Composting in winter
Composting does not stop in winter, but it slows down a great deal. The organisms that break material down are most active in warm conditions, and as the weather cools their work slows until, in a hard freeze, it pauses almost completely. That is not a problem to fix. A heap that sits quietly through the cold months picks up again in spring, often with a burst of activity as the weather warms. The aim in winter is simply to keep collecting material in a way that will compost well later, and to avoid the problems that cold, wet weather can cause.
Why heaps slow down
The bacteria that do the fast early work in a heap slow down as their surroundings cool. A large, active heap generates some of its own warmth and can keep working in its centre for a while into the cold season, but a small heap loses heat to the air quickly. Once the material freezes, nothing much happens until it thaws. Worms in a garden heap move deeper or slow down too.
Keep collecting
Kitchen scraps keep arriving all winter, and they are worth keeping. You can go on adding them to the heap, where they will freeze and wait. Freezing actually helps a little, because ice crystals break down the walls of plant cells and make the material easier for organisms to eat once it thaws. Cover each addition with browns, just as you would in summer, so the heap does not become a mat of wet scraps by spring.
If the heap is far from the house and the weather is unpleasant, a lidded bucket by the back door can hold scraps for a week or two between trips. Some gardeners keep a second bin near the house just for winter.
Keep browns ready
Autumn is the season when browns are easy to find. Bag up fallen leaves while they are dry and keep them somewhere they will stay dry, such as a shed or under a cover. You will want them in winter to balance kitchen scraps, and again in spring when grass clippings start. Shredded cardboard from the holiday season is another good source.
Protect the heap from too much water
Winter rain and melting snow can soak an open heap, and a soaked heap loses its air and turns sour. Covering the heap with a lid, a piece of old carpet, a tarpaulin or a thick layer of straw keeps the worst of the water out while still letting air in. In very cold places, a cover also holds in a little of the heap's warmth.
Insulating for a little more activity
If you want a heap to keep working longer into the cold season, size helps most. A heap of a cubic metre or so holds heat far better than a small one. Some gardeners also surround a bin with bags of leaves or bales of straw as insulation. These measures will not keep a heap hot through a deep freeze, but they can stretch the working season a few weeks at each end.
Worm bins in winter
Composting worms are sensitive to cold. A worm bin outdoors in a cold winter needs to come into a shed, garage or other sheltered place, or the worms will slow down and may die. Indoors, they keep working through the winter at normal room temperatures. Feed them a little less if they slow down, and do not let the bin sit next to a heater, since overheating harms them as well.
Turning in winter
There is little point turning a frozen heap, and turning a heap in cold weather lets out what warmth it has. Most gardeners leave the heap alone through the coldest months and give it a thorough turn in early spring, mixing in browns if it has become wet and compacted.
Getting ready for spring
When the weather warms, turn the heap, check its moisture, and add some fresh greens if it is mostly winter scraps and leaves. Compost that was nearly finished in autumn may be ready to use by spring after its winter rest. Sieve it and use it as your beds wake up, and put anything unfinished back into the heap. Our page on using finished compost explains how to tell when it is ready.