A cold frame is a box frame with a clear lid on top for the extension of the growing season inside. Cold frames are used to start seedlings earlier in the spring, to help harden tender plants in early summer, and to carry cropping vegetables well into the winter. Normally constructed from wood, cold frames made from cinder blocks or bricks are also common. Similar to cold frames, but larger, are greenhouses.
The clear lid lets in sunlight and traps warm air inside to protect plants from inclement weather. The lid is ideally sloped towards the midday sun to allow for the maximum light and warmth.