How to Build a Hot Bed or Cold Frame

Vegetable gardening is addictive and once bitten, many gardeners look for ways to expand their gardening enterprises beyond the traditional growing season. Enter the hot bed or cold frame. Through these simple structures, gardeners are able to propagate vegetable plants from seed and even grow cool-season vegetables almost year-round.

Easy Hot Bed and Cold Frame Construction

Hot beds and cold frames are identical in construction; the only difference is that hot beds are heated and cold frames are not. Hot beds are generally used to start seeds. Cold frames protect young plants and harden them off until they can be transferred to a vegetable garden. Both hot beds and cold frames are simply a sloped frame that is deeper at the back than the front. To make a basic hot bed/cold frame, you’ll need:

• Materials for the frame, such as rot-resistant lumber, bricks or masonry block.
• Glass, plexiglass or polyethylene plastic film in 4 to 6 ml weight. An old window sash can also be used.
• Drainage materials, including coarse gravel and sand.
• A heat source (for hot beds) — typically electric heating cables — although manure or light bulbs have also been used
• Potting soil or a pasteurized garden soil

Location of Hot Bed or Cold Frame

First, decide where to place the structure. Choose a site that receives southern or western exposure, plenty of sunshine, and protection from the wind. The site must also be near a water source (and an electrical outlet if you’re using electric heat). Decide how large you want it, based on your ambitions, available space and budget. Most home gardeners find that a bed 3 feet wide by 6 feet long is ideal. If you make the bed too wide, it is difficult to access the plants. It is generally 18 inches tall in the back, sloping to 12 inches tall in the front.

Thermostat Controls for Hot Beds

Electric heating cables are the simplest solution. Most systems come with a thermostat to control the heat. To germinate seeds, set the thermostat at 75 degrees Fahrenheit. Once seeds emerge, set the thermostat based on the needs of the plants. Cables also come in different wattages. In cold climates, you’ll need to space your cables to provide at least 12 watts per square foot.

Seal Out Weather for Hot Beds and Cold Frames

Glass or plexiglass panels are more expensive to install, but are long-lasting and provide the best insulation from harsh winter weather. Install them with a latch and a system to prop them open for ventilation. Plastic films are inexpensive and can be simply installed, but lose heat rapidly and must be replaced yearly. Use staples to hold plastic films securely in place.

Tips for Using Hot Beds

• Set the thermostat at 65 degrees in the winter to grow cool season greens and root vegetables. Raise the temperature to 75 degrees when germinating seeds. To convert the hot bed to a cold frame, simply turn off the heat.
• Don’t sow seeds too early. Although the hot bed will keep them toasty warm, they should be transplanted when they are 3 to 4 inches high. If you plant seeds too early, they’ll become tall and leggy before the garden soil warms enough to transplant them. Six to eight weeks before the last expected frost is usually a good time to start seeds.
• Sow seeds in rows and label the rows with the type of plant. Sowing in rows keeps the plants neat and simplifies transplanting.
• Water as needed to keep the soil evenly moist, but not soggy. Seeds will not germinate if the soil dries out; they may rot if the soil is too wet.

Want to learn more about building a hot bed and cold frame?

Building and Using Hotbeds and Coldframes from University of Missouri Extension
Cold Frames & Hotbeds from Cornell University Cooperative Extension