How to Compost in Winter

Compost in winter by continuing exactly as you do in summer, with three adjustments: build the pile bigger before the cold arrives, chop and bury kitchen scraps rather than piling them on top, and stockpile a bag of dry browns -- leaves, shredded cardboard -- somewhere they will not freeze solid, so you always have something to cover scraps with. A pile that freezes has not died. The microbes go dormant, decomposition pauses, and everything resumes in spring. The only genuine winter risk is a wet, airless, all-greens pile that goes sour and attracts animals.

Composting is microbial work and microbes slow as temperature drops. Around freezing, activity in the outer layers effectively stops. A large pile keeps a warm core for much longer than a small one, because the heat it generates has further to travel to escape -- which is why volume is the single most effective winter strategy.

Tumblers are the setup that disappoints people in winter. They are small by design and raised off the ground, so they lose heat on every side and freeze faster than anything else. They are still worth filling -- the material composts rapidly in spring -- but do not expect it to work in January.

1. 1

   Build the volume up in fall

   A pile roughly three feet in each direction has enough mass to hold a warm core. Fall is also when leaves are free and abundant, which is the only time of year browns are not the limiting factor.

2. 2

   Stockpile dry browns under cover

   Bag shredded leaves or keep a sack of shredded cardboard in the garage. In January, every brown material outdoors is either frozen or soaked, and you need something dry to cover each addition of scraps.

3. 3

   Turn it one last time, then stop

   Turning in deep cold vents the warm core and does more harm than good. Give it a final turn in late fall and leave it alone.

4. 4

   Insulate the sides if you want to extend activity

   Straw bales, bagged leaves stacked around the bin, or a tarp over the top. None of this is essential; all of it buys a few more active weeks.

5. 5

   Keep it covered against rain and snow

   A waterlogged pile in spring is slower to restart than a damp one, and a lid or tarp is the whole fix.

6. 6

   Clear a path to the bin

   Trivial and decisive. The reason winter composting stops is almost always that walking to the bin through snow in the dark stopped being worth it.

On the worst weeks the honest answer is not to compost at all but to store. Three options, in order of how little work they are:

• Freeze them. A container in the freezer holds weeks of scraps with no smell and no fruit flies, and it thaws into a pile that breaks down faster for having had its cell walls burst by freezing.
• Fill a lidded outdoor tote. A second bin, or a bucket with a tight lid by the back door, collects through a cold snap and gets emptied into the pile when you can reach it.
• Trench compost in a bed. Dig a hole in an empty raised bed, bury the scraps, cover. It works until the ground freezes hard, and the material is incorporated into the bed by spring.

A kitchen caddy is doing more work in winter than in summer, because trips to the bin are rarer. A smooth-walled pail that rinses out easily and seals properly is the difference between a habit that survives January and one that does not.

If you want compost production rather than scrap storage through the cold months, bring it indoors. A stacking-tray worm bin in a basement, garage or utility room runs at room temperature, processes kitchen scraps continuously, and produces castings that are among the best amendments available for a raised bed in spring.

Worms stop feeding below about 50 degrees Fahrenheit and die if the bin freezes, so an outdoor worm bin must come in or be heavily insulated. Composting worms covers species and conditions, and the best worm composting bins compares the systems.

When the pile thaws, it will usually be wetter and denser than it was in fall, and it will have a layer of recognizable winter scraps through the middle. Turn it thoroughly, add a generous volume of dry browns to correct the moisture and the carbon balance, and it will heat up within days. A winter pile that looks like a disaster in March is often finished compost by June.

That spring compost arrives at exactly the right moment, which is the argument for bothering at all. How to refresh raised bed soil is where it goes.