A Simple Guide to Composting Kitchen and Garden Waste

Why it is worth composting at all

Every UK household throws away a remarkable amount of stuff that could be quietly turning itself into next year's soil. Potato peelings, tea leaves, grass clippings, the stalky remains of a summer courgette plant — all of it is food for a compost heap. Get the mix roughly right and the heap does the work for you, no fancy equipment required.

The rewards are practical: a free, steady supply of dark crumbly compost for mulching beds, filling pots and topping up raised beds. It also saves you bin bags and trips to the tip, and it gives back to a garden that has spent the season giving to you. If you grow in