How to Build a Compost Heap That Heats Up

Why a Heap That Heats Up Is Worth the Effort

A cold compost heap will still break down eventually, but it will take a year or more and it will attract slugs, foxes and the neighbour's curious cat along the way. A hot heap is a different creature altogether. When the conditions are right, the bacteria and fungi working away inside it generate real, measurable warmth — often between 55°C and 65°C at the core — and that heat does three useful things at once. It speeds decomposition so you get usable compost in eight to twelve weeks rather than twelve months, it kills many weed seeds and plant pathogens, and it signals loud and clear that your balance of materials is right.

If you have ever shoved grass clippings into a bin and watched them turn into a slimy, smelly mess, you already know that heat is not automatic. It is a product of getting several simple things right at the same time. Here is how to do it.

The Greens and Browns Balance

Everything you compost falls into one of two camps. Greens are wet, nitrogen-rich materials: grass clippings, kitchen vegetable peelings, coffee grounds, tea leaves, comfrey leaves, annual weeds before they seed, and fresh manure from vegetarian animals. Browns are dry, carbon-rich materials: cardboard, newspaper, straw, autumn leaves, woodchip, sawdust, shredded paper and dead plant stems.

The ratio that reliably heats up is roughly two to three parts browns to one part greens by volume. Volume matters more than weight here — a bucket of dry leaves is mostly air, while a bucket of grass clippings is mostly water.

If your heap is too green, it goes anaerobic: wet, sour-smelling and slow. If it is too brown, it sits there doing nothing for months. When in doubt, add more browns. It is almost impossible to have too many.

Building the Heap in Layers

A heap needs to be at least one cubic metre to hold heat properly — a metre wide, a metre tall, a metre deep. Anything smaller loses warmth to the surrounding air faster than the microbes can generate it.

Build it like a rough lasagne:

• A 10cm base of coarse twiggy prunings or woodchip to let air in from below.
• Alternate 15cm layers of greens and 30cm layers of browns.
• Scatter a handful of soil or finished compost every few layers to introduce the right microbes.
• Water each layer lightly as you go if the materials are dry.
• Finish with a brown layer on top to seal in moisture and keep flies away.

Chop everything smaller than your thumb as you build. Surface area is everything to decomposers, and a heap of intact cabbage stalks will sulk for months.

Turning, Air and the Heat Cycle

Within three or four days of building, push a metal bar or old broom handle into the centre and leave it for ten minutes. When you pull it out, it should feel warm — sometimes genuinely hot — to the touch. That is your heap working.

After a week or so the temperature will start to drop as the microbes exhaust the easiest food and the oxygen in the core. This is your cue to turn the heap. Fork the outside edges into the middle and the middle out to the edges, rebuilding it roughly as it was.

Turn every two to three weeks for the first two months, then monthly until it stops heating. Each turn should give you another heat spike. If turning produces no warmth at all, the problem is almost always one of three things: too dry, too wet, or too little nitrogen.

Moisture: As Damp as a Wrung-Out Sponge

This is the single most common reason a heap fails to heat. Microbes need water, but they also need air, and those two needs pull in opposite directions.

The test is simple: squeeze a handful of material from the middle of the heap. It should feel like a wrung-out sponge — moist enough that a few drops appear on your palm, but not so wet that water streams out. If it drips freely, the gaps between materials are filled with water and the heap is going anaerobic. If it crumbles dryly, everything has stalled.

To fix a wet heap, turn it thoroughly and mix in dry browns. To fix a dry one, water it as you turn, layer by layer, until the whole mass feels like that sponge. In a dry summer you may need to water a heap weekly. In a wet winter, cover it with a tarpaulin or an old carpet to stop it becoming a bog.

What to Add and What to Leave Out

Keep the heap well-fed and it will keep working. Good additions include vegetable and fruit scraps, tea bags, coffee grounds, eggshells crushed up, shredded cardboard and paper, grass clippings in thin layers, and spent plants from the veg patch.

Leave out cooked food, meat, fish, dairy and dog or cat waste, which attract rats and smell unpleasant. Avoid perennial weeds such as bindweed, ground elder and couch grass roots unless you are confident your heap reaches a genuine 60°C, and never add diseased plant material or anything treated with weedkiller. Glossy magazines, plastic tea bags and the stickers from bananas all belong in the bin.

Get these basics right and you will know within a week whether your heap has caught. That first warm fistful of half-finished compost, dark and sweet-smelling, is one of the small, unglamorous pleasures of a productive garden — and it costs you nothing but a bit of attention.