Is it a feasible/good idea to heat a greenhouse with compost?

When vegetable matter decays, it generates heat. But it also generates methane and CO2. So if I put a big pile of compost in a greenhouse, enough to heat it up to prevent frost in winter, would it release enough methane to risk an explosion, or enough CO2 to poison me when I entered? Would it kill the plants? I am thinking of a 50 m² greenhouse, well insulated, little draft. Or would it never be enough to heat it up anyway?

A compost heap shouldn’t produce appreciable methane gas unless it’s so deep a pile that it doesn’t get turned over sufficiently, thus resulting in anaerobic breakdown.* I doubt CO2 levels sufficient to be harmful would occur unless there’s an enormous amount of compost present in a confined space along with CO2 plants are giving off during the night.

On the other hand, compost is not likely to serve well as a main heating source. This article talks about using containers of manure for supplemental heating - a feasible idea if you don’t mind the fragrance.

*Garden writer Allen Lacy once described problems that arose when he enthusiastically plunged into the making of compost, creating an enormous heap that resulted in a stench sufficient to keep neighbors indoors during nice summer weather.

You want elevated CO2 levels in your greenhouse anyway to increase plant growth. Most commercial greenhouses are heated with natural gas precisely so the emissions can be piped into greenhouses.

Jackmannii: Manure? Yes, I could get some from the farmers nearby. Thanks for the tip!

Yes, I see. But how much would be too much? And how would I know?

Yes, CO2 is beneficial to plants but excessive amounts can be directly toxic to humans.

As mentioned previously, I highly doubt that decaying compost or manure in a greenhouse would be harmful to people due to CO2 production unless there was a shit-ton of it in a confined area.

Side note: I have a DIY CO2-generating setup bubbling gas into a 20-gallon planted aquarium in my bedroom, and am not dead yet.

The sweet spot is 1000 - 1200ppm. You can measure it via a air quality monitor although be warned that many consumer versions are of dubious quality. For a home greenhouse where you’re not obsessed with maximizing that extra 2% of yield, I wouldn’t stress too much about it but just know that a little bit of CO2 enrichment is not a bad thing.

Try searching on “hot beds”. Using decomposing manure as a heat source for growing plants is a very old technique; and current instructions are available.

(That’s about as much as I myself know about it. But just knowing the right search term can be really useful.)

No obsessing (I hope), it’s supposed to be a hobby, just for fun, and for growing in cold Berlin some plants I remember from my youth in Spain. Artichockes, Fig trees, tomatos of course… and some experimenting. I am thinking of pawpaws, which are completely unknown in Europe.

I will, thank you very much.
ETA: Good that I searched “hot beds for plants”, as my search engine suggested. Without the plants the results are… not what I was looking for. Not all of them, anyway. But it seems manure is a better option than compost.

At the “Lost” Gardens of Heligan in Cornwall, they grow pineapples like the Victorians did, using manure as a heat source.

Eight facts about Heligan Pineapples

Everybody likes a good pineapple fact, so here are 8 of our favourites.

  1. Our pineapples are grown in specially designed pits heated by a winter supply of fresh decomposing manure and an emergency back up heater, the heat of these warms the air that enters the pits through vents in the wall.

  2. Although we use horse manure to heat the Pineapple Pits, our pineapples do not taste of manure or urine, they are actually some of the best tasting pineapples outside of the Tropics.

  3. The Queen was gifted the second pineapple ever grown at Heligan. Heligan Gardeners ate the first one just to check that it didn’t taste of manure… which it didn’t.

  4. Prince Charles came to visit the Gardens in 1997 to see the first budding pineapple fruit. Image seen below.

Whoops! – I guess my search was affected by what else I’d recently been looking for; the top hits, at least, were the right sort.

You’ll need plenty of room to have several trees in hopes of getting a male and a female.

FWIW, pawpaws are native to South Carolina (where I live) and they are pretty much unknown here, too, outside of reruns of The Beverly Hillbillies. I’ve never seen the fruit for sale or growing anywhere.

Note that paw paws really aren’t a “tropical” fruit. They need a winter dormancy period, typically with at least 400 chill hours (at temps of 45F or below).* So a warm or even temperate greenhouse probably wouldn’t give you fruit.

I grew paw paws in central Ohio. Tasty fruit, though the custardy consistency (and flavor) isn’t for everyone.

*total chill hours needed varies by variety.

A local community garden put in a greenhouse a decade or so ago, with the goal that it would be completely passively self-sustaining. The temperature, they managed: Even through the depths of a Cleveland winter, it never gets colder than the high 50s Fahrenheit, just from passive solar heating. But it turns out that plant growth in it is very limited, due to shortage of carbon dioxide.

Ironically, one of the sponsors of the project was a local microbrewery. In the brewery building, they routinely have carbon dioxide levels of close to 1%, and if they had built the greenhouse on site, it wouldn’t be too hard to pump some of that carbon-enriched air into the greenhouse.

Thermal mass alternatives
to compost and manure.