Experienced gardeners have many tools at their disposal for extending the growing season and protecting tender young plants. Some gardeners use traditional full-size greenhouses with air pumps and heating elements, while others prefer the cheaper and easier-to-maintain method of using cold frame greenhouses. A cold frame greenhouse is simply a four-sided structure that is low to the ground with a clear top that is designed to let in the sun’s rays as a means of keeping the plants inside protected and warm in colder weather. This kind of greenhouse is easy to make from recycled materials, or it can be purchased commercially in a kit.
The basic structure of a cold frame greenhouse includes four low walls, with two slanted side walls that are higher in the back than in the front. The slanting serves two purposes for the cold frame greenhouse: First, it enables snow, rain, and debris to slide off the lid to keep the top panel clear; second, it ensures that the plants inside receive maximum sunlight exposure even late in the day when the sun begins to set.
Cold frame greenhouses should always face the south to allow for maximum sun and should not be located beneath trees or other objects that might cast it in shade. Often, cold frames are built on the south-facing side of a house, garage, or shed, which allows the wall of the existing structure to act as the rear wall of the cold frame. If the building is heated, it will help keep the cold frame warm on cloudy days.
The most important element of a cold frame greenhouse is the transparent top panel. Gardeners who make their own cold frames typically use a recycled storm window as the lid, with hinges screwed in to make opening and closing the lid easier. Another material that can be used for the lid is heavy-duty, clear plastic that is strong enough to withstand wind and winter storms.
While the cold frame should be kept closed to prevent heat from escaping during very cold weather, it should be able to be open on warmer days so the plants inside do not become overheated. The cold frame greenhouse should also be situated in a spot that receives excellent drainage so plants do not become waterlogged and drown in wet weather. The sides of the cold frame can be made of bricks, concrete blocks, or a type of wood that is resistant to decomposing.
Gardeners use cold frames in the autumn to extend the summer season, protecting plants from falling temperatures and ensuring harvests of herbs, salad greens, and root crops into the winter months. In late winter into early spring before the last freeze, cold frames are used to start annual plant seeds to get a head start on blooming or to harden off tender seedlings that were started indoors. If temperatures are especially low, cold frames can be covered in blankets or an insulating material such as hay to keep the plants warm until the sun reappears.