The short version: A compost heap does not die in winter, it slows down. Cold air pulls the heat out of a small heap. Across the southern states, where most of us garden, the bigger problem is winter rain, because a waterlogged heap runs out of air and turns sour. Build bigger, keep the rain off, add plenty of coarse dry material, turn less often than you would in summer, and aim to use the compost in spring.
The heap that steamed on a January morning goes quiet some time in May. Last week's peelings are still recognisable peelings, and lifting the lid releases a whiff of drains rather than the usual earthy smell.
Most of us then do one of two things: give up until spring, or keep tipping wet scraps onto a sodden heap and quietly make it worse. Neither is necessary. Nearly everything that goes wrong with an Australian winter heap comes down to two things: a heap too small to hold its own warmth, and a heap too wet to breathe.
What actually slows down
The microbes slow down, not the process. Composting is done by microorganisms, and like most living things they work faster when they are warm. The mesophilic organisms that do most of the work in a home heap are active from about 0°C to 40°C. The hot thermophilic ones take over above roughly 40°C, which a small backyard heap rarely reaches at any time of year.
Decomposition slows once the temperature drops, but it does not stop unless the material freezes solid, which almost never happens in an Australian backyard. Expect a slower heap, not a dead one, and judge it by what the material looks like rather than by the calendar.
Why a small heap is the real problem
Size is the single biggest thing you control. The smaller the heap, the greater its surface area compared with its volume, and the faster it loses the heat its microbes make, by conduction and radiation. A big heap insulates its own middle. A small one is nearly all edge.
Australian guidance puts the practical minimum for a heap that will heat up at about one cubic metre, roughly a metre each way. The South Australian Government's composting guide gives the same figure as the bulk needed for hot composting to happen at all.
Most home bins hold well under that, and a standard black cone is unlikely to run hot in any season. That is not a failure, it is a cold heap, and a cold heap still makes good compost. It simply takes longer, up to six months by Gardening Australia's reckoning, and it needs cleaner ingredients. If you do want winter heat, stockpile material through autumn and build the whole cubic metre in one go, rather than feeding the heap a bucket at a time.
Insulation you can actually manage
- Site it in the sun, on soil. The shade that saved the heap in January works against it in July, and bare ground drains better than paving.
- Keep a lid or cover on. One cover does two jobs, holding warmth in and keeping winter rain out.
- Put a blanket on top. A thick layer of straw, or bags of dry leaves laid over the surface, slows heat loss where it is fastest.
- Pack around the sides. Straw bales, bagged leaves or hessian stacked against a bin work much the same way as a jumper.
If the autumn leaf fall has given you more material than the heap can take, our guide to putting autumn leaves to work covers the other jobs they do, including leaf mould.
The wet problem is the bigger one
In the southern states a winter heap is far more likely to drown than to freeze. Composting proceeds best at a moisture content of 40 to 60 per cent by weight. Above that range, excess water fills the pore spaces, blocks oxygen from diffusing through the material and tips the heap into anaerobic conditions.
Aerobic composting wants oxygen above about 10 per cent. Without it the process turns anaerobic and gives off the rotten-egg smell of hydrogen sulfide. That sour bin in the corner is not lazy, it is drowning. Four fixes, in the order worth trying:
- Cover it. A lid, a pegged-down tarp, or a sheet of corrugated iron propped on a slope. Keeping rain out is easier than getting water back out.
- Add coarse dry material. Straw, twiggy prunings, torn cardboard and dry leaves hold the structure open so air can move. Fine wet material collapses into a mat and seals itself.
- Fix the drainage underneath. A bin in the low corner where water pools will stay wet all winter. Move it to higher ground, or start the heap on a layer of coarse twiggy material.
- Rebuild a sodden heap. Fork it out onto a tarp, mix dry browns through as you go, and stack it back up. One afternoon of this beats months of waiting.
Carbon and nitrogen when leaves are everywhere and clippings are not
Winter hands you browns and takes away greens. Microbes want a carbon to nitrogen ratio of around 30 to 1. Too much carbon and the heap stays relatively cool and breaks down slowly. Too little and nitrogen is lost as ammonia gas, which is the other bad smell a heap can produce.
