Cold frames and mini-greenhouses give low-cost, space-saving plant protection.
Cold frames
A cold frame is simply a box with a sloping glazed lid. Glass lasts longer than plastic glazing. The sides may be made of masonry, timber, metal or glazed. Typically, they are 45–60cm (18-24in) deep and the lid can be opened to varying degrees to control the temperature. They are seldom heated, but the addition of a soil-heating cable in a sandbed at their base can greatly enhance their usefulness for propagation and overwintering tender plants.
Cold frames are a traditional means of protecting plants that supplement, not replace, glasshouses. They are especially used to harden off plants raised in glasshouses as the lid can be used to accustom young plants to increasing exposure before planting out in the garden. Other uses include overwintering plants, for sowing seeds that need cool conditions but dislike waterlogging, and for rooting cuttings. Summer crops of low-growing cucumbers and melons, winter lettuces and salad leaves are particularly well suited to cold frames.
Mini-greenhouses
Popular for small gardens, mini-greenhouses are taller, usually plastic-covered boxes (with shelves) and are open at the front.
Mini-greenhouses are much better than windowsills for raising seedlings, and ideal for summer crops of plants too tall for cold frames, such as aubergines, peppers and tomatoes.
They have their disadvantages, however. Replacement covers can be difficult to find, and heating is impractical. Drips and trickles of water can damage plants on lower shelves, which are also shaded by plants above. The openings often fail to give the fine control over temperatures that cold frames offer, and they offer less winter protection than a cold frame. Their light weight can lead to them blowing away in winter gales unless securely anchored.