Cheating the climate gods

Photos by Dan Bostan

The unheated greenhouse under a blanket of snow.

Engineer Dan Bostan says you can grow peaches in Winnipeg and pomegranates in Toronto by using his methods to build a “Plus4Zones” greenhouse. Does it sound too expensive for you? Well get this: it requires no outside energy at all, relying instead on various types of passive heating.

Dan lives in Montreal. Over the past several years, he’s built six different greenhouses in his urban back yard to grow apricots and sweet cherries. Then a couple of years ago he found an old barn with, walls of corrugated metal and plywood, to change into a greenhouse. He shares his experience in a few videos on Youtube, the most complete at the QR code

The unheated greenhouse under a blanket of snow.

His principles are these: Snow is a good insulator. Air is a good insulator. Earth is a good insulator.

To build his first greenhouse in his back yard, he dug down three feet next to the house, and built walls six to eight feet high from the bottom of the excavation. Not one to do things by halves, the total area of the greenhouse was 14 feet along the house by 9 feet. He left the bottom of the walls as exposed earth and above that installed plywood with spray insulation on the inside. Outside the walls he used the earth that was excavated to build up berms.

Inside the greenhouse is an abundance of life.

For a roof, he installed plywood with spray insulation, having run out of time and needing to get a bunch of sweet cherry saplings protected. There was no light, but his cherries survived. Though the temperature dipped to -21 Celsius, inside the greenhouse it did not go below freezing, bottoming out at 1.7 Celsius.

At the beginning of March, he removed the plywood roof and installed a roof of two layers of plastic sheeting separated by two-by-fours. The cherries came out of dormancy with the sunlight, blooming by end of March.

The next winter, Dan kept the double plastic sheeting roof and found the temperature in the greenhouse got as low as -2.9 Celsius. It’s below freezing, but still perfectly acceptable for sweet cherries; I’ll add that it’s also warm enough for figs and even limes. What amazed Dan that year was that after the roof was covered in six inches of snow, the greenhouse was a few degrees warmer than when it was covered with only an inch of snow.

Peaches.

Figs.

Cherries.

Grapes.

In building another greenhouse in his yard, he discovered that vertical gardening on the walls of a greenhouse in summer is an excellent insulator for the building in the winter. Vegetables like lettuce, spinach and radishes will grow quite happily on a south-, east- or west-facing wall. The activity of the microorganisms in the soil keeps things warmer through the winter and through the growing season, you can really increase your planting space.

When he came upon the barn that would become his biggest greenhouse, it had a metal roof and a concrete floor. The first year, he had to remove the concrete floor and bring in compost to plant his trees before winter. The trees spent the winter in the ground under the metal barn roof. In May, he removed the metal roof leaving only the rafters throughout the growing season. The trees bloomed and produced fruit, and for the next winter, Dan covered the roof with plastic sheeting and then installed a ceiling of plastic sheeting below that. He also installed shelves on the inside walls to enable planting and filled them with compost and soil. These would capture the sun’s heat during the day and keep the greenhouse warmer at night.

Dan has found he is able to raise his USDA Zone 4 garden to Zone 8, which is Canadian Zone 5 to Zone 9 (roughly speaking). I’ve used the USDA Zones because that’s what Dan uses and because they only take temperature into account. Canadian hardiness ratings take snow cover and length of time the coldest temperatures occur into account; nevertheless, adding 1 to the USDA Zone gives a rough equivalent.

He has been able to grow sweet cherries, apricots, figs and peaches in the greenhouse and managed to do it all organically.