Build an Earth-Sheltered, Energy-Efficient Greenhouse

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


| February/March 2004



202-083-01

A friend spreads growing medium in Mike Oehler's original earth-sheltered, energy-efficient greenhouse.


Photo by Mike Oehler

Building an Energy-Efficient Greenhouse

Ninety frost-free days here in the mountains of Idaho close to the Canadian border are 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 and how they prolonged their grow season. "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 "grow-holes." 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.

It seemed logical and practical to raise the angle of the glass and to make the pit tall enough on the north side so I could work on the plants from the inside. I saved the soil from the excavation and mounded it on the north side for insulation. Finally, my plans had evolved from a humble growhole into a modest, earth-sheltered greenhouse.

Unfortunately, this design had weaknesses, too, so I went back to the drawing board. I wanted to dig the planting area inside the earth-sheltered greenhouse as deeply as possible to take advantage of the Earth's warmth: The ground maintains a steady, moderate year-round temperature 8 feet down, and I wanted to get as close to that as possible.

samnjoeysgrama
1/7/2016 12:53:54 PM

My frost level at 8500 ft is 7 ft deep. So insulating the garden with hay or straw bales does nothing. The rules change in the high Rockies. But it's beautiful.


samnjoeysgrama
1/7/2016 12:47:48 PM

Any house financed with an FHA or VA loan must have double glass sliding patio doors replaced if there is steam between the two layers of glass. The window companies that replace these doors have no use for them and they function perfectly for a green house. I can get the used ones for free or for $20 each from Habitat's Restore. Great article! I'm at 8500 ft in Colorado and planning to build one.


elaine walton
5/15/2011 10:28:27 PM

Suggested modifications to design: 1) whole structure is above ground; 2) structure is insulated with double thick straw bales instead of earth; 3)growing surface is sloped or terraced inside structure, instead of level; 4)Water collection barrels are just below rain gutter level, but above level of hose-end sprayer so gravity will discharge water instead of siphoning. Note about points 1) + 2): if you have ever covered your row of carrots and beets in the garden with straw bales for winter use, the warmth from the earth comes into the space and keeps the vegetables from freezing . . because of the insulation, not because of depth in the earth. So the only digging required would be the "heat sink" walkway, and that dirt may be enough to build the sloped growing surface inside. Dig trench to elbow depth (about 40"); put dirt behind short retaining wall, building "steps" (maybe 2 or 3) until high enough, or as high as you can reach.






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