The temperature dropped to minus four overnight, and by morning the wormery on the balcony had a thin crust of ice across the top tray. I lifted the lid expecting the worst. The worms had retreated to the bottom layer, which was a few degrees warmer, and were balled up in the corner like a tangled handful of brick-coloured spaghetti. They were alive but slow, and they did not move when I disturbed them. I closed the lid, threw a wool blanket over the bin, and brought it inside the porch for the rest of the winter. The worms were fine.
That was an early lesson. Worm bins that ran beautifully through summer can collapse in two weeks of December cold if you don’t prepare them. The good news is that the preparation is not complicated. There are five things to get right, and worms are forgiving creatures provided you give them what they need. the EPA’s home-composting guide
Why winter is the problem
Composting worms (Eisenia fetida) are subtropical in origin. They thrive between about 15 and 25 degrees centigrade. Below 10 they slow significantly. Below 5 they almost stop eating, breeding and moving. Below freezing they die.
What kills worms in winter is rarely the cold itself, in the sense that they could survive a single very cold night. USDA composting overview What kills them is sustained cold, especially when combined with too much water, too little food, or fresh scraps that fail to break down in time. A worm bin in November is fundamentally a different organism from the same bin in July, and treating it the same is the most common winter mistake.
Five things to do before the cold arrives
Ideally these happen in October, before the first proper frost.
1. Move the bin into shelter. A wormery outdoors in a UK winter is at the mercy of the weather. Move it into an unheated outbuilding, a porch, a garage, or even a kitchen corner if space allows. The temperature in a garage rarely drops below 5 degrees even in deep winter, which is enough to keep worms alive.
If you absolutely can’t move the bin, insulate it. Wrap the sides with hessian, old blanket, foam pipe insulation cut to length, or even bubble wrap (the more layers, the more insulation). Tuck a thick layer of straw or shredded card around the top, under the lid.
2. Add a thick top blanket. Inside the bin, on top of the bedding and food, lay a thick (two to three centimetre) blanket of shredded paper, hessian, or coir. This traps the worms’ own metabolic heat and acts as a temperature buffer. A worm bin in winter without a top blanket loses heat through the lid every time you open it.
3. Reduce feeding. This is the bit people get most wrong. In summer your worms might eat a kilo a week. In winter they will eat maybe a third of that. Adding the same amount of food into a slower bin means uneaten scraps sit on the surface, ferment, drop the pH, and create exactly the conditions that finish off cold worms. Halve the feeding in October and quarter it in December.
4. Switch the food. Winter worms prefer pre-decomposed scraps to fresh ones. Freezing scraps for two days before adding them helps enormously, it breaks down the cell walls and lets the worms get to the contents faster. Also lean on bedding-style foods: shredded card and paper, slightly aged compost from a heap, the soft inner bits of cabbage and lettuce.
5. Check moisture more often. Cold air is dry. A wormery indoors can dry out faster in winter than in summer, particularly if it’s near a heat source. Squeeze a handful of bedding from the middle tray weekly. It should feel like a wrung-out sponge. If it crumbles, mist it with a spray bottle or add a small amount of damp shredded card.
Where to put the bin in winter
Some options, in order of how well they work.
- Heated indoor space, kitchen, utility room. Worms thrive. Bin operates almost as normal.
- Unheated indoor space, porch, conservatory in winter, indoor garage. Worms slow but stay alive. Probably the most common winter setup.
- Insulated outbuilding, shed with good insulation, garage with a draught-free door. Worms slow significantly but survive most UK winters.
- Sheltered outdoor spot with wrapping, north-facing wall, against a heated house wall, under a porch. Worms slow heavily, may freeze in severe cold. Workable in mild winters; risky in cold ones.
- Exposed outdoor, open balcony, middle of the garden. Worms likely to die in any winter that drops below minus two for several days.
The signs of a bin in trouble
Three warning signals to watch for between October and March.
Worms balling up in corners. A small ball is sometimes normal (worms cluster in optimal microclimates). A large ball, with most of the population in one spot, means most of the bin has become uninhabitable. Move the bin somewhere warmer immediately.
Uneaten food piling up. A summer bin polishes off food within days. A winter bin might leave the same food for weeks. If old scraps are visibly accumulating, stop feeding and let the bin catch up. Don’t add to a pile that isn’t being eaten.
Sour or fermented smell. Cold bins where food sits too long start to smell of cider, then vinegar, then worse. Stop feeding, add a generous handful of shredded card to absorb moisture and balance pH, mix gently.
What about over-wintering an outdoor bin you can’t move
If you really can’t bring the bin inside, the best plan is to convert it temporarily into an insulation cocoon.
Surround the bin with bales of straw or hay. Wrap a tarpaulin around the whole thing. Place a thick layer of straw or dry leaves on top (under the lid if possible). Stop feeding entirely from December through to mid-February, yes, entirely. The worms will live off existing bedding and slowly reproduce. They will not breed as fast as in summer, but they will survive.
I have done this with a stacked-tray bin in a windy north-east garden, with the bin lashed against the house wall under a porch overhang and surrounded by straw. It got down to minus seven that winter and the worms came through, slower in spring than usual but alive.
The spring restart
Around mid-March, when overnight lows reliably stay above 5 degrees, the bin will start to wake up. The worms become visible at the top of the bedding again. They start eating fresh food. The bin smells more, in a healthy woodland way.
Start by adding small amounts of food and slowly increasing. Don’t try to make up for three months of light feeding in a week, overload now will set the bin back to where it was in December.
By late April the bin should be running roughly at summer capacity. The worms that came through the winter will be smaller and fewer than they were in October, but they will breed back to full population over six to eight weeks.
Mine are back at it now. The balcony bin is on the balcony again, the porch ones are insulated but not panicking, and the worms are doing what they always do, slowly and quietly, in a brick-coloured tangle just under the surface of the bedding.
Frequently Asked Questions
Will my worms really die in a UK winter?
They can. Sustained temperatures below 5 degrees slow them severely, and below freezing kills them. The good news is that most UK indoor and sheltered locations stay warm enough — the danger is fully exposed outdoor bins in cold winters.
Can I keep a worm bin in an unheated garage?
Yes, in most cases. Garage temperatures usually stay above 5 degrees even in cold weather, especially if the bin is well-insulated and tucked away from draughts. The worms will slow but should survive.
How much should I feed worms in winter?
Roughly a quarter to a third of summer rates. Reduce gradually from October, watch how fast they eat, and never add new food on top of uneaten old food. Underfeeding is far safer than overfeeding in winter.
Should I move the bin indoors at the first frost?
Ideally yes, or at least into shelter. A single frost won't kill worms, but a series of cold nights can. Moving early into a porch or garage is easier than rescuing a bin that's already in trouble.
Will the worms eat their own bedding if I stop adding food?
Yes, slowly. Composting worms eat bedding material (shredded card, paper) along with food scraps. A well-bedded bin can sustain its population for several weeks without new food, particularly in cold months when their appetite is reduced.
Why are my worms clustered in a ball?
Small clustering is normal — worms find microclimates. Large clustering suggests most of the bin is uncomfortable for them, usually due to cold, wet, or wrong pH. Check conditions and move the bin if needed.
