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Get A Jump On Spring Planting — Spring 2009 | Out Here Magazine

Cold frames and hotbeds help extend your garden's growing

By Christopher J. Starbuck
Illustration by Tom Milner

Cold frames and hotbeds — a heated cold frame — are simply a protected plant bed that can help get your garden started much earlier than nature allows.

A cold frame has no artificial heat added. The temperature difference between inside and outside the frame is generally not more than 5 to 10 degrees. There are times, however, when a few degrees can be very important.

A cold frame is used to provide shelter for tender perennials, to "harden off" seedling plants or to start cold-tolerant plants such as pansies, cabbage, or lettuce earlier than they can be started in open soil.

A hotbed is, in many ways, a miniature greenhouse. It is most often used to give an early start to warm-season vegetables such as tomato, pepper, or melon.

Location. Hotbeds and cold frames should have a southern exposure to receive maximum sunlight. To reduce the cost of heating, use a north or northwest windbreak — a building, bales of hay or straw, tight board fence, or hedge.

Excellent drainage is essential to keep water out during heavy rains. If natural drainage is not good, use drainage tile or a thick layer of coarse gravel.

Constructing the bed. Basically, the hotbed or cold frame is a rectangular box with the back higher than the front, covered with a transparent roof. Often scrap lumber and old window sash may be used, thereby reducing costs.

Sash and coverings. The sash available will determine the bed's dimensions. Glass sash is the conventional frame covering and generally the best.

Used window sash is satisfactory and often can be obtained at little or no cost. Adjust dimensions of the cold frame to fit the sash. Remember, whatever the size, the slope from back to front should be about 1 inch per foot.

If glass sash is not available or too expensive, the frame may be covered with fiberglass or clear polyethylene (4 or 6 mil) stretched on wooden frames.

Materials. The sides of the structure may be wood, brick, masonry blocks, concrete, or metal. Metal must be well insulated, or heat loss is high.

The average hotbed or cold frame is constructed of wood. It is easy to work with and more temporary, but also more flexible if the bed needs to be enlarged or removed. Lumber should be treated with a preservative nontoxic to plants. Don't use wood treated with creosote or pentachlorophenol; accumulation of fumes from these materials in a closed frame can cause plant damage.

Preparing the bed area. The area for the bed must be leveled. Although temporary frames may be set on the soil surface, excavation is required for more permanent structures. Most home hotbeds are heated with electric coils, but where fresh manure is available, it may be used. A deeper excavation is generally required for establishing a manure-heated bed.

For electric heating, and where drainage material is required, excavate to a depth of about 14 inches.

Dr. Christopher J. Starbuck is in the Division of Plant Sciences at the University of Missouri Extension, from which this was reprinted, in part, with permission.