Making compost: a simple guide for UK gardens

Making compost is simply giving garden and kitchen waste the right conditions to rot: a balance of soft, nitrogen-rich “greens” and dry, carbon-rich “browns”, enough moisture and some air. Get that mix roughly right in a bin or bay and microbes, worms and other small creatures do the rest. In most UK gardens you can expect crumbly, usable compost in six months to a year, sooner if you turn it regularly.

Home-made compost is the single best thing you can add to your soil. It feeds the soil life, improves drainage on clay, helps sandy soil hold water and gives you a free mulch for borders and vegetable beds. It also keeps a lot of material out of the council green-waste lorry.

Choosing a bin and a spot

Any container works as long as it holds the heap together and keeps it warm and damp. Plastic “dalek” bins suit small gardens. If you have more space, two or three timber or pallet bays side by side are ideal: fill one, let it rot while you fill the next, then turn the first into an empty bay. Each bay is best at least a metre each way, because a bigger heap holds heat and rots faster.

Stand the bin directly on soil rather than paving, so worms can move in and excess water can drain away. A spot in light shade is best. Full sun dries the heap out in summer, and deep, cold shade slows it down. Put it somewhere you can reach easily with a wheelbarrow and a full caddy in the rain, or you will stop using it.

What to put in: greens and browns

Greens are wet, soft and quick to rot. They include grass clippings, vegetable peelings, fruit scraps, tea leaves and paper tea bags, coffee grounds, spent bedding plants, young weeds without seed heads, and soft prunings. Clippings from summer-pruned shrubs such as lavender can go in if they are chopped small.

Browns are dry, tough and slow. Think shredded or torn plain cardboard, egg boxes, scrunched newspaper, straw, dry autumn leaves, sawdust from untreated wood, and woody prunings that have been cut or shredded small. Fallen leaves from trees such as silver birch are a free supply each autumn, though large quantities of leaves are better stacked separately to make leaf mould.

Aim for roughly equal volumes of each, mixed rather than in thick layers. A heap that is mostly grass clippings will turn slimy and smell. A heap that is mostly cardboard and twigs will sit there for years. If you are adding a big load of one, mix in some of the other at the same time.

What to keep out

Some things cause trouble. Leave out meat, fish, dairy and cooked food, which attract rats and flies. Leave out cat and dog waste. Avoid glossy or coated paper and anything treated with chemicals.

Be careful with diseased plants. Most home heaps do not get hot enough to kill disease spores reliably. Do not compost roots affected by clubroot; the spores can survive in soil for many years and you would spread them around the whole plot. Potato and tomato foliage with blight is a debated case: the disease needs living tissue to survive, but infected tubers left in the heap can sprout and carry it over, so bin any blighted potatoes. Roots and wood infected with honey fungus belong in green waste or should be burnt where allowed, not in your heap.

Perennial weeds such as bindweed, couch grass and ground elder roots can regrow from tiny fragments. Drown them first in a bucket of water with a lid for a few weeks until they turn to a brown sludge, then add them. Weeds that have set seed are best left out unless you run a very hot heap.

Building and looking after the heap

Start with a layer of coarse, twiggy stems at the bottom to let air in and water out. Then add material as you get it, mixing greens and browns. Chop bulky things such as cabbage stalks, sunflower stems and courgette plants into short lengths with a spade; small pieces rot much faster.

Moisture matters more than most people realise. The heap should feel like a wrung-out sponge. If it is dry and nothing seems to be happening, water it as you add layers. If it is wet and smelly, add more browns. A lid or a piece of old carpet keeps rain off in winter and holds in warmth.

Turning is optional but speeds things up enormously. Every few weeks, fork the heap out and put it back, or move it into the next bay, bringing the drier outer material into the centre. A freshly turned heap that is well mixed will often feel warm to the touch within a day or two.

How long does it take?

In the UK it depends heavily on the season and how much effort you put in. A heap built in late spring and turned a few times can be ready by autumn. A “cold” heap that you just keep adding to and never turn might take a year or more. Rotting slows almost to a stop in a cold winter and speeds up again as the weather warms.

Compost is ready when it is dark brown, crumbly and smells pleasantly earthy, with most of the original ingredients no longer recognisable. The bottom of a bin is usually ready first. A few twigs or eggshells are fine; sieve them out or throw them back into the next heap.

Using your compost

Spread it as a 5cm mulch over borders in early spring or autumn, around fruit bushes such as blackcurrants, and on vegetable beds before sowing. Hungry crops like courgettes and potatoes love a generous helping dug into the planting area. Roses and other shrubs benefit from a layer after their spring prune; see pruning basics for the timings.

Garden compost is usually too rich and variable to use on its own for seed sowing. For that, see our guide to sowing seeds indoors, which explains what to use instead. The October garden jobs page is a good reminder to empty and restart your bins as the leaves come down.

Common mistakes

Too much grass at once. A thick layer of mowings turns into a smelly, airless mat. Mix each load with cardboard or straw.
Letting it dry out. A dry heap does nothing. Water it in summer.
Never emptying the bin. The finished compost at the bottom is the bit you want. Lift the bin or open the hatch once or twice a year.
Adding cooked food. It draws rats. Stick to raw peelings.
Expecting it to be fast without turning. Patience or a fork, one or the other.

Troubleshooting

It smells of rotten eggs or ammonia. Too wet and too many greens. Fork in plenty of torn cardboard and turn it to let air in.

Nothing is happening. Probably too dry or too many browns. Add greens such as grass clippings, water it and turn.

There are flies. Fruit flies are harmless but annoying. Bury fresh kitchen scraps in the middle of the heap and cover with a layer of browns.

Rats have moved in. Stop adding food scraps for a while, keep the heap moist (rats prefer it dry), and stand the bin on fine wire mesh to block tunnels.

It is full of woodlice, worms and slugs. That is normal and a sign the heap is working. Slugs in compost are mostly breaking down waste, though you may want to check mulch near seedlings; see slugs and snails for ways to protect young plants.