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Missouri Environment & Garden


David Trinklein
University of Missouri
Plant Science & Technology
(573) 882-9631

Season Extenders: Cold Frames and Hotbeds

David Trinklein
University of Missouri
(573) 882-9631

Published: November 1, 2010

The end of the growing season is rather somber event for those of us who are avid gardeners. The thought of having to endure a long, cold winter before we can again work with plants outdoors is enough to dampen the spirit of even the most optimistic amongst us. There is a way, however, to “squeeze” a bit more gardening into the calendar year. It involves the use of “season extenders” such as cold frames and hotbeds to allow us to gain as much productivity and enjoyment out of growing plants as possible. Properly designed and operated, season extenders can allow for the production of cool season vegetables (e.g. lettuce and spinach) throughout the winter and make gardening in Missouri a 12 month activity.

A cold frame is a simplistic plant growth structure designed to protect tender plants during the winter, harden started plants before setting them outdoors or start cool-season plants earlier than they can be started without protection. Cold frames take advantage of solar heat for warmth. Soil and other materials in the cold frame absorb heat energy during the day; the protective covering and insulated sides of the cold frame help to retain this energy at night. A hotbed simply is a cold frame supplied with additional heat energy, usually from electric heating cables. In many ways hotbeds are miniature greenhouses that provide many of the same benefits but at minimal expense. They are most often used to give an early start to warm-season annual and vegetables but can be used for other purposes such as overwintering tender perennials.

Because they rely heavily on solar input, cold frames and hotbeds should have a southern exposure to maximize sunlight. They often are dug into a south facing slope to take advantage of the insulative nature of soil against the northern wall. The location selected should have good natural drainage so there will not be unwanted water in the structure. If the cold frame is dug into the ground, excellent drainage is necessary to keep water from accumulating. This can be accomplished through the use of perforated plastic drainage pipe located under a thick layer of gravel or sand. Finally, the cold frame should be located where it will be easy to attend since ventilation is accomplished manually.

Cold frames are usually rectangular wooden frames constructed with sidewalls between 8 and 16 inches in height and covered with glazing materials such as glass, fiberglass, acrylic, polycarbonate or plastic film. The size of the structure often depends on materials and funds available for construction. Wood used should naturally resist decay (e.g. cypress or redwood) or be treated to resist decay. ACQtreated wood currently is the most desirable type; avoid creosote or penta-treated wood since these preservatives are toxic to plants. The use of bricks, cinder blocks or poured concrete can make the structure more permanent but adds significantly to the cost of construction.

Glass sash is the traditional glazing material used to cover cold frames although the newer twin-walled, rigid plastics are gaining popularity because of their insulative nature and light weight. Standard glass sash manufactured for cold frame use is 3 by 6 feet; this limits the cold frame to be about 6 feet wide and any dimension in length that is divisible by 3. Surplus or salvage windows are an inexpensive alternative but the dimensions of the cold frame will have to be adjusted to accommodate the dimensions of the window. Covering frames can be constructed that substitute fiberglass, twin-walled acrylic, twin-walled polycarbonate or polyethylene film for glass. Such frames are light in weight and must be secured somehow to prevent winds from lifting them during storms. Glass substitutes tend to yellow with age and will need to be replaced in time, depending on their summer storage. This is especially true of polyethylene film which has a life expectancy of four years or less.

Hot beds are simply heated cold frames. If the structure is to be operated as a hotbed, supplemental heating will be required. Heating methods include manure, electric cables, light bulbs, hot water and steam. Electric heating cables are probably the most practical choice for most Missouri gardeners due to their availability and ease of installation and operation. Hotbeds in Missouri should be equipped with 12 watts of electricity per square foot of hotbed area. Heating cables vary in wattage rating per linear foot; the two most popular options being 6.7 and 3.5 watts/foot. Each is available in different lengths as well with popular lengths being 6, 12, 24, 36 and 48 feet. To determine the size of cable needed for a particular hotbed, simply divide the total square footage of the hotbed by the wattage rating per foot of the cable to be used. If that length is not available, chose the next longest size. For example, suppose we have a hotbed 6 x12 that will be heated with cable rated at 3.5 watts/ foot. This results in the need for 20.6 feet of cable in which case would choose one 24 feet long. To install, space out the cable on a 2 to 3 inch layer of sand excavated and leveled into the floor of the hotbed. It never hurts to have extra protection for the hotbed ready in case of extremely cold weather. Old blankets, straw or burlap bags are effective ways to provide additional insulation to the hotbed on frigid nights. Bales of hay or straw can be stacked against the sides of the structure to further prevent heat loss.

Both cold frames and hotbeds require proper temperature control, ventilation and watering. Plants can be grown in amended soil covering the floor of the structure given there is adequate drainage. Another alternative is to grow in containers (pots or flats) filled with a soilless medium. In either case, the root zone temperature should be between 70 and 75 degrees for seed germination. Once seeds have germinated the temperature should be adjusted according to the requirements of the species being grown. Basically, one can group most herbaceous plants into one of two temperature categories. Cool-season plants prefer air temperatures in the neighborhood of 65 to 70 degrees during the day and 55 to 60 degrees at night whereas warm-season plants prefer days 5 to 10 degrees warmer. Lower temperatures can be maintained to “harden” cool-season plants before setting them outdoors.

Ventilation is most critical when days are sunny but cool. When ventilation is required, raise the sash on the leeward side of the prevailing wind. This will prevent the introduction of massive amounts of cold air that could damage tender plants. On days when additional cooling is required the sashes can be opened wider or removed totally. A well-placed thermometer is the best guide to determine how much ventilation or cooling is needed. Since cold frames totally rely on solar energy for heat, it is important that their sashes be closed immediately after the danger of excessive heat buildup passes to allow for the storage of solar energy for the ensuing night.

Water is critical for plant growth. Watering should be done early in the day to allow foliage to dry as quickly as possible after watering. Also, avoid opening the sashes widely to water on cold days to avoid plant damage. Fortunately, relatively little water is required while plants are small and outside temperatures are cool. As plants grow and the season becomes warmer more water will be required and the frequency of watering will need to be increased. As a general rule, one should wait until the surface of the growing medium is dry to the touch before watering. Wilting is not a good indicator for the need for water since plants frequently wilt when sunny conditions follow several overcast days even when soil moisture is adequate.

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REVISED: July 27, 2012