Cold frames allow you to start plants early in spring and keep them out longer in fall, extended your planting and harvesting dates.
An inexpensive foray into greenhouse gardening, a cold frame is practical for starting plants six to eight weeks earlier in the spring growing season and for hardening off seedlings. Cold frames can also be used, however, to protect plants from the upcoming fall frost.
Basically, a cold frame is a box set on the ground and topped with glass or plastic. Although mechanized models with thermostatically controlled atmospheres and sash that automatically open and close are available, you can easily build a basic cold frame from materials you most likely have around the house.
The back of the frame should be about twice as tall as the front so the lid slopes to a favorable angle for capturing sun rays. Build the frame tall enough to accommodate the maximum height of the plants before they are removed. The frame can be made of brick, block, plastic, wood or just about any material you have on hand. It should be built to keep drafts out and soil in.