The cost of heating a greenhouse in the north is probably the biggest reason that most gardeners living in a cold climate don’t have one. There are ways around that issue, like passive heat walls and below ground greenhouses, but not everyone has a hill they can use as the insulation needed to block the northern wind chill. Then there’s the problem of snow accumulation that could cover the top of a sunken greenhouse design, which is great for places that don’t get lots of snowfall but do experience temperature drops that garden plants won’t grow in.
The least expensive approach will naturally be black barrels full of water that maintain warmer temperatures inside thanks to the sun, however, not everyone has tons of sunshine in winter, and there are more reliable ways to heat a greenhouse without resorting to giving the electric or gas company all or your money. There is something called geothermal heat, but not quite like the system needed to heat and cool a house, this is Low-Grade Geothermal. It’s a lot more simplified and has proven to work very well. And you also have the options of boiler heat using an outdoor wood or corn burning furnace. Corn is not as sustainable as wood if you live in a place with plentiful forest, because bugs kill trees in any woodland, and burning dead timber for heat is not just affordable – it’s downright cheap.
In an urban situation, it’s likely you will not want to use the outdoor furnace option, but that’s okay, you won’t face neighborhood regulation or zoning issues if you go the geothermal route.
Here’s Dr. Nate Storey of Bright AgroTech explaining how they heat their aquaponics tanks and 5,000 plant greenhouse with wood and water throughout frigid Wyoming winters:
One thing to note about geothermal heat, is the severity of the cold, annual average snowfall, and the length of the cold season where you are. There’s a farmer in Nebraska who raises citrus fruit in his geothermal greenhouse with the growing space being below ground level. I believe it is only partially submerged, but the images available a year ago or so have disappeared. Probably because now he’s selling his geothermal report. Now, this form of heat alone will limit your crop selection to cool weather plants, so growing things like tomatoes, peppers, and other hot summer crops will require heat assistance in the coldest months – and probably grow lights too to boost sun energy when the sun is weak in mid-winter. Here’s a tour of the Citrus In The Snow greenhouse in Nebraska:
Heat assistance to raise temps 20 degrees or so at night, and even give what you’re growing warmer day temps will be vastly less expensive with a geothermal system in place. Depending on the cold season averages where you live, it might be as simple as adding black barrels of sun-warmed water or going further below ground with your air exchange system. Plus you have the added bonus of geothermal cooling in the greenhouse in summer, making your hoop house more of a year around gardening space – in the new summer climate of the north, this could be a necessity if things continue as they’ve been the past several years.