Wednesday, November 29, 2006

Building a Cold Frame and a Hot Bed

Hot beds and cold frames are both used for seeding and growing vegetables, flowers, and ornamental pants for your home and garden. Hot beds are used for starting plants from seeds and cold frames temper or harden the new plants to outdoor conditions before they are transplanted. This ensures that they have a good chance to grow outdoors. Actually, the same frame can serve both purposes, the main difference between the two being hot beds use an artificial heat source and cold frames do not.

Traditionally, hot beds and cold frames were built flush with or slightly below ground level, and then covered by glass sash. Today, these frames are built often completely above ground and covered with plastic because film is generally available at cheaper cost than is glass.

Frames and Their Location

Hot beds and cold frames should always be built on well-drained soil that is completely free from flood during the heaviest rains. A location with a southern exposure and good wind protection on the north and west sides is perfect. Also locate your frames close to water and heat sources.

Frame Size

Almost any size frame can be heated with electricity, but most hot beds are 5 to 6 feet wide by 6 to 12 feet long. The proper size depends on planting requirements, plant species, number of plants and their spacing. If a glass sash will be used to cover the frame, then the length of the frame is usually in multiples of 3 feet; however, plastic coverings do not limit dimensions at all, because plastic comes in rolls that can be cut to size.

Construction Materials

Most frames are built with wooden sidewalls, but permanent beds can be made of poured concrete or masonry blocks used as a base. Wooden walls and supports should be painted or treated with copper napthenate, but not creosote or pentachlorophenol, because both are harmful to plants.

Sashes and Covers

Excavate a hot bed area 8 inches deep. After walls are built, apply 6 inches of gravel or coarse sand for drainage. Add a layer of burlap or other coarse material to prevent sand from sifting down. Add a 2-inch layer of sand on which to place heating cables. Two more inches of sand should be applied over the heating cables and the sand covered with 1/2 inch hardware cloth to protect the cables. Next, fit either propagating medium (potting soil, etc) or flats over the hardware cloth.

Build the back or north walls 18 inches above the level at which the heating cable is placed. This is important. Sidewalls usually slope toward the front about 1 inch per foot of width and a 6-foot wide bed will be 12 inches high in front. The footer for concrete or block walls must be placed below the frost line. This is also very important. Nail a 1 x 4 inch board to the outside top edge of the back and side walls. The sashes extend over the edge of the front wall to slough away water.

The boards serve as weather-stripping and reduce heat loss between the s and sashes. Next, bank soil up against the outside walls to prevent air leakage. The sash or plastic-covered frames, can be hinged at the back, and lifted in front and braced open for ventilation.

Above Ground Beds

Above ground units can be of form from an arch, an A-frame, all the way to a to a Quonset design, with the structure made of wood or thin-wall electrical conduit. These frames are inexpensive to build and easy to construct. They are covered with 4-mil thick, clear polyethylene plastic film designed to be rolled down the ridge or up the sides to allow for adequate ventilation.


Although steam, hot water, and manure heat hot beds, most gardeners use electric cables. A thermostat is needed to control the temperature in the bed. Although heating cables operate on 240 or 120 volts, beds of 10 feet or less can be operated on a 120-volt system. One 60-foot long cable is required for a 6 x 6 foot bed and two 60-foot cables are used for a 6 x 12 foot bed. The cables should be arranged carefully in the beds. Cables and thermostats are available from mail order, hardware outlets, and garden supply centers.

Using the Beds and Frames

A soil temperature of 70 to 75 degrees F is required for germinating most seeds. After germination, you need to adjust the temperature to suit the particular plant. Cool-season crops such as lettuce, cabbage, and cauliflower require daytime air temperatures of 60 to 65 degrees F. Warm-season crops like tomatoes, peppers, and melons need an air temperature of 65 to 75 degrees F. Night temperatures are usually 5 to 10 degrees F lower than daytime requirements.
If the air temperature in the bed goes above 85 degrees F, ventilation will be necessary. The beds require ventilation on all mild, sunny days. Electrically heated beds tend to dry the growing medium fast, and attention to watering is mandatory. The soil should be kept moist at all times, but not wet. Remember to apply water in the morning so the plant foliage will dry before evening.

Cold Frames

A cold frame is a great natural way to harden off seedlings started indoors or in a hot bed. By hardening off we mean preparing them for harsher conditions outside beyond the greenhouse. Cold frames collect sunlight and provide protection and ventilation at once, and are considered the halfway houses for young plants before they move out into the garden.

You can start hardening plants as early as four to six weeks before your growing zone is cleared frost-free. A cold frame therefore extends the vegetable growing season. Closed, you can also use it as a part-time greenhouse, and with the top open or removed, as an extra vegetable or flowerbed.

Another, cheaper alternative to building the wood and glass or plastic cold frame is the option to put together a simple, old-fashioned structure from bales of hay and an sturdy old window. Use just four rectangular bales of hay and an unbroken window frame with the glass still intact. Then arrange the hay bales to form a square with a space left in the center that can be covered by laying the window frame flat on top of the hay bales.

The hay will provide insulation while the window frame will let in daylight and sunshine. When it is sunny, it is important to prop the window open with a brick or a rock to provide adequate ventilation, or your plants will cook. Water the plants according to seed instructions.

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