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Composting through winter

Composting is driven by living organisms, and like most of the garden, their activity drops off when temperatures fall. A heap that was steaming and shrinking fast in September can look almost frozen in place by January. That’s expected, not a sign something has gone wrong.

Quick fix

Why it slows down (not stops)

The bacteria and fungi that do most of the work in a compost heap are far less active in cold temperatures, and some effectively go dormant below a certain point. A pile that was hot in the middle over summer will cool right down in winter, and visible breakdown will slow to a crawl. Underneath that quiet surface, though, slower activity is usually still happening, especially toward the centre of a decent-sized pile, which holds onto some of its own heat even in cold weather. Don’t mistake “slow” for “dead” — most heaps come back to life on their own once the weather warms.

Keep adding through winter

It’s tempting to stop adding to a heap that looks like it’s doing nothing, but that’s actually the wrong instinct. Keep adding your usual kitchen scraps and garden trimmings through the colder months. You’re not necessarily going to see them break down quickly, but you’re building up a stock of material that will kick off fast once temperatures rise and the organisms wake back up. If you stop adding material over winter and only start again in spring, you’re delaying that restart by however long it takes to build the pile back up.

Stop turning as often

During the warmer months, turning helps by adding air and mixing material. In winter, though, opening the pile up releases the heat it’s managed to hold onto, which can set it back further. It’s generally better to leave a winter heap mostly undisturbed, turning only occasionally rather than on your usual summer schedule, so the core retains whatever warmth it can generate.

Insulate the outside

A layer of insulation around the outside of the pile — dry leaves, straw, even an old piece of carpet or blanket — helps retain the heat generated inside and buffers the pile against hard frost. This matters more for an open heap than an enclosed bin, since a bin already has some shelter built in, but even bins benefit from a bit of extra insulation in a genuinely cold spell. Just make sure whatever you use isn’t sealing out all air; a loose covering breathes better than something wrapped tight.

Bin versus open heap in the cold

An enclosed bin generally holds heat better than an open heap of the same size, simply because it has walls between the compost and the cold air. If you’re starting a new compost setup and know you get hard winters, a bin is worth considering over an open pile for that reason alone. That said, an open heap isn’t a bad option in winter either — building it a bit larger than you might in summer helps it hold more of its own heat, since a bigger mass loses heat more slowly relative to its size.

Stockpile your browns in autumn

Dry browns — leaves especially — are abundant in autumn and much harder to come by once winter sets in properly. It’s worth setting aside a stash of dry leaves or cardboard in a dry spot (a bag in a shed or garage works fine) before the season turns, so you’ve got browns on hand through winter to balance out the kitchen scraps you’re still adding. Running short on browns in the middle of winter, with wet weather and few dry leaves left on the ground, is a common reason winter heaps turn wet and smelly rather than just quiet.

What to expect in spring

As temperatures rise, most heaps restart on their own: you’ll notice the pile shrinking again, maybe some visible steam on a cool morning, and a return to that familiar earthy smell. Give it a turn once the weather has properly warmed to mix in whatever you added over winter and get air moving through the whole pile again.

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