Build an Earth-Sheltered, Energy-Efficient Greenhouse

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


| February/March 2004


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.

CarolAnn
1/19/2018 10:24:33 AM

Great Article! I used to live in Troy, Mt.and now live in Hot Springs. We built an earth sheltered home that works perfectly and I'm ready to build an earth sheltered greenhouse. Would it be possible to come visit and talk pros and cons. etc. Csharp


CarolAnn
1/19/2018 10:24:31 AM

Great article! I used to live in Troy, Mt. and now live in Hot Springs. We built a SE facing earth sheltered home and love it. But I would like to excavate an area for a green house next to it. Would it be possible to visit you and see your greenhouse? CSharp


geodejerry
12/9/2017 12:53:56 AM

Thanks for your nice design, it seems very practical & well-thought-out to me. I'd do the same, but our soil depth here won't dig that deep -- our soil is a pocket to the east of the lave ridge where we built our house. We've done various things with raised beds sitting on decomposing straw bales, and our black 55-gallon drums now have several goldfish swimming in them. We created a design for concentrating solar collectors just behind the south glazing, they only obscure 20% or so of the area so we don't miss much. The circulating pump uses only 75 Watts, and runs only when the sun warms up its thermostat. Right now (December 2017) the fish are hibernating down in the bottoms of the tanks, but we look for their return as soon as the sun can warm up the water again. Eventual mixed fish/greens cultivation, is our plan. If you care to converse further, please email me at geodejerry@gmail.com.






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