With colder temperatures, questions about composting in winter tend to come up: What’s happening to microbes when the compost appears frozen? Can worms survive freezing temperatures? How best to protect the compost pile and keep adding materials throughout the winter? The following discussion may be helpful.
By John Zarola, Bernalillo County Extension Master Composter, January 2021
When the temperature within a composting setup goes below 55 F, microbial activity slows down as does the decomposition of organic materials in the bin. Heat loss from any setup will be by radiation and convection and is dependent on the impact of the ambient air temperature. Evaporation of moisture will decrease at lower temperatures.
The internal production of heat in any composting setup comes from microbial enzymatic breakdown of the carbohydrates (plants) which have been added. Heat energy from the sun is stored in plants during photosynthesis, which produces carbohydrates. When those plants are decomposed, that energy is released in the form of heat.
Hot and Cold Process Composting
Two unique composting methods are the cold process and the hot process. If one is not following the specific guidelines for hot process composting, then all other methods fall into the cold process category. Each process will be briefly discussed.
Hot process composting is a specifically constructed batch of at least a cubic yard of organic materials organized all at once. Attention to the hot process recipe insures the internal production of heat. The appropriate mix of greens and browns with moisture and air provides for the rapid growth and proliferation of decomposing microorganisms. Millions of microorganisms quickly breakdown the organic material thereby releasing heat stored in the carbohydrates. A well managed hot pile will produce appreciable and sustainable internal heat (100 F to 140 F) in any season, including winter. This process requires attention to detail and timely turning and churning.
A cold process composting setup is a frequent choice for home composting. This process is straightforward, easy, and effective. The process may be static; that is, no turning is required as long as coarse bulking material is added as the pile is built to avoid compaction and facilitate convective air flow. Moisture is maintained in all seasons.
Composting in the High Desert
Download a brochure which explains the particulars of hot and cold process composting.
The cold process involves occasional additions of whatever organics may be available at any time. It is not usually a batch method. Often only a small amount of internal heat will be produced. Heat loss from a cold process setup by radiation and convection is influenced by the ambient air temperature. Temperatures in a cold process setup will equilibrate with ambient air temperature changes. External heat may be added by placing the bin in a sunny location in the colder months. In general the decomposition process slows in the winter, then speeds up in spring and summer. Though the process is slowed, it is OK to continue to add organics to the bin. When the temperature rises in the spring the materials that have been added will begin to decompose at a faster rate.
Burying organic material directly into garden soil is a useful cold technique. Freeze-thaw cycles in the soil will mechanically break up plant fibers. Microbial decomposition will be slow until the soil temperatures rise in the spring. Mulch on top of soil, to which organics have been added, will help the maintain moisture required for decomposition.
Outdoor worm composting setups will perform well in the urban areas of central New Mexico as long as the bin is protected from a sustained frost. Insulation may be in the form of straw bales or thick “blankets” of several layers of cardboard. Composting worms, i.e., red wigglers, may be added to a cold process compost bin as long as the bin is protected from freezing.
Sheet mulch composting may be started in the fall and maintained throughout the winter. Periodic watering will be necessary. Decomposition will proceed slowly in winter, then improve in the spring.
Select a composting process which fits your lifestyle. Continue to organize and recycle your organics in a composting setup in all seasons, including during the winter. Maintain moisture in the bin in all seasons. Chop, shred, cut materials before adding as this will improve moisture absorption. Add coarse bulking material as the pile is built to reduce compaction and improve convective air flow.
Homemade Compost Bins for the High Desert
Explore several inexpensive composting set ups for the home composter, appropriate for the desert.
E-mail your questions about winter composting along with your zip code to
The Bernalillo County Extension Master Composters freely share desert home composting information with the interested public through public classes, on-line presentations, demonstrations, consultations and an informative website: nmcomposters.org