What Happens to My Compost Heap in Winter?

Composting does not stop completely in winter, but it slows dramatically as temperatures fall. The bacteria and other microorganisms responsible for decomposition are most active between 20–60°C, and their activity falls off sharply below 10°C. In a hard frost, the heap may become frozen and apparently inactive entirely. This is normal — the material is not harmed, and composting resumes as temperatures rise in spring. Understanding what to do (and what not to do) over winter prevents problems and sets the heap up for a productive spring restart.

Keep adding material through winter

Even though decomposition is slow, keep adding kitchen waste and soft garden material to the heap through winter. The material will sit largely unchanged until spring but accumulates to give the heap a large, nitrogen-rich mass that heats up quickly when temperatures rise. Do not abandon the heap in winter — material added over the cold months contributes to a productive spring flush of decomposition. Continue to layer greens with brown material to maintain a good structure.

Managing excess moisture

Winter rain can waterlog open compost heaps, driving out the air that aerobic composting needs. A saturated, airless heap develops unpleasant anaerobic conditions and smells strongly. Cover the heap with something waterproof — a piece of old carpet, cardboard topped with a tarpaulin, or a fitted cover — to keep rain off while still allowing air to reach the sides and base. If the heap is already very wet, add dry brown materials (cardboard, dry straw) to absorb excess moisture.

Spring restart

As temperatures rise above 10°C in early spring, composting activity resumes spontaneously. Speed the restart by removing the cover, turning the heap to reintroduce air, and adding a nitrogen-rich activator if needed — a shovel of compost activator, a bucket of nettles, or a generous layer of fresh grass clippings. Within two to three weeks of a spring turning, an active heap will be noticeably warm at the centre and well on its way to producing usable compost.