What can I put in a compost bin, and what should never go in?

Most compost advice reads as a long list of prohibitions, and it puts people off. In practice the rule is simple. Greens are wet and nitrogen-rich, browns are dry and carbon-rich, you need roughly equal volumes of each, and the exclusions are about vermin, disease and the law rather than about chemistry.

Safety

Step by step

1. Add greens: the wet, nitrogen-rich half.Raw vegetable and fruit peelings, grass clippings, soft prunings, annual weeds without seed heads, coffee grounds, tea leaves, spent flowers, nettles and comfrey. These rot fast and heat the heap.
2. Add browns: the dry, carbon-rich half.Torn cardboard, shredded paper, egg boxes, dry leaves, straw, wood shavings from untreated timber, small woody prunings, dead stems. These provide structure, air and the carbon the process runs on.
3. Know the ones that surprise people, in the fine list.Hair and nail clippings, vacuum cleaner dust from wool or cotton carpets, natural fibre string, wood ash in small quantities, hamster and rabbit bedding from herbivores, cut flowers, and untreated sawdust in moderation.
4. Handle teabags and packaging carefully.Many teabags contain plastic in the seal and leave a mesh skeleton behind. Split them and compost the leaves, or check the box. Compostable packaging is usually certified for industrial composting only and will not break down in a garden heap.
5. Keep cooked food out of an open heap.Cooked food, meat, fish, dairy, bread and oil attract rats and go slimy. A sealed bokashi system or an in-vessel hot bin can take them; a dalek or an open bay cannot.
6. Never compost cat or dog waste.It carries parasites, including toxoplasma and roundworm, that a garden heap does not get hot enough to kill. Herbivore pet waste and bedding are fine.
7. Keep diseased material out.Blighted potato and tomato haulm, clubroot-infected brassica roots, white rot on onions, honey fungus, rose black spot on a serious scale. A cold heap preserves these and you spread them next spring.
8. Keep perennial weed roots and seed heads out of a cold heap.Bindweed, couch grass, ground elder, dandelion and creeping buttercup regrow from fragments. Weeds already in seed leave you a seed bank. Drown them in a bucket of water for several weeks first, then compost the sludge.
9. Never compost invasive listed species.Japanese knotweed and giant hogweed among them. Causing them to spread is a legal matter as well as an ecological one, and they go through a licensed disposal route rather than into a heap or a green bin.
10. Keep glossy paper, ash from coal and treated timber out.Glossy and laminated paper does not break down. Coal ash and barbecue briquette ash carry sulphur and heavy metals. Treated, painted or preserved timber carries preservatives into your soil.
11. Tear, chop and mix as you go.Whole flattened cardboard boxes form airtight mats, and whole cabbage stalks sit unchanged for a year. Everything breaks down faster in small pieces mixed together.
12. Balance by volume, and keep browns to hand.Greens arrive daily from the kitchen; browns do not. A sack of torn cardboard beside the bin is what stops a heap going slimy.

Tips

• Citrus peel, onion skins and rhubarb leaves are all perfectly fine in a garden heap, whatever you have read.
• Wood ash in thin dustings is a useful potash source; in quantity it turns the heap alkaline and claggy.
• Weeds in a bucket of water with a lid on turn into a foul but excellent liquid feed within a month, and the residue is safe to compost.

Common mistakes

• Adding only kitchen greens — It goes slimy, sour and anaerobic within weeks. Every bucket of peelings needs a comparable volume of torn cardboard or dry leaves.
• Composting compostable packaging — Almost all of it is certified for industrial composting at temperatures a garden heap never reaches. It comes out looking almost unchanged a year later.
• Putting blighted tomato and potato tops on the heap — You reintroduce it to the same beds next year. Bin it, or burn it where that is permitted locally.

Questions people ask