Cold composting is a method of decomposition that requires minimal effort, relying on natural processes over an extended period. This passive approach transforms organic waste into a soil amendment without the need for high temperatures or frequent turning, making it an excellent choice for beginners or those with limited time. Unlike the high-heat method, which uses thermophilic bacteria to rapidly process materials, cold composting involves a slow breakdown by fungi and other microorganisms. The trade-off for this simplicity is time, as the process takes months or even a year to complete.
Selecting and Preparing Materials
A successful cold compost pile begins with the right blend of materials, categorized as “Greens” and “Browns.” Green materials are nitrogen-rich, providing the protein necessary for microbial growth, and include kitchen vegetable scraps, fruit waste, coffee grounds, and fresh grass clippings. Brown materials are carbon-rich, offering the energy source and bulk structure, typically consisting of dry leaves, shredded paper or cardboard, straw, and small twigs. For cold composting, maintaining a higher volume of browns is recommended, often aiming for about three to four times the amount of brown material as green material by volume. This abundance of carbon helps prevent the pile from becoming overly wet or anaerobic, which can lead to unpleasant odors.
Minimizing the size of inputs is the only preparation needed, as smaller pieces decompose much faster. Shredding paper, crushing eggshells, and chopping large vegetable scraps create more surface area for microbes to act upon, which accelerates the process.
However, certain items should never be added to a cold system because they can attract pests, create foul smells, or introduce pathogens. These items include meat, bones, dairy products, oils, and grease, as their slow decomposition in a cool pile will likely invite rodents or other scavengers. It is also important to exclude diseased plants, weeds that have gone to seed, and pet waste. Since cold compost does not reach the high temperatures of 131–160°F found in hot composting, it cannot reliably kill harmful pathogens, parasite eggs, or weed seeds.
Constructing the Cold Compost System
The physical setup of a cold compost system can be very simple, requiring little more than a designated space or container. Choose a site that is level, well-drained, and ideally located near a water source for occasional re-moistening. While an open pile is possible, a simple structure like a wire mesh cage, a repurposed plastic bin, or a wooden enclosure will help contain the materials and maintain a tidy appearance. Placing the system directly on bare earth allows beneficial microorganisms and earthworms to colonize the pile from the soil below.
Begin the construction with a base layer of coarse, carbon-rich material, such as wood chips or loosely piled straw, to ensure good drainage and airflow at the bottom. This initial layer prevents the material above from becoming compacted and waterlogged.
Upon this base, start adding the prepared materials in alternating, thin layers of Greens and Browns. A common practice is to cover every layer of nitrogen-rich kitchen scraps with a layer of carbon-rich dry material, like dried leaves or sawdust, which helps to contain odors and deter pests.
As new material is added to the system over time, it is beneficial to keep pieces small and consistently maintain the ratio of approximately three to four parts Brown material for every one part Green material. Consistent layering is more effective than mixing large batches, as the cold method is designed for continuous, gradual additions.
Passive Maintenance and Harvesting
The defining characteristic of cold composting is its minimal maintenance, as the process does not require the frequent turning associated with hot composting. While turning the pile occasionally will introduce oxygen and speed up the breakdown slightly, it is not strictly necessary for the process to complete. The single most important maintenance factor is moisture content, which should be consistently damp like a well-wrung sponge. If the pile feels dry, particularly during long rainless periods, lightly watering it will reactivate the microbial activity.
This low-effort method requires patience, as the decomposition relies on a cooler, slower biological breakdown. Finished, usable compost typically takes anywhere from six months to a year, or sometimes longer, to develop. The material is considered “finished” when it has a dark, uniform color, an earthy smell, and a crumbly texture, with most of the original inputs unrecognizable.
Many cold composting setups operate as a continuous system, where new scraps are consistently added to the top layer. When the bottom layer is ready, the finished material can be harvested without disturbing the ongoing process above. This is often accomplished by using a bin with a removable door at the base or by temporarily moving the unfinished material from the top to one side to access the mature compost underneath.