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Why Cold Frames And Hot Beds Are A Useful Addition To Your Greenhouse

Dec 6, 2007
Cold Frames:

A cold frame is an outdoor growing "area" built without a bottom but with a solid-sided frame of wood, cement or brick, and a removable hinged top, glazed with glass, Fiberglas, or plastic. Cold frames are invaluable. For instance, they take some of the spring bulge from a greenhouse. By using them for growing greenhouse-started annuals and perennials, you make under-glass room for a new crop of salable plants.

Then there are plants such as delphiniums, pansies, and Oriental poppies, to be planted in the frame in late summer and kept there over winter. The cold frame makes an excellent "cold-42 conditioning" rooting area for the spring-flowering bulbs you wish to force.

You can purchase material and build your own cold frame, buy ready-fitted supplies from a greenhouse dealer and assemble it, or you can buy a ready-made cold frame of wood or aluminum with plastic "lights."

How to Build a Cold Frame

The frame should face south. If you are going to have but one frame you might want to attach it to your south greenhouse wall. If you plan on a number of frames, build them in rows either free-standing in the garden or attached to the greenhouse, garage, or other building.

In cold-winter areas the frames should be provided with a cover of matting, either the roll-up kind or straw mats. Wooden slats, cheesecloth, and shading paint compounds help protect plants in the frame from summer sun.

Standard-sized sash for use on the frame come 3 by 6 feet. If you purchase this, you will have to govern the width and length of your frame accordingly. However, you can use any kind of window frame, and with so many home owners converting wooden window frames to aluminum, you may be able to get wooden storm sash for little or no cost from almost any window or wrecking company, or through a want ad in your local paper. It may be easiest for you to obtain the sash and then construct the frame around it.

Here's how we built our cold frame. For the back we used the cement wall of our garage. The frame is 18 inches high in the back, sloping to 8 inches in front, to allow water to run off. Lumber, 2 by 12 inches, 14 feet, forms the front. The sides are 28 inches long.

The lights (three storm sash) are hinged on a 2 by 4 wooden strip which is nailed to the garage wall.
If you live in a cold climate and plan on using the cold frame for year-round growing, build it on a concrete or brick foundation which extends below the frost line. In my area the building code specifies that the frost line is 42 inches deep.

On sunny days, even in midwinter, you'll have to be careful about ventilation. Heat can build up rapidly in the confinement of a cold frame and "cook" the plants. A notched stick will make it easy to raise the sash cover as needed.


A hotbed, obvious as it may sound, is basically a cold frame with heat. While cold frames receive all of their heat directly from the sun, hotbeds are heated with electric soil cables, stable manure or steam, or hot water heated with flues. The hotbed can be used earlier in the spring and later in fall and early winter than the cold frame.

Hotbeds are constructed just the same as cold frames, with a slope to the south to admit heat from the sun and to allow water or snow to run off. Plants growing in these frames are protected on cold spring nights with the same kind of mats suggested for cold frames.

Hotbeds are usually built to be permanent structures, with the frame of wood, concrete, or brick extending into the ground below the frost line. As with the cold frame, you can build it yourself, purchase a kit of materials for building it, buy a ready-built one, or have someone construct the entire thing for you.

A soil-heating cable furnishes the simplest kind of heat for the hotbed and these cables come in a variety of sizes and prices. The type used for hotbeds is insulated and enclosed in lead or plastic sheathing. The cables are made in several lengths but the most useful sizes are 40, 60, or 80 feet, all adapted for use with an ordinary electric service of 110 volts.

A 60-foot cable will heat a 6- by 6-foot hotbed. You should reckon your cable to suit your space. Each 60-foot cable carries an electrical load of approximately 400 watts. In our area the cost of operating such a cable on a continuous 24-hour basis is about 1 cent per hour. You should have a thermostat to regulate air temperature and another to regulate soil temperature.

However, you will find that during many hours of the day the sun will heat the hotbed enough so the thermostat shuts off the current. As spring nears, the outdoor temperature rises and the artificial heat will be on for shorter periods of time.

You can conserve heat by making certain that all construction is tight. Bank the sides of the hotbed with earth and check the sash--it should fit tightly. If it doesn't, weather-strip the top of the frame. Make sure that all glazing is well puttied and that it laps J4 inch at joinings. Keep the glass clean to admit maximum light. In my area it is not practical to use a hotbed before March first.

As the spring temperature increases, start ventilating the hotbed by raising the sash a crack. This applies equally to cold frames. From midday until mid-afternoon on warm spring days, you will have to ventilate more. Be sure to close the frame before the temperature falls at night.

Owners of home greenhouses invariably have one problem in common. They do not build them large enough. This is an especially knotty situation for those of us who have profit in mind. If you are in this boat, you will welcome ideas on obtaining more growing space with the use of "auxiliary growing facilities," such as cold frames, hotbeds, and lath houses.
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