Cold frames are an ideal place to harden off plants so that they can become acclimatised to the outside conditions before they are planted out. The soil and air temperatures in cold frames are warmer than in open soil, which means they provide a perfect 'halfway-house' between the greenhouse and the outdoor world.
A cold frame can also be a good place to sow seeds of early vegetables or hardy perennials, which although hardy, can benefit from a little more warmth at germinating time. Some cuttings, particularly hardwood, can also be grown in a cold frame.
Using a Cold Frame
When placing newly grown plants in a cold frame, the lid should be opened successively higher each day to air the plants, and then closed at night. During particular cold or frosty nights, the lid should be insulated with old carpet, hessian or several layers of thick, clear plastic or bubble wrap. The plastic insulation has the advantage that it may be left on during cold days as it still transmits light to the plants.