Composting tips, advice and science.

My garden in the snow.

Frozen Compost? Don’t Panic—Here’s What’s Happening Inside

Even in my relatively mild climate, winter temperatures occasionally dip as low as -7°C (19°F), and the surface of compost can freeze. While it’s not a frequent occurrence here, I know many composters live in much colder regions and wonder what happens when their compost piles freeze over.

Fortunately, even when your compost seems to freeze on the outside, it’s far from lifeless. In this article, we’ll explore:

  • What happens to bacteria in your compost (and who steps up when the cold hits)
  • The activity that continues in the core of your compost heap
  • The surprising benefits of freeze–thaw cycles
  • The possible role of fungi in winter composting
  • My personal experience with winter composting (spoiler: the worms survived!)

Microbial Activity in the Cold

Digital art image of thermophilic bacteria.

Even when temperatures plummet, your compost remains alive with microbial activity. The thermophilic (heat-loving) bacteria that usually drive the composting process do go dormant, but they’re replaced by their cold-loving counterparts: psychrophilic bacteria.

Research has identified psychrophilic bacteria capable of breaking down organic material efficiently in low temperatures. While we may not have direct access to these specific strains, the point is clear—microbial life adapts, even in the cold.

Many of these microbes can function at temperatures below 68°F (20°C) and remain active near freezing. While their metabolism is slower than thermophiles, they still generate small amounts of heat, keeping the decomposition process ticking over.

Learn more: The Mind-Boggling Role of Bacteria In Compost

Activity in the Core of the Heap

Thanks to this microbial activity, composting can often continue in the core of the heap, especially in larger piles or bins. The increased volume of organic material acts as insulation, maintaining warmth in the center even when the outer layers freeze.

You can help this process by insulating the compost, which effectively expands the core:

Mini-infographic showing how the core keeps compost warm.

This warmth can also help with vermicomposting (composting with worms). Indeed, worm composters in colder climates sometimes use larger containers packed with organic matter to help generate enough heat for worms to survive the winter. 

The Power of Freeze–Thaw Cycles

Snow lies on plastic covering my compost.
Above: A dusting of snow on my pallet bin.

Another unsung hero of cold-weather composting is the freeze–thaw cycle. As temperatures fluctuate, water inside organic materials freezes and expands, physically breaking apart leaves, vegetable scraps, and other compostable items. When the material thaws, these fractures create more surface area for microbial activity, accelerating decomposition when warmer weather returns.

Freezing and thawing also release nutrients locked within organic matter, making them more available for microbes and, eventually, for plants. That said, most compost heaps are warmer in the center, so this may only affect the outer part of the compost heap. 

Could Fungus Play a Role?

Fungus growing in insulated compost bin.
Above: Fungus growing in an insulated compost bin on 31st December. It was snowing outside but still warm in the bin.

When temperatures cool down, fungi often take center stage in the composting process. But could they be active in freezing conditions too?

While there’s limited research on the role of fungi in cold composting, a Chinese study identified specific fungal strains capable of speeding up composting in low temperatures, while other researchers have found fungus growing in the arctic. 

And when I go for a walk in the woods in the winter, I’ve noticed cold-adapted fungi and mushrooms thriving on everything from fallen logs to leaves. I find it hard to imagine that there isn’t similar fungi working in the protected core of the compost heap.

Even if fungi aren’t active in the frozen outer layers, they’re likely contributing in the warmer core of the pile, continuing to break down organic material in tandem with psychrophilic bacteria.

My Experience: Proof in the Pudding

As I write this, it’s late January. Just last weekend, during a mild spell, I spent some time mulching my garden beds with compost I’d finished last summer. The compost was beautifully broken down and crawling with worms—a sure sign of a healthy heap.

Even more encouraging, the last compost heap I started before winter is progressing nicely and should be ready for spreading by spring. That’s nothing new – typically, I find compost finished in the autumn is ready for spreading by spring. 

So if freezing temperatures have you worried, don’t be discouraged. While the process might slow down, your compost pile is still alive and active in ways you might not see—and the cold may even bring unexpected benefits!

Related articles

Sources

Kim, Y. J., Kim, J., & Jung, J. Y. (2023). Responses of dissolved organic carbon to freeze–thaw cycles associated with the changes in microbial activity and soil structure. The Cryosphere, 17(7), 3101–3114. https://doi.org/10.5194/tc-17-3101-2023

Hou, N., Wen, L., Cao, H., Liu, K., An, X., Li, D., Wang, H., Du, X., & Li, C. (2017). Role of psychrotrophic bacteria in organic domestic waste composting in cold regions of China. Bioresource Technology, 236, 20–28. https://doi.org/10.1016/j.biortech.2017.03.166

Robinson, C. H. (2001). Cold adaptation in Arctic and Antarctic fungi. New Phytologist, 151(2), 341–353. https://doi.org/10.1046/j.1469-8137.2001.00177.x

Shi, W., Dong, Q., Saleem, M., Wu, X., Wang, N., Ding, S., Huang, J., Wang, X., Zhou, B., & Gao, Z. (2022). Microbial-based detonation and processing of vegetable waste for high-quality compost production at low temperatures. Journal of Cleaner Production. https://doi.org/10.1016/j.jclepro.2022.13327