Hot or cold composting
Gardeners often talk about hot and cold composting as if they were two different crafts. They are really two ends of one range. Both rely on the same organisms eating the same greens and browns. The difference is how much material you gather at once and how actively you manage it. Hot composting asks for more work and gives you compost sooner. Cold composting asks for very little and gives you compost later. Neither is the right answer for everyone.
What makes a heap hot
A heap heats up when a large mass of well mixed material, with enough moisture and air, feeds a fast growing population of organisms all at once. Their activity releases heat, and a big enough heap holds that heat in its centre. To get there, gardeners usually build the whole heap in one go rather than adding a little at a time, chop or shred material so it breaks down evenly, keep the mix of greens and browns balanced, keep it as damp as a wrung out sponge, and turn it every few days to a week so the cooler outer material moves into the centre and air reaches every part.
A hot heap feels clearly warm when you push a hand into it, and gardeners who use a compost thermometer often look for the centre to stay well above the outside air temperature for a few days at a time. Extension services that publish home composting guidance generally describe a working hot heap as reaching temperatures in the range that kills many weed seeds and plant diseases, which is one of the main reasons people choose it.
What a cold heap looks like
A cold heap is built gradually. You add kitchen scraps and garden material as they come, mix in browns, keep it from drying out, and otherwise leave it alone. It never heats up much because the material arrives in small amounts and is never all fresh at once. Over many months the same organisms, along with worms and other larger creatures, turn it into compost from the bottom up.
Cold composting is far less work. It fits around a busy life, and it suits people who compost steadily rather than in big batches. Its drawbacks are time and thoroughness. It can take a year or more, and because it never gets hot, weed seeds and plant diseases can survive it.
Comparing the two
The practical differences come down to a few questions. How quickly do you want compost? A hot heap can be ready in a few months, while a cold heap is slower. How much material do you have at once? Hot composting needs a large batch, which is easy in autumn and hard in a small kitchen. How much time can you give? Turning a hot heap regularly is real work, and skipping it lets the heap cool. What are you putting in? If you want to compost weeds that have set seed or plants with disease, a hot heap gives you a better chance of killing them, while a cold heap should not get that material at all.
A middle way
Many gardeners end up somewhere between the two. They add material as it comes, like a cold heap, but turn it now and then and keep an eye on moisture and the mix. That warms the heap a little, speeds it up and keeps smells down, without the commitment of turning every few days. A three-bay system makes this easy, because each bay can be at a different stage.
Signs it is working
Whichever method you choose, the signs of a healthy heap are the same. The material shrinks. It smells mild and earthy, not sour. It darkens over time and the original ingredients become harder to recognise. When a heap stops showing those signs, the cause is almost always one of four things: too dry, too wet, too little air, or the wrong mix. Our page on fixing common compost problems goes through each one.
Is it finished?
Finished compost is dark, crumbly and smells like a forest floor. You should not be able to pick out most of the original material, apart from a few woody pieces that you can sieve out and put back in the heap. A hot heap that has just cooled may look finished before it is. Giving it a few more weeks to rest and cure lets the last stages run their course, and compost that has cured is gentler on young plants. Our page on using finished compost explains how to test it.