Jade McgeeMost gardeners cover their compost heap at the start of winter and consider the job done until spring. This is understandable, but it’s also wasteful. A pile that stays at least partially active through the cold months arrives at the new growing season weeks ahead of where it would be if left completely dormant. On the Highveld, where winter frosts can drop to well below zero, complete freezing is possible, but even a partially active pile is better than one that’s been abandoned entirely.
Why winter slows the compost
Decomposition is driven by microorganisms that slow down significantly in cold conditions. The bacteria responsible for breaking down organic matter are most productive between 15 and 65 degrees Celsius. Below this range they continue working, but more slowly. Below freezing they pause almost completely, though they don’t die, and resume as soon as temperatures lift.
The goal of winter composting is not to replicate a summer pile but to maintain enough biological activity at the core to keep the process moving, however slowly. A few adjustments to how you build, manage and protect the pile make this achievable without a great deal of extra effort.
Build the pile bigger
A larger pile retains heat better than a small one because the decomposition process itself generates warmth, and a bigger mass insulates this heat more effectively against cold outdoor temperatures. Building the pile to at least one and a half to two metres wide and tall before winter begins, or consolidating material from multiple smaller heaps into a single large one, gives the microorganisms the best conditions to maintain activity through colder nights.
Insulate the heap
A compost pile needs protection from the cold in the same way garden plants do. Covering the heap with a thick layer of cardboard, straw, old hessian or a heavy-duty tarpaulin traps the heat generated internally. On the Highveld, where winter nights can be severe, this insulation can keep the core of the heap several degrees warmer than the surrounding air temperature.
Position the heap where it will receive winter sun, particularly morning sun, which allows the pile to warm up from early in the day. A sunny, sheltered position against a north-facing wall is ideal.
Balance your green and brown material
Kitchen scraps continue through winter, providing the green, nitrogen-rich material that feeds the microorganisms. The challenge in winter is sourcing enough brown, carbon-rich material, which is less abundant once autumn leaves have broken down. Stockpile brown material in autumn: dry leaves, torn cardboard boxes, egg cartons, paper bags and dry garden clippings can be stored near the heap in bags or a covered container and added through winter to maintain the balance.
A compost pile that receives only kitchen scraps without carbon to balance it becomes wet, compacted and slow. Layering brown material between additions of green scraps keeps the pile aerated and correctly balanced.
Avoid turning the pile
The instinct to turn compost regularly, which is good practice in summer, works against you in winter. Each turn releases heat and draws in cold air, cooling the pile’s core. In winter, add new material on top in layers and leave the internal structure undisturbed. This keeps the warmth at the core, where the most active decomposition is happening.
What to expect
A winter compost pile won’t produce finished compost as quickly as a summer one, but it will continue to break material down, and a well-managed heap will be noticeably advanced by the time temperatures lift in August. When warmer spring conditions arrive, a single good turning will reinvigorate microbial activity, and the heap will move quickly toward finished compost, ready to apply just as the garden needs it most.
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