I hit -41C a few days ago, with 3 days into the low -30sC. To grow peaches and sweet cherries, it can’t be below about -30C at the coldest, or the trees simply die of cold. While I can keep the temperatures to no colder than about -25C with a 100 watt light bulb I plug in, even at -40C outside air temperature, I wanted to try WITHOUT any electricity, extension cords, light bulbs, fans, pumps, batteries, solar panels, etc. My teepees are simply a double wrapped insulated tarp over a teepee frame. They are about 8 ft. tall and have a base of about 4ft. by 4ft. Trees on dwarf or semi-dwarf rootstock are used so they can fit into the teepees by bending the branches inside.
The frost in my area decends an average of 7 ft. 10" into the ground by March 1, but the soil under the frost level is +5C. My thinking is "why not drop a geothermal bore hole down about 9 ft, and see if the +5C warmth will rise up into the teepee via the “stack effect”. No fans, no heaters, no electricity, just heat naturally rising up into the teepee via the stack effect law of thermodynamics (warm air is lighter and rises up).
So far it is working well! Here is the data so far from the coldest days:
Coldest Outside Air Temperature Vrs. Coldest Inside Teepee Temperature
-37C (Dec. 21) but inside the teepee -26.2C, so 10.8C warmer
-38C (Dec. 20) but inside the teepee -26.8C so 11.4C warmer
-41C (Dec. 22) but inside the teepee -29.0C so 12.0C warmer
The hole as per the photo is dug with a manual post hole digger with an extension added. It is 8 inches wide by 9 ft. deep. Will this still work for the second half of winter as the frost goes deeper into the ground? I think so, but I will know in about 6 more weeks. Our really cold potential weather, the -40C stuff, is pretty well over by mid February each winter. Its a fun experiment and so far it is working and I expect the sweet cherry tree inside will be fine and bloom and fruit for me this coming spring/summer.
