cold frame, in horticulture, sun-heated board frame covered with a removable top of glass or other transparent material and sunk into the ground. The top may be solid or slatted or screened for shade. The cold frame is used to start seedlings in early spring (four to six weeks before the average frost-free date), to harden seedlings or plants removed from greenhouses or hotbeds, and to protect plants during the winter. A hotbed is an artificially heated cold frame.
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