Earth-sheltered Greenhouse

Enlarge Image Image Gallery Diane A. Rader

Issue # 202 February/March 2004

Garden & Yard

Triple the length of your growing season with this simple, energy-efficient design.

By Mike Oehler

Ninety frost-free dayshere in the mountains of Idaho close to the Canadian borderare not enough to raise most vegetable crops. For several years, I watched in frustration as my tomato and corn crops succumbed to frost in early September. Even the hardy Swiss chard and cabbage would call it quits in October. I asked my old-time neighbors what they were doing about the problem. "Plant root crops," they told me. "Potatoes and carrots. Put 'em in a root cellar and they'll keep all winter."

I tried that and it did work. I found other "keepers," too, like apples and squash, onions and garlic. The cook at my favorite restaurant astonished me by keeping cabbage fresh for months by pulling it up and hanging it upside down by the roots in her own root cellar. But these weren't fresh-picked foods. I wanted fresh, organic greens in seasons other than summer.

In past centuries in early spring, wise farmers would start their garden vegetables in 3-foot deep pits known as "growholes." These pits were filled with three layers of organic material: The lowest layer was a foot of fresh horse manure; the second, a foot of topsoil; and the third, a foot of growing space for the vegetables planted in the soil. The grow-pit was traditionally covered with old storm windows or other wood-framed glass. The glass trapped both the sun's heat and the heat rising from the decomposing horse manure. The soil around the pit acted as a passive heat-sink for the sun's energy, absorbing it during the day and releasing it at night.

I built a small grow-hole, and it did work. I got a jump on my traditional garden, increasing the growing season from three to four months. It was a good start, but some weaknesses inherent in this method were apparent. First, to water or weed the plants, I had to open the pit and expose them to the air. Most plants will not grow in temperatures lower than 40 degrees; so, on a 30-degree day I was destroying the cozy, sun-baked 50- or 60-degree environment, shocking the small starts. Second, having the glass lay flat on top of the pit bothered me. When the sun hit the glass at a low angle, such as we have in early spring, much of the radiant heat bounced off like a stone skipping over water.