A cold frame is an enclosure which traps solar radiation for the purpose of growing plants in cold weather. Typically, a cold frame is a simple open-bottomed frame made from wood or other construction material with a clear glass or hard polycarbonate sheet cover which lets the sunlight pass through. A cold frame is generally not used as a hot house but instead used to shelter plants from the elements to extend a growing season in the early spring and late fall.
Cold frames are inexpensive to build or to purchase. They also tend to be small enough to be moved about. Most frame-and-glass type cold frames are used for small scale agriculture such as gardening. In larger commercial agriculture, floating row covers made from hoops and sheeting are more economical solutions. Many gardeners use cold frames to start crops earlier and to extend the growing season. In milder regions, the use of cold frames can allow year-round gardening.
In the early spring a cold frame can be used to harden off seedlings started indoors. In this manner, tender seedlings can be introduced to cold weather in the cold frame for a few hours for the first few days. After the first few days, the seedlings can stay in the cold frame overnight until the weather is mild enough and plants are mature enough to transplant into the garden. Alternatively, larger cold frames can be set up directly over seedbeds in the garden. In this manner, many gardeners are able to start their cold weather crops weeks earlier than without the use of cold frames.
In warm weather, the temperature inside a cold frame can get too warm for cold weather plants. It is advisable to vent a cold frame during these warm spells, often by simply raising the cover during the hours of greatest daylight. Even on an overcast day, the temperature inside a cold frame can be over ten degrees Fahrenheit warmer than outside.
In the fall, a cold frame can be placed directly over a seeded bed of cold weather crops. The crops in these beds will be sheltered from cold winds and freezing temperatures within the cold frame. The cold frame can insulate the soil and keep it from freezing until the coldest months or, in some regions, all winter long. Many gardeners, in this manner, can plant successive crops of carrots, spinach, lettuce, radishes, beets and other cold weather crops to be harvested from late fall through the next spring.
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