Composting organic waste is an activity that does not stop when the temperatures drop. While decomposition in cold weather is possible, the process changes significantly from the active, rapid breakdown seen in warmer months. Winter composting is a storage method or a period of slow decomposition, rather than a rapid “hot” composting system. The goal shifts from producing finished compost quickly to simply reducing the volume of waste and preserving the materials until spring.
Understanding the Cold Composting Process
The primary reason composting slows down in winter is the reduction in microbial activity. Composting relies on microorganisms, which thrive in high heat, typically between 113 to 160 degrees Fahrenheit. When the internal temperature of the compost pile drops significantly, these heat-loving microbes become dormant or die off.
Once the pile temperature falls below about 68 degrees Fahrenheit, the decomposition process is mainly taken over by psychrophilic bacteria. These microbes work much slower than their warm-weather counterparts, which extends the time required for material breakdown. This slower process is known as “cold composting.”
The goal shifts from active decomposition to volume reduction and material storage. Even if the pile freezes solid, the organic material is preserved, and freeze-thaw cycles can help mechanically break down plant fibers. While the decomposition rate is dramatically reduced, the materials are waiting for warmer temperatures to resume microbial action.
Essential Setup and Insulation Techniques
The external setup must retain residual heat and protect the pile from extreme cold. A large pile size is the most effective way to maintain internal heat, as greater mass minimizes surface heat loss. Building the pile to a minimum of three to four feet in width and height helps to protect the active core.
Enclosed bins are better suited for winter as they provide more thermal protection than open heaps. For open piles or three-sided pallet systems, external insulation is a practical method to mitigate heat loss. Surrounding the bin with bales of straw, thick layers of dried leaves, or even piled snow can create an effective thermal buffer.
Positioning the compost on the sunniest side of a building or in an area naturally sheltered from prevailing winter winds helps to keep the temperature slightly elevated. Minimizing the turning of the pile during sustained cold periods is also important. Turning a pile releases internal heat that the microbes have generated.
Managing Winter Feedstock and Ratios
Have a large reserve of carbon-rich “brown” materials, such as dried leaves, shredded wood chips, or straw, stockpiled before the cold season begins. These browns are essential for balancing the daily addition of kitchen waste.
When adding high-moisture kitchen scraps, or “greens,” they should be immediately mixed with the dry carbon materials. Adding high-moisture material directly to the pile can lead to it freezing into solid, impenetrable blocks. This mixing prevents waterlogging and maintains the necessary air pockets for future aerobic activity.
Avoid adding large quantities of frozen material, as this can dramatically drop the internal temperature of the pile and halt any remaining microbial function. To maximize the surface area available to microbes when the pile eventually thaws, shredding or chopping all inputs before adding them is recommended. Smaller pieces of material will decompose much faster once the temperatures rise and the microbial community reactivates.
Transitioning the Pile Back to Spring
Once the threat of a deep, sustained freeze has passed, the stored winter materials must be “woken up” to resume active decomposition. The first action should be to completely turn the material, which introduces much-needed oxygen and breaks apart any frozen clumps.
This turning process allows for an assessment of the pile’s moisture level. If the pile is excessively wet, adding fresh, dry carbon sources, such as sawdust or more shredded leaves, will help absorb the moisture. After turning and adjusting the moisture, the pile needs a fresh source of nitrogen to kickstart the thermophilic process.
Adding a batch of fresh, nitrogen-rich “greens,” like grass clippings or fresh food scraps, will provide the fuel for the heat-loving microbes to rapidly multiply. With proper turning and moisture content, the internal temperature of the pile should begin to rise, signaling the return to active composting. The compost will be ready for use after this active heating phase is complete and the material has had time to cure.