Troubleshooting a Compost Heap That Will Not Heat Up

You have built the heap, layered it with care, and waited. A week later you plunge a fork into the middle and find... nothing. No steam, no warmth, just a cool, slightly guilty pile of peelings and prunings. It happens to every allotmenteer, usually in that first hopeful year. The good news is that a cold heap is rarely a dead heap. It is a heap that is trying to tell you something. Listen closely and you can usually coax it back into life within a fortnight.

Why Heat Matters (and When It Doesn't)

Heat is the visible signature of the microorganisms that break down your waste. When bacteria and fungi are working hard, the centre of a well-built heap can reach 60°C or more, hot enough to kill weed seeds and speed everything along. But a heap that never gets hot will still rot down eventually. It just takes longer, and it may attract more slugs and flies. So if your heap is cool but crumbling, do not panic. If it is cool and slimy, or cool and bone dry, it is time to intervene.

Add More Greens

Nine times out of ten, a cold heap is short of nitrogen. Nitrogen-rich greens are the fuel that fires the microbial furnace. If your heap is mostly cardboard, straw, dead leaves and woody prunings, the microbes have plenty of carbon but not enough protein to multiply quickly. The fix is simple: add more green material.

• Grass clippings, but only in thin layers — a thick mat of clippings turns to slime and blocks air.
• Kitchen scraps: vegetable peelings, fruit cores, tea leaves, coffee grounds. Avoid meat, fish, dairy and cooked food if you have rats or foxes about.
• Soft prunings, annual weeds before they set seed, and spent flowers.
• Comfrey leaves, nettles, and the leafy tops of brassicas. These are compost accelerators par excellence.
• A handful of poultry manure or a sprinkle of blood, fish and bone if you want to give the pile a real boost.

Mix the greens through the heap rather than leaving them in a lump. Aim for roughly one part greens to two parts browns by volume. If in doubt, add greens, then turn.

Turn the Pile

Composting is an aerobic process. The bacteria that generate heat need oxygen, and a settled heap soon runs out of it. If your pile has gone cold and compacted, turning is the single most effective remedy. Fork the whole heap from one bay into the next, breaking up clods and fluffing it as you go. This introduces fresh air, distributes moisture, and mixes the greens and browns you just added. For a stubborn heap, turn it once a week for three weeks. You should feel warmth within days.

Chop the Bulky Stems

Big, woody stems and whole cabbage stalks are a common hidden cause of a cold heap. Their tough skins resist microbes, and they create air pockets that let heat escape. Before they go on the heap, chop them. A sharp spade, a pair of secateurs, or a run over with the lawnmower will reduce them to pieces the size of your thumb. The smaller the pieces, the more surface area the microbes have to work on, and the faster the heap heats up.

Keep It as Moist as a Wrung-Out Cloth

Moisture is the other half of the equation. A dry heap cannot compost; a sodden one turns anaerobic and stinks. Push your hand into the middle. It should feel like a wrung-out cloth — damp but not dripping. If it is dry, water it with a can or a hose on a gentle spray, turning as you go. If you have a water butt, so much the better, but tap water is fine. If it is soggy, add shredded cardboard, newspaper, or dry leaves and turn thoroughly. Cover the heap with a lid, a tarpaulin, or a piece of old carpet to keep the moisture in and the rain out.

Size, Mass and a Little Patience

Small heaps struggle to heat up. A pile smaller than about a cubic metre simply cannot retain enough heat, especially in a British winter. If your heap is on the small side, save up materials until you have enough to build a proper batch, or insulate the sides with straw bales, cardboard, or bags of leaves. A dalek bin in full sun will do better than a cold, shaded corner. And remember that even a perfect heap will slow down in January. If everything else is right, leave it alone and let the worms take over. Time is the one ingredient you cannot rush.