Earth-sheltered Greenhouse
(Page 2 of 3)
February/March 2004
By Mike Oehler
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.
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But the lower the growing bed was, the more shade the south earthen wall created a definite problem.
One solution was to create a path on the south side from which to work on the plants, but that side was where the glass was the lowest.
To work on the plants with any comfort, I had to utilize the area along the north wall, which was prime growing space. I was losing growing area on the south because of the shade and on the north because of the path. Hmm. Finally, I hit on a plan to dig a trench on the south side deep enough to walk in while working on the plants in the growing bed. That would eliminate the path on the north side and make use of the shaded area on the south. It would also eliminate working in a bent-over position. By designing the grow-bed so it was only 3 feet across, I could reach all the plants comfortably without having to kneel.
So, that was the greenhouse I built. I didn't realize at the time I'd stumbled across the "cold-sink" concept.
The cold-sink is the walkway trench on the south side of the growing bed. Now, instead of the coldest air in the greenhouse sinking and settling around the plants, it spills over and down into the trench, where the Earth warms it.
I heard of a Massachusetts man who built an earth-sheltered greenhouse similar to mine but without the cold-sink. To keep the cold air from settling on his plants, he installed a fan to keep the air moving. But a fan takes energy, can fail and does not warm the air. The cold-sink will never fail, allows the air around the plants to stay heated and uses absolutely free, passive energy.
I decided to glaze my greenhouse with fiberglass rather than regular glass. Fiberglass is more flexible with a snow load, and it was the most readily available material at the time. It is also easy to handle. This earth-sheltered greenhouse doubled my growing season and would have done much better had it not been in the shade of a hill, which blocked the sun November through February.
Rather than fight the shade, I built another greenhouse on the south slope of the hill, one so successful it has tripled the length of my growing season. I often harvest tomatoes into the second week of December, and sometimes hardy kale and cabbage through the winter. While those plants aren't growing during the cold, they are living and harvestable. And some of them will send up new shoots in late February, when springtime comes to the greenhouses' interiors. A diagram of the hillside structure is shown on Page 82.
