Summer is the compost heap's busiest season, and the season when it most needs attention. The garden produces more material in summer than at any other time — grass clippings, prunings, spent crops, weeds, kitchen scraps — and the heat that makes the garden grow also makes a compost heap work fast, provided it has the right balance of materials, enough moisture, and enough air. A heap left to itself in summer will usually dry out, slow down, or turn slimy and smelly, while a heap given a little regular care will produce usable compost by autumn. This guide covers what to compost in summer, how to keep the heap balanced and moist, and how to fix the common problems. Check your local extension service for any local rules on composting.
What goes on the heap in summer
Summer gives you the two things a compost heap needs in abundance: green material, rich in nitrogen, and brown material, rich in carbon. Green material includes grass clippings, soft prunings, spent vegetable plants, weeds before they seed, and kitchen scraps such as vegetable peelings and coffee grounds. Brown material includes straw, shredded cardboard, dry leaves saved from autumn, woody prunings chopped small, and shredded paper. The heap needs both, in roughly the right balance, and summer is the season when green material is so plentiful that the balance tips too far towards green — which is the most common cause of a smelly heap.
Add material in layers, mixing green and brown as you go, rather than dumping a large amount of one thing at once. A whole barrow of grass clippings added in one go will collapse into a slimy, airless mat, while the same clippings mixed with shredded cardboard or straw will break down cleanly. Chop or shred woody material before adding it, because thick stems take a long time to break down, and tear cardboard into small pieces so it does not form wet sheets. The compost ratio calculator can help you get the balance right.
Water, air and heat in summer
A compost heap in summer needs water. The heat and the dry air dry the heap out, and a dry heap stops working — the microbes that break material down need moisture to live. A heap should feel like a wrung-out sponge: damp all the way through, but not dripping. Water it when you add dry brown material, and check it weekly in hot weather by pushing a hand into the centre; if it feels dry, water it thoroughly and cover it to hold the moisture in. A lid, a piece of old carpet, or a sheet of plastic over the top keeps both moisture and heat in.
Air matters as much as water. The microbes that make good compost need oxygen, and a heap that is compacted and airless will turn smelly and slimy instead. Turn the heap every few weeks in summer with a fork, moving the outside material into the centre where it is hottest, and push a few air holes into it with a broom handle or a garden fork if you cannot turn the whole heap. A heap that is hot in the centre — too hot to hold your hand in comfortably — is working well, and the heat kills weed seeds and disease, which is one of the main benefits of a hot heap.
What not to compost
Some things should never go on a home compost heap. Diseased plants, perennial weed roots and weeds that have set seed should go in the green waste bin, because a home heap rarely gets hot enough, for long enough, to kill them reliably. Meat, fish, dairy and cooked food attract rats and other pests and should not go on an open heap, though they can go in a sealed hot composter or a wormery designed for them. Woody material thicker than a finger takes a long time to break down and is better shredded or used as a mulch or a habitat pile.
Avoid adding large amounts of any single material, and avoid material treated with weedkiller or pesticide, which can survive composting and damage plants when the compost is used. Ash from a wood fire can be added in small amounts, but not in large quantities, and never add ash from treated wood or coal. If you are unsure about a material, leave it out — a heap with a small amount of safe material is far better than a heap contaminated with something that ruins the whole batch.
Fixing a heap that has gone wrong
A smelly heap is usually too wet, too green, or too airless. Fix it by turning it, adding dry brown material such as shredded cardboard or straw, and leaving the cover off for a day or two so it can dry out a little. A heap that smells of ammonia has too much green material, especially grass clippings, and needs brown material and turning. A heap that is dry and not breaking down at all needs water and more green material, and a good turning to mix it through.
A heap full of flies is usually a heap with food scraps too close to the surface; bury kitchen scraps under a layer of brown material, and cover the heap. A heap that is not heating up may be too small — a heap needs a certain volume to hold its heat — or too dry, or short of green material. Add green material, water it, and give it a week; if it still does not heat up, it will still break down, just more slowly, and slow compost is still good compost. For more on using compost, see our summer mulching guide and summer soil preparation.
Using summer compost
Compost made over the summer is often ready by autumn, and the test is simple: it should be dark, crumbly, and smell of earth, with no recognisable pieces of the original material left in it. Use it as a mulch around plants, fork it into beds before autumn planting, or spread it over the vegetable garden as a winter cover. Compost that is not quite finished can still be used as a mulch, where it will carry on breaking down on the surface, but it should not be dug into the soil or used in seed trays, where unfinished compost can rob nitrogen and harm seedlings.
Keep the cycle going: as soon as one heap is finished, start the next one, because the garden never stops producing material. A garden with two or three heaps or bins — one being filled, one breaking down, one ready to use — will never be short of compost, and it will never have to buy in what it can make for itself. Compost is the garden's own fertiliser, its own mulch and its own soil improver, and a summer spent keeping the heap working is a summer invested in every season that follows.
A summer composting checklist
Add material in layers, mixing green and brown, and chop woody material small. Water the heap when you add dry material, and check its moisture weekly in hot weather. Turn it every few weeks, keep it covered to hold heat and moisture, and bury kitchen scraps under brown material. Keep diseased material, seeded weeds and meat out of the heap, and note what you added and when, so you know when the heap should be ready.
Summer composting is a habit, not a project: a few minutes a week, a fork, a watering can and a supply of brown material are all it takes. The reward is a heap that works through the hottest months and a garden that feeds itself from its own waste. For the wider summer routine, read our summer mulching guide and browse the guides hub and the planting calendar for the full seasonal series.
Frequently asked questions
Why does my compost heap smell in summer?
A smelly heap is usually too wet, too green, or too airless — often all three, from a large amount of grass clippings added at once. Turn it, add dry brown material such as shredded cardboard or straw, and leave the cover off for a day or two so it can dry out.
How often should I water a compost heap in summer?
Check it weekly by pushing a hand into the centre: it should feel like a wrung-out sponge, damp but not dripping. Water it when you add dry brown material, and keep it covered to hold moisture and heat in.
Can I compost grass clippings in summer?
Yes, but mix them with brown material and add them in thin layers rather than a large amount at once. A barrow of clippings dumped in one go collapses into a slimy, airless mat; the same clippings mixed with shredded cardboard break down cleanly.
What should never go on a compost heap?
Diseased plants, perennial weed roots, weeds that have set seed, meat, fish and dairy, and material treated with weedkiller. Put diseased material and seeded weeds in the green waste bin, because a home heap rarely gets hot enough to kill them.
When is compost ready to use?
When it is dark, crumbly and smells of earth, with no recognisable pieces of the original material left. Use finished compost as a mulch or forked into beds; unfinished compost can be used as a surface mulch but should not be dug in or used in seed trays.