Composting is the managed decomposition of organic matter, transforming scraps and yard waste into a nutrient-rich soil amendment. A frequent question is whether the compost pile should be covered. The decision depends entirely on environmental conditions and the specific goals of the composting process. Covering the pile manages the internal environment, controlling the critical factors that drive microbial breakdown. Understanding the specific function of a cover determines when and why it is a beneficial management technique.
The Core Functions of Covering
Covering a compost pile serves two primary functions related to controlling the internal environment necessary for microbial activity. The first is moisture regulation, as microorganisms require a damp environment to thrive. A cover prevents the rapid evaporation of water from the surface, especially during warm or windy weather, helping the pile maintain ideal moisture content, often described as feeling like a well-wrung sponge.
The second function is to retain the heat generated by microbial metabolism within the pile. As bacteria and fungi break down organic materials, heat is released, raising the internal temperature. A cover acts as insulation, minimizing heat loss to the surrounding environment. This helps achieve the high temperatures needed for “hot composting,” accelerating decomposition and destroying pathogens and weed seeds.
Situations Requiring Coverage
Specific external circumstances make covering the compost pile necessary to protect the composting process. During heavy or prolonged rainfall, a cover shields the pile from excessive moisture, preventing saturation that displaces oxygen. Overly wet conditions lead to an anaerobic environment, which slows decomposition and results in unpleasant odors.
Conversely, covering is important during hot, dry, or windy conditions to combat desiccation. Intense sun and wind quickly strip moisture from the surface, creating a dry outer layer that stalls microbial activity. A cover acts as a shade and windbreak, protecting the pile from these drying effects and conserving internal water content.
In colder climates, a cover helps retain internal heat, allowing the thermophilic process to continue longer. Its insulating effect keeps the core temperature elevated, offering protection against rapid heat loss. A cover also assists in pest deterrence, suppressing odors that attract scavengers and physically blocking access.
When Not to Cover the Pile
While covering a compost pile offers many benefits, there are specific situations where it can be counterproductive. The most common negative outcome is when the pile is already too wet, and adding an impermeable cover traps the existing moisture. This excess water saturates the material, leading to anaerobic conditions. The resulting material will be slimy and release foul-smelling gases.
A tight or non-porous cover can also impede the necessary airflow into the pile, which is required by the aerobic microorganisms. Although covers help retain heat, they restrict the passive exchange of gases, reducing the oxygen supply. If a pile is covered, it often necessitates more frequent turning or aeration to introduce fresh oxygen.
Covering the pile can also be inconvenient when active management is required, such as the routine addition of fresh “green” materials or monitoring internal conditions. Leaving the pile uncovered during dry, temperate periods allows for natural air circulation and easy access for maintenance.
Selecting the Right Cover Material
The cover material should be selected based on the specific goal: insulation or allowing air and water exchange. For maximizing heat retention and insulating against cold, thick, heavy, or dark materials are suitable choices. These materials minimize heat transfer and block wind effectively. Examples include:
- Heavy black plastic tarps
- Old carpet scraps
- A deep layer of straw
When the goal is moisture control while allowing breathability, lighter, more porous options are better suited. Permeable landscape fabric, shade cloth, or loosely placed plastic sheeting can shed heavy rain while allowing air to filter through. This protects the pile from saturation while mitigating the risk of excessive wetness.
A third, temporary method involves using a thick layer of finished compost or soil, known as a bio-layer. This organic layer suppresses odors and discourages pests by creating a physical barrier. The choice of cover material is dynamic, shifting based on current pile conditions and prevailing weather patterns.