The sight of steam rising from a compost pile on a cold winter day is sure to warm the heart of every organic gardener. It's a sign that when spring comes, you'll have a batch of fresh compost to use for getting seeds and transplants off to a healthy start in your garden. Frigid weather outside can slow the decomposition process, but you can maintain an essential core of heat, which indicates that crucial microbial activity is occurring inside the pile. "The outside layer of the pile will be ambient temperature," says Mark Van Horn, a researcher at the University of California-Davis, "but if things are right, the inside of the pile will be hot." These hints from the experts we spoke to will help keep your compost cooking through winter in any region.
Even in winter, a compost pile is alive, an ecosystem in flux. "Microorganisms such as bacteria, fungi, and actinomycetes account for most of the decomposition activity in a compost pile," explains Dave Wilson, research agronomist at the Rodale Institute. The microbial action in decomposition is exothermic, which means that heat is a by-product of the chemical process of breaking down the material.
The microbes' metabolism slows down as the temperature dips, which explains why food keeps in a refrigerator or freezer. In winter, your goal is to create an ideal habitat for microbes. Think of it as "micro-husbandry."
Feeding. Microbes need a balanced diet, a mix of carbon- and nitrogen-rich materials (also known as "browns" and "greens"). Your kitchen scraps, such as vegetable and fruit peelings, coffee grounds, and houseplant trimmings are handy sources of nitrogen-rich ingredients. You can also compost eggshells and even domestic waste products like shredded newspaper. Manure from chickens or rabbits is loaded with heat-generating nitrogen. If you don't have access to that, you can add alfalfa pellets (sold as rabbit food) or blood meal to give your pile a nitrogen boost.