Firewood Greenhouse Heat

I built a 12'x16' greenhouse in 2019. I figured the main expense would be heating over the winter. I decided to insulate the building as much as possible, and calculated it would take around 400 pounds of propane per heating season to keep the building from freezing. That was actually pretty close to my real world experience.

It takes about 400-500 pounds (a little less than 100 gallons) of propane to make it through the whole winter. 400 pounds of propane is somewhere around $300-400. That's not too bad, but that's just enough heat to keep the building 53F, which is too cold to be productive.

It doesn't take much effort at all to get $400 of value out of the greenhouse. This year, for example, I'm doing blueberry clones. I'll have 30 just this year. If I bought those, it'd probably be about $200 paying retail prices. If they grow over a couple years, and I sold them I'd probably get around $1000. On top of that, I've got 36 roma tomato plants, and a shitload of peppers. This year there will probably be around $1000 worth of plants that go in the garden from the greenhouse. It's essentially a passive income producer--well not quite passive. Maybe I spent 10 minutes a week planting things, and a few minutes a day watering them. I don't monetize any of that production, though. It's just a hobby at this stage.

Most of the plants really only start germinating and growing in mid-February though, like when the Robins return, which is really pretty interesting. (Actually right now the redwing blackbirds are back, the robins are back, and the raccoons and chipmunks are out of their torpor.) Only Kale grows in the cold, and even that grows too slowly. Once February rolls around the building gets warmed by the sun, even on an overcast day.

It's really not worth planting stuff in December/January if it's only going to be 53F in the building all the time. If it were warmer, they'd grow using artificial light.

Many commercial greenhouses use very low cost fuels for heat. Heating is probably the primary cost of operating a greenhouse in a cold climate. Wood heat can be very cheap. Some greenhouses have enormous wood-chip boiler facilities that heat a large complex, for example. Woodchips from tree maintenance are a commodity that's marginally useful and are very cheap--they're even cheaper than firewood. Large scale heating with wood is like having your own mini sun. Since the wood is cheap fuel, it makes sense to build a less insulated cheaper greenhouse, too, and it's plausible to expand to a larger area.

I'm in a transition from doing stuff like operating a greenhouse as a hobby to trying to do it commercially and I'm seeing it's plausible with the resources I have at hand. My 12x16 greenhouse cost maybe $5000 in materials. It's really more of a lab than a thing for doing "production". Ironically, a hoop-house type building will maybe be $1000, for a much larger area, like maybe 10x32 or larger. I'll probably build two smaller ones this season to get my feet wet. Then I'll add a wood fired heater before next winter. It seems fairly plausible that I can generate some passive income from just some pretty simple and cheap infrastructure.