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I'm fiddling with a weird idea... Let's say I built a two shelf planter: concrete. Bottom shelf is a side-opening compost bin. Top shelf holds the base of an avocado tree. Three sides of this top shelf are bermed into the rest of the garden; one side stays flat so I can get compost in there and turn said compost.

Think this could keep a tree warm enough to survive in winter? What problems am I failing to consider?

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    See: gardening.stackexchange.com/q/14651/6806 - how hot it gets depends mostly on the size of the pile. The outside freezes, but if the pile is large enough the inside gets as hot as compost gets. Based on the large pile of hot horse manure, I doubt your avocado will survive standing on top, because the outer foot or so of the pile froze solid; so I expect a tree sitting on top would freeze solid too.
    – Ecnerwal
    Commented Apr 29 at 2:42
  • I do recall an article about someone who used huge compost piles with tubing run through them to hydronically heat their house.
    – Ecnerwal
    Commented Apr 29 at 2:50
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    Compost that hasn't been composted yet, especially if it contains manure, is not in a state (chemically) that I would want that close to my tree roots. Commented Apr 29 at 10:56

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I love weird ideas!

The good folks at Cornell Composting say:

A well-designed indoor compost system, >10 gallons in volume, will heat up to 40-50°C in two to three days. ...commercial or municipal scale compost systems may take three to five days to heat up and reach temperatures of 60-70°C.

Here they have a very nice graph of a compost pile's temperature over time. They marked where they turned the pile with an arrow.

A graph of a compost pile's temperature over 120 days, starting with a sharp climb to almost 140 degrees F and slowly declining, with a jump after each of five turning events

But more than temperature, you need energy. Looking at Energy from Waste: Reuse of Compost Heat as a Source of Renewable Energy, they tell us:

A recent study reports that during high-temperature phases (∼60°C) of municipal waste composting, on average 1,136 kJ/kg of heat was released [10]. Similar values (961 kJ/kg) have been reported earlier with an average compost moisture content of 52.7%[11]. Heat produced during the composting of wheat straw and poultry droppings was approximately 17.06 MJ kJ/kg [12] and 12.8 MJ kJ/kg [13], respectively.

For comparison, 1 gallon of gasoline (3.8kg) holds 131.8 MJ. You would need about 8kg wheat straw compost (that's all? wow!) or 116kg 'average municipal compost' to provide that same heat energy. From the other direction, 250kg of compost gives you as much energy as 2.2-32 gallons of gasoline, depending on what's in it. Can you keep your tree warm on that?

So yes, your compost pile will be a source of heat. If you size and insulate it appropriately, tent over the tree when needed, and commit to actively managing/turning/aerating the pile in response to cold spells, I could see it working for mild cases. Awful lot of work for a little bit of captured heat. But that's the charm of it.


Their citations:

[10] E. Klejment and M. Rosi ´nski, “Testing of thermal propertiesof compost from municipal waste with a view to using itas a renewable, low temperature heat source,” BioresourceTechnology, vol. 99, no. 18, pp. 8850–8855, 2008.

[11] N. Guljajew and M. Szapiro, “Determining of heat energy vol-ume released by waste during biothermal disposal,” in SbornikNaucznych Robot, pp. 135–141, Akademija KommunalnowoChozjajstwa, Moskow, Russia, 1962.

[12] A. Stainforth, Cereal Straw, Clarendon, Oxford, UK, 1979.

[13] A. T. Sobel and R. E. Muck, “Energy in animal manures,”Energy in Agriculture, vol. 2, pp. 161–176, 1983.

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