Why Heat Matters in a Compost Heap

A compost heap that never gets warm is really just a slow, tidy rubbish pile. It will rot down eventually, but it takes a year or more, it attracts more slugs and mice than you'd like, and weed seeds often survive the journey. A properly built heap, on the other hand, will hit 55–65°C within a few days and hold that heat for a fortnight or more. At those temperatures the thermophilic bacteria take over, breakdown speeds up dramatically, and most weed seeds and many plant pathogens are killed off. On a smallholding this matters. You get usable compost in six to ten weeks instead of next winter, and you can safely spread it on the veg patch.

The Green and Brown Balance

Hot composting is really about feeding the microbes the right diet. Greens are the nitrogen-rich, sappy materials — grass clippings, kitchen vegetable scraps, comfrey, nettles, young weeds before they seed, manure from chickens or horses. Browns are the carbon-rich, dry, fibrous materials — straw, cardboard, shredded paper, dead leaves, woodchip, spent bedding. The classic target is roughly two to three parts brown to one part green by volume. Too many greens and the heap goes slimy and smelly; too many browns and it sits cool and dry.

• Greens: grass clippings, veg peelings, tea leaves, coffee grounds, fresh manure, green plant trimmings
• Browns: straw, cardboard, newspaper, woodchip, dead leaves, hay, sawdust, shredded twigs
• Avoid: cooked food, meat, dairy, diseased plants, perennial weed roots like bindweed or couch grass, pet waste

Layer them rather than dumping — a hand's depth of green, then a couple of hands of brown, like building a very rough lasagne. Breaking up thick stems and tearing cardboard into palm-sized pieces helps the bacteria get stuck in quickly.

Size, Air and the Shape of the Heap

The single biggest reason a heap never heats is that it's too small. You need a minimum of about one cubic metre — roughly a metre wide, a metre high and a metre deep — for the centre to insulate itself and hold warmth. A pallet bin made from four pallets wired together is ideal, and free from most farm suppliers or builders' yards. A three-bin system lets you turn one heap into the next while a third matures, which suits a working smallholding better than a single plastic cone.

Air is just as important as size. The heap should be built loosely, not trodden down. If you can push a broom handle or a length of scaffold pipe into the middle, that's a good sign. Some gardeners add a few horizontal pipes with holes drilled through, but a good turn every week or so usually does the job.

Building the Heap Step by Step

Start with a 15cm base of coarse browns — woody prunings, woodchip or scrunched cardboard — so air can move up from below and any liquid can drain away. Then build your alternating layers, watering lightly as you go if materials are dry. Aim for the texture of a wrung-out sponge: damp enough that a squeeze produces a few drops, not a stream. Sprinkle a little soil or finished compost every few layers to inoculate the heap with the right microbes, then cap it with a final layer of browns to keep the rain out and the heat in. A sheet of old carpet or a tarpaulin over the top helps in wet weather, but lift it on dry days.

Turning, Moisture and Knowing It's Working

Within two or three days you should be able to feel real warmth by pushing your hand into the centre — sometimes too hot to hold comfortably. If it isn't warming, the usual culprits are too dry, too many browns, or too small a heap. Add a bucket of water or some grass clippings and rebuild it, fluffing everything up as you go.

Turn the heap every seven to ten days. Move the outside to the middle and the middle to the outside, which is the bit people skip and then wonder why nothing cooks. Each turn should give the heap a fresh burst of heat as oxygen reaches the microbes. After three or four turns the temperature will start to fall — that's your cue that the first, hot phase is done. Give it another four to six weeks to mature, turning occasionally, until it's dark, crumbly and smells of woodland floor rather than ammonia.

• Too dry: add water and more greens, then turn
• Too wet or smelly: add browns, turn, and cover against rain
• Cool but damp: likely too many browns — add grass clippings or manure
• Warm only at the edges: the heap is too small or not turned properly

Countryside Practicalities

Site the heap on soil, not concrete, so worms and microbes can move in from below. Full sun is fine in most parts of the UK, though a little shade in high summer stops it drying out too fast. Keep it away from hedgerows where rats run, and turn it often enough that it never becomes a nesting site. If you keep poultry, the spent bedding and droppings are excellent green material — just balance them well with straw or woodchip.

Get the mix, the size and the moisture right, and the heap will do the rest. A fortnight of real heat in the middle of a British summer turns a mixed pile of clippings, cardboard and muck into something you'd happily pay for at a garden centre — and it costs you nothing but a bit of wheelbarrow work and a pair of sturdy gloves.