Cold Frames - Mini Greenhouses for New Seedlings

Notes From the Gardener

Richard Reading The Seed Catalogue
Richard Reading The Seed Catalogue


In this climate of ours where winters linger and springs are short, many gardeners employ cold frames to get a jump on the growing season. Cold frames are mini-greenhouses used to start new seedlings, or protect them from the cold of early spring. In fact, they are sometimes called "poor man's greenhouse" because they are cheap and simply constructed.

A cold frame can be something as simple as a wooden box with a sheet of glass placed over it, but most are constructed of four boards nailed together into a rectangle. To maximize the exposure to the sun, cold frames should be located in a southern exposure, preferably on a slope. Covers can be made of glass or plastic, but must shut tightly to protect young plants from freezing temperatures. Depending on the severity of the cold additional covering might be necessary, such as blankets, tar paper or pine needles.

Cold Frames

If you really want to get a jump on the season you can add a heat source to the cold frame. Here are three suggestions:

1.You can build your cold frame next to your house, adjacent to a cellar window. On very cold nights all that will be necessary to do is open the cellar window to allow heat from the house into the covered cold frame.

2.You can also deliver heat to a cold frame by stringing incandescent light bulbs into the interior. The number of bulbs you use and their wattage depends on the size of the cold frame. I have used 40 watt bulbs and found them sufficient.

3.Another method I have had success with, but requires more labor, is using the natural heat given off by composting manure. First, dig a pit 18" deep where the cold frame is to be located. Then fill this with moistened, fresh stable manure if available. After a few days turn the manure, moisten it if necessary, and cover with a few inches of soil. Locate the cold frame over the compost and heat from below will keep the soil and plants warm.

Cold frames must be watched carefully for cold is not the only danger. On a sunny day the heat in a closed cold frame can shrivel up tender seedlings in a very short time. So you must be prepared to remove or prop open cold frame covers on a sunny day and even provide shade using netting or cheesecloth if necessary.