clear plastic is a good alternative. These should slant in a southward direction for better solar gains.
Cold frames are best sited in a warm, sheltered area; well-drained soil should be provided. Frames may benefit from having their walls padded with foam insulation, especially those with thin glass or plastic walls. Or, if nights are very cold, use old blankets or bales of hay to help hold warmth inside. Walls should be well-sealed, and keep drafts out.
In summer, however, good ventilation and some provision for dappled shade is essential, as plants can be quickly baked alive. Sashes should be propped open when temperatures of 80-85F/27-31C are reached, and earlier for cool-weather crops. In cooler areas, warmth-loving plants like tomatoes, peppers and melons, may be kept in the cold frame throughout the summer, provided protection from very hot temperatures is provided.
As mentioned, hardening off is easily accomplished with the cold frame, over a period of six to eight weeks. Allow new indoor-started seedlings a week or two to adjust, then begin opening the sashes for increasing periods of time, first during the day, then day and night, until finally the sash may be removed completely. Keep plants in the cold frame for at least a week, with sashes left off, prior to transplanting to the garden.
Seed also may be started early in the cold frame; especially those plants that require a dormant period or double dormancy are easily grown from seed in the cold frame. Again, these seedlings must be hardened off gradually. While initial growth may be somewhat slower, plants will be strong and healthy.
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