The decision to cover a compost heap during the colder months balances the need for heat retention against the risk of poor aeration. While the decomposition process does not halt entirely in winter, it slows considerably as the ambient temperature drops. Maintaining an active pile requires a different approach than summer composting, and the necessity of a cover depends heavily on your local climate and composting goals. The primary challenges involve regulating internal heat, maintaining optimal moisture levels, and ensuring sufficient oxygen flow to the microbes.
Why Insulation Matters
The core of composting is driven by microbial activity, and these organisms are highly sensitive to temperature fluctuations. Below approximately 55°F (13°C), the mesophilic bacteria responsible for much of the breakdown become less active, causing the process to decelerate. To keep decomposition going, a cover acts as a thermal blanket, trapping the metabolic heat generated by the remaining microbes. This insulation prevents the outer layers of the pile from freezing solid, which would otherwise halt the activity completely.
Even in near-freezing conditions, specialized microorganisms called psychrophiles can continue to break down organic matter, preferring temperatures around 55°F. A properly insulated pile helps sustain this activity by keeping the internal temperature high enough for these cold-tolerant bacteria to work. This retention of warmth ensures that decomposition proceeds at a slower pace, rather than entering complete dormancy. A large pile, typically one cubic yard or more, generates enough internal heat to benefit significantly from external insulation.
Controlling Moisture and Oxygen
Covering the compost heap is a direct way to manage the moisture content, which is a common problem in winter due to rain and snow. Excessive water saturates the material, displacing air pockets and leading to anaerobic conditions, which produce foul odors and slow decomposition. The ideal moisture level for composting material is similar to a wrung-out sponge, generally between 50% and 60%. A waterproof cover, such as a tarp, prevents the pile from becoming waterlogged by external precipitation.
However, a cover that seals the pile too tightly can obstruct the necessary exchange of gases, starving the aerobic microbes of oxygen. This balance requires a cover that provides shelter from the elements while allowing for some breathability, or one that is easily removed for periodic aeration. Regularly checking the material is important, as a covered pile can also become too dry from internal heat evaporating moisture. An overly dry pile will see microbial activity slow down, so the cover serves as a tool to regulate water.
Adjusting Winter Composting Habits
Maintaining an active winter pile requires several adjustments to material preparation and management. To give the microbes a head start, all new organic materials should be chopped into smaller pieces, ideally less than two inches in size, to increase the surface area available for decomposition. It is also helpful to add material with a high carbon-to-nitrogen ratio, such as dry straw or leaves, around the edges of the pile to provide additional insulation and structure.
When adding new nitrogen-rich “green” materials like kitchen scraps, bury them deep into the center of the pile to fuel microbial activity directly where the heat is concentrated. This practice helps quickly recharge the pile’s internal temperature after adding cold material. Turning the compost, which is done frequently in summer to introduce oxygen, should be significantly reduced in winter to minimize heat loss. Instead of weekly turning, an active winter pile should only be aerated every one to two weeks, or when a thermometer indicates the temperature is dropping significantly.
When a turn is necessary, it should be done quickly to conserve the trapped warmth, and fresh green material should be added simultaneously to reignite the decomposition process. The physical location of the heap can also serve as a natural insulator. Placing the pile in a sheltered spot, perhaps near a wall or fence, or using straw bales to create a windbreak, will help minimize heat dissipation from cold air and wind.