A cold frame is a transparent-roofed enclosure, built low to the ground, used to protect plants from cold weather in agriculture and gardening. The transparent top admits sunlight and prevents heat escape via convection that would otherwise occur, particularly at night. Essentially, a cold frame functions as a miniature greenhouse season extension device.
Historically cold frames were built to be used in addition to a heated greenhouse. The name, itself, exemplifies the distinction between the warm greenhouse and the unheated cold frame. They were frequently built as part of the greenhouse's foundation brickwork along the southern wall (in northern latitudes). This allowed seeds to be germinated in the greenhouse and then easily moved to the attached cold frame to be hardened-off before final planting outside.
Cold frames are found in home gardens and in vegetable farming. They create microclimates that provide several degrees of air and soil temperature insulation, and shelter from wind. In cold-winter regions, these characteristics allow plants to be started earlier in the spring, and to survive longer into the fall and winter. They are most often used for growing seedlings that are later transplanted into open ground, and can also be a permanent home to cold-hardy vegetables grown for autumn and winter harvest.
Cold frame construction is a common home or farm building project, although kits and commercial systems are available. A traditional plan makes use of old glass windows: a wooden frame is built 30cms or a foot or two high, and the window placed on top. The roof is often sloped towards the winter sun to capture more light, and to improve runoff of water, and hinged for easy access. Clear plastic, rigid or sheeting, can be used in place of glass. An electric heating cable, available for this purpose, can be placed in the soil to provide additional heat.
One of the advantages of using a cold frame is that you can raise a huge variety of crops from seed through to maturity within the frame - so it's possible to have vegetable crops ahead of their normal season when they are extremely expensive to buy. Some crops suitable for growing in a cold frame include lettuces, parsley, salad onions, spinach, radishes and turnips etc. One vegetable crop can occupy the whole of a cold frame or you can grow a combination of crops so that they mature in rotation - so you'll get a wide range of different vegetables throughout the year from one cold frame.
- How to Build a Cold Frame, Andrew Czernek, February, 2010.
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