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Cold Frames "The Magic Box"
Excerpt from Four Season Harvest, by Eliot Coleman
Gardeners should dedicate a monument to the cold frame. It is the simplest, most flexible, and most successful low-tech tool for modifying the garden climate. It's simple because it is basically a box with a glass top and no bottom that sits on the soil. It's flexible because it can be made as long, as wide, or as tall as the gardener wishes. And it's successful because it is a tried-and-true garden aid that has been used in one form or another since ancient times (sheets of mica predated glass). The cold frame was the foundation for the early development of intensive commercial horticulture.
My first experience with cold frames occurred when I was a child, long before I began growing plants. A gardening neighbor had a small cold frame in which she grew hardy flowers for early and late blooms. I can remember going over to her yard every so often to see the "magic box." The drab tones of fall and winter prevailed in the outdoor world, but inside the frame, a riot of bright colors and green leaves existed. It was like looking into a warm, friendly house on a cold, snowy night. I never knew what flowers she grew in there - possibly calendulas and chrysanthemums, strawflowers and anemones - but their beauty stuck in my memory. I experience that same fascination today when I look into the green and growing world of our vegetable cold frames. The magic is created with a single sheet of glass and the careful selection of hardy cultivars.
Comparative investigations of different cold frame designs back in the 1970's showed that the standard old-time model - a bottomless box made of 2" thick planks, 12 inches high at the back and 8 inches high at the front, and covered with glass frames - was still the best. Our cold frames look just like that and would be familiar to a gardener of one hundred years ago.
Since growing in cold frames is nothing new, it's interesting to ponder, as we did so often on our trip to France, why the idea of a winter garden has never caught on in the U.S. For those gardeners without the "tradition" of winter gardening, it may be that the timing involved takes some getting used to. You can't wait till winter to plant the winter garden. Most of the crops must be planted in late summer and early fall. That's because the rate of plant growth diminishes with the shortening days of fall until it almost stops around November 15. If you don't plant till then, it's too late. By then the plants need to be grown to the size for harvesting. They will hibernate successfully in the shelter of the cold frame. A friend has described this process as storing them in a large translucent refrigerator crisper drawer. The vigorous growth that took place in the fall, plus some slow winter regrowth, provides plenty of food to harvest until February, when the days have lengthened enough for serious new growth to begin again.
Occasionally, gardeners have told us that by fall they are just tired of gardening and glad for a break. But that is the best part. You can have your break and eat it too. There is little gardening - soil preparation, sowing, cultivation, etcetera - taking place after late summer and early fall. Furthermore, the weeds aren't growing and the pests aren't pestering in the winter garden. Just some watering of the frames until November and then no more of that until spring. It is necessary to vent the frames to prevent overheating in fall and spring, but ingenious, temperature-activated ventilating arms can be installed to look after that for you. We aren't gardening in the winter, just harvesting, which in the final analysis is the aim of all this activity.
Back when I started with winter gardening, I soon realized that success lay not in technology, but in biology - in the selection of the crops. Surprisingly, the successful crops, many of which may seem exotic to gardeners today, would also have been familiar to gardeners of one hundred years ago, since Fearing Burr's 1863 classic, Field and Garden Vegetables of America, describes all of them and gives cultural directions. I didn't know which of the cool-season crops would work best at first so I began by experimenting. Nothing scientific. I just planted selected crops and observed how long they could be harvest. The results were judged at the dinner table.
How the Cold Frame Works
The cold frame lessens climatic stress in a number of ways:
Temperature: A single layer of glass creates a micro-climate in which the nighttime temperature inside the frame can be as much as 20° F. warmer than the temperature outside, although the average difference is 7° to 10° F. The daytime temperature inside the frame, even on a cloudy, early spring day, will be 10° to 15° F. warmer than outdoors. On a sunny spring day, the temperature can rise high enough to cook the soil and the plants if you don't vent off the extra heat. Both daytime and nighttime temperature differences depend on the time of year, the angle and intensity of the sun, the rate of outdoor temperature change, and the initial temperature in the frame.
Moisture: Much of the havoc that freezing can wreak on winter vegetables is a function of how wet the plants are. High humidity helps protect plants from cold but plants sitting in a puddled soil just soaked by a rain before freezing will be more stressed than one that is drier. The glass roof of the cold frame protects the crops inside from pounding winter rains.
Wind: The wind can make a cool day feel very cold. Weather forecasters always mention the windchill factor. The same conditions affect plants. Wind cools by removing ambient heat and evaporating moisture. The stress of winter wind alone can mean the difference between life and death of hardy vegetables. Even the slightest windbreak will help. That was proven by two beds of spinach planted a few Septembers ago to winter over outdoors. One was covered lightly with a mulch of pine boughs and the other left uncovered. Even though you could look through the thin layer of pine boughs and clearly see the spinach, that minimal amount of wind protection was significant. Ninety percent of the protected spinach survived the winter, compared to ten percent of the unprotected crop.
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