Autumn fills the yard with leaves, then the lawn stops needing a mow, so a winter heap drifts carbon-heavy and sits there. Kitchen scraps become your most reliable green, and they are worth chopping smaller than you would in summer. The other nitrogen sources that suit a cold heap are coffee grounds, soft winter weeds pulled before they set seed, aged poultry manure, and a light scatter of blood and bone at the rate on the bag. Gardening Australia suggests blood and bone, or a seaweed solution watered in, to speed a heap along, with keeping it damp the most important thing of all.
One winter caution. Do not tip a whole catcher of wet lawn clippings in as a single slab, because sodden clippings mat together, exclude air and turn slimy quickly in the cold. Mix them through in thin layers with plenty of dry material.
Turning: less often, not more
Turn a winter heap less than you would in summer. Every turn opens the middle of the heap and lets out the warmth it has built up, which is why the University of New Hampshire extension service says there is no need to turn a heap through the coldest months.
Air is not optional, though. A heap that is sodden and sour needs oxygen more than the small amount of heat it is holding. Fork it through, mix in coarse dry browns and rebuild it, on a mild dry day rather than the morning after a downpour. The winter rule is simple: leave a heap that is warm, damp and sweet-smelling, and open up one that is cold, wet and smelly.
Gloves, a mask and a damp heap
Compost, potting mix, mulch and other soil products can carry Legionella longbeachae, the bacterium behind one form of Legionnaires' disease, which people catch by breathing in dust. NSW Health advises putting on a face mask and gardening gloves before you handle the material, wetting it first so the dust does not blow into the air, and washing your hands with soap before eating or drinking. A winter heap is usually damp already. Dry straw and bagged leaves are not, and they throw dust of their own, so shake them out in the open air and damp them down as you go.
When a heap is not the answer
Some winters the honest answer is to stop feeding the heap and put the scraps somewhere they will do more good.
- Trench composting. Bury the scraps in a bed you are resting over winter. Choose it when you have a spare bed and want the organic matter in the root zone by spring.
- Dig and drop holes. A spade-deep hole every few days, moved along the bed each time. Choose it when the garden is full and there is no spare corner for a heap.
- A worm farm in a sheltered spot. Worms slow right down in the cold, so move the farm against a north-facing wall, into a carport or into a shed, insulate it and feed less. Choose it when the scraps are mostly soft kitchen waste.
- Bokashi as a pre-treatment. A sealed bucket ferments scraps whatever the weather, and the fermented material then goes into a trench or a compost hole to finish. Choose it when the heap is too wet to take anything more.
Our guide to composting in a small garden sets out how the worm farm and bokashi options are run day to day.
A cold heap does not sterilise anything
A heap that does not heat up will not kill weed seeds or plant pathogens. Gardening Australia makes the point plainly about cold composting: killing weed seeds and plant diseases is something it cannot do. Sustained heat is the mechanism, and temperatures of 55°C and above are where many human and plant pathogens are destroyed.
So be fussier in winter than in summer about what goes in. Keep out seed heads, runner grasses such as couch and kikuyu, nutgrass with its tubers, and any leaves or fruit showing disease. The council green bin is the right destination for those, because commercial composting reaches temperatures a backyard heap does not.
Rats and mice
A compost heap can hand a rodent everything it needs at once: food, cover and somewhere dry. SA Health lists composts among the things feral rats and mice will eat, and names rubbish heaps and thick vegetation as places they shelter and nest. It advises against open compost heaps and against composting any animal products. That sits awkwardly with the advice earlier in this article to build a big open bay, so let your own situation decide. Where rats are already about, use a lidded bin on mesh and accept a slower cold heap, or close the bay in with fine mesh across the base and sides and a solid lid on top. The winter twist is that the insulation you have just added is also good nesting material, so keep straw and bagged leaves on the heap or stacked tidily, not in a loose pile against a fence.
The measures that last are the dull ones. Use a bin with a close-fitting lid and sit it on fine galvanised mesh folded up around the base. Bury fresh scraps under a layer of browns, and keep meat, fish, dairy, cooked food and pet droppings out entirely. SA Health notes that chemical control is generally short term and that rodents return if food and shelter are still available, so baiting on its own solves very little.
Winter composting across Australia's climate zones
These zones are broad groupings. If you garden between two of them, or a few hundred metres higher than the nearest town, lean on local cues instead: your own first and last frost dates, and whether your wettest months fall in winter or summer.
- Tropical (Cairns and far north Queensland, Darwin and the Top End of the Northern Territory, the Kimberley in Western Australia): winter is the dry season, not the cold season. Darwin Airport averages 1.7 mm of rain in June and 1.1 mm in July, with mean July minimums above 19°C, so the heap stays warm and active and the risk is that it dries out. Keep it covered and water it when you water the garden.
- Subtropical (Brisbane and coastal Queensland, Coffs Harbour and northern New South Wales): mild winters, so a heap keeps working with little help. Moisture rather than cold is the thing to watch, and vinegar flies keep breeding, so cover fresh scraps with browns.
- Arid and semi-arid (inland Queensland, Broken Hill and Dubbo in New South Wales, Mildura in Victoria, inland South Australia and Western Australia, Alice Springs in the Northern Territory): cold nights and dry days. Alice Springs Airport averages a July minimum of 3.9°C and a maximum of 19.9°C, with under 15 mm of rain a month through winter, so the heap loses its heat overnight and dries out by day. Insulate it, keep a lid on and water it.
- Temperate (Sydney and New South Wales, Melbourne and Victoria): the classic slow, damp winter heap, with rain and low sun arriving together. Cover it, feed it coarse browns, and build it up in autumn while there is still plenty of material about.
- Mediterranean (Perth and southern Western Australia, and Adelaide and the Adelaide Plains in South Australia): winter is the wettest part of the year. Perth Metro averages 147 mm of rain in July against about 16 mm in January, so drainage and a cover matter here far more than insulation does.
- Cool and alpine (Hobart and Tasmania, Canberra and the ACT, Ballarat in Victoria, Orange and Armidale on the New South Wales tablelands): cold and wet at once. Hobart averages a July maximum of 11.8°C with steady rain right through the year. Build big, insulate, cover, and accept that the heap is mostly storing material until spring.
When will winter compost be ready?
Aim at spring. A cold heap takes six months or more to finish, and longer through a cold winter, so material added in June or July is unlikely to be usable before the weather warms. A heap built up in autumn often comes good just as the beds want feeding. Running two heaps, one filling and one finishing, saves stirring fresh scraps into a batch that is nearly done, and readiness is judged by look and smell rather than by date: dark, crumbly and earthy, with nothing recognisable left.
Frequently asked questions
Should I turn my compost in winter?
Less often than in summer, because each turn releases the warmth held in the middle of the heap. Leave a heap that is warm, damp and sweet-smelling alone, and open up only one that is cold, wet and sour, because that heap needs air more than heat.
My bin smells sour and is sitting in water. What do I do?
It has gone anaerobic. Fork the contents out onto a tarp, mix plenty of straw, dry leaves or torn cardboard through, move the bin out of the wet spot, rebuild the heap and cover it. Expect the smell to fade within a week or two.
Can I keep adding kitchen scraps all winter?
Yes, but cover each addition with dry browns and chop the scraps smaller than usual. If the heap is already sodden, hold the scraps in a bokashi bucket or bury them in a compost hole until it has dried out.
Will a winter heap kill weed seeds and disease?
No. A heap that does not reach high temperatures will not kill weed seeds or plant pathogens, so keep seeded weeds, runner grasses and diseased material out of it through the cold months.
Winter composting is mostly about lowering your expectations and raising your standards: fewer turns, cleaner ingredients, a cover on top and a bigger heap than feels necessary. If you are setting a heap up from scratch, our guide to composting for beginners covers the greens, browns, water and air that a heap needs in any season.