Building and Using Hotbeds and Coldframes
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A coldframe is a protected plant bed. It has no artificial heat added. The temperature difference between the inside and outside of the frame is generally not more than 5 to 10 degrees. A mat or blanket may be placed over the frame on cold nights to conserve heat, but this increases temperature by only a few degrees. There are times, however, when a few degrees can be very important.
A coldframe is used to provide shelter for tender perennials, to "harden off" seedling plants or to start cold-tolerant plants such as pansies, cabbage or lettuce earlier than they can be started in open soil. It may also be used to overwinter summer-rooted cuttings of woody plants.
A hotbed basically is a heated coldframe. In many ways it is a miniature greenhouse, providing the same benefits with limited space at minimal expense. It is a means for extending the growing season. It is most often used to give an early start to warm-season vegetables such as tomato, pepper or melon. It may also be used to root cuttings of some woody plants.