Why has my compost heap stopped working in hot weather?

It is counter-intuitive that a hot summer slows composting down, since warmth is supposed to speed it up. But the decomposition is done by bacteria and fungi living in a film of water around the particles, and when that film dries the whole process stops. A heap in August is frequently bone dry a few centimetres in, with grass clippings on top that have turned to hay rather than compost.

Step by step

1. Check the moisture properly.Dig into the middle rather than judging by the surface. A working heap feels like a wrung-out sponge — damp enough to hold together when squeezed, not wet enough to drip. Dusty, crumbly, or material that still looks recognisable after months means it is too dry.
2. Water it as you turn it.Watering the top of a dry heap achieves nothing, because dry material sheds water exactly as dry compost in a pot does. Turn the heap and water each layer as you go, so moisture is distributed through the whole mass rather than running down the outside.
3. Add green material to restore the balance.Dry summers produce a surplus of brown material — dry stems, straw, cardboard, spent crops — and greens are what carry both nitrogen and water. Grass clippings, kitchen waste, soft prunings, weeds before they seed and comfrey all restart a stalled heap. Mix them through rather than laying them on top, where they mat into a slimy layer.
4. Cover the heap.An old carpet, a sheet of cardboard, a tarpaulin or a proper lid keeps moisture in and stops the surface baking dry. This is the single easiest change and it also keeps winter rain from waterlogging the heap, so it is worth doing permanently.
5. Chop material smaller.Smaller pieces have more surface area, hold moisture better and break down faster. Dry woody summer prunings composted whole will still be there in two years. Shred, chop or run the mower over them.
6. Keep the heap in some shade if you can.A bin in full sun dries out from the sides and top continuously. A shaded position, or shading the sunny side with a board, holds moisture far better. It does not need to be cold — just not baking.
7. Use grass clippings properly.Clippings are the most useful summer ingredient and the most commonly misused. In a thick layer they mat into an airless slimy mass; mixed through the heap with brown material they are excellent, providing both nitrogen and a great deal of water.
8. Do not water it into a swamp.The failure in the other direction is just as real. A sodden airless heap goes anaerobic, smells of ammonia and rot, and stops working too. Wrung-out sponge is the target in both directions.
9. Be patient with a cold heap.A small domestic heap that never gets hot still composts, just slowly. Not reaching high temperatures is normal for most garden bins and is not a fault — it only matters if you are relying on heat to kill weed seeds and pathogens, which a domestic heap generally will not do.

Tips

• Ants in a compost bin are the most reliable dryness indicator there is. If you see them, it needs water.
• Save some dry brown material through the summer — cardboard, dry stems, shredded paper — because autumn brings a flood of green material and you will want it then.
• A heap built all at once composts much faster than one added to gradually, because it reaches a working volume immediately.
• Urine is an excellent free compost activator, providing nitrogen and moisture at once, and it is genuinely effective on a stalled dry heap.
• Two bins beats one: fill one while the other finishes, so you are never adding fresh material to a heap that is nearly ready.

Common mistakes

• Watering the top of a dry heap — Dry material sheds water down the outside. The heap has to be turned and watered layer by layer for moisture to reach the middle.
• Adding grass clippings in a thick layer — They mat into an airless slimy sheet that stops air and water passing through. Mixed through with brown material they are the best summer ingredient available.
• Leaving a heap uncovered in a dry summer — It dries continuously from the top and sides. A carpet, cardboard or tarpaulin over it costs nothing and keeps it working.
• Assuming a heap that does not heat up has failed — Most small domestic heaps compost cool and slow. That is normal and produces perfectly good compost — it simply does not kill weed seeds.

If it doesn't work

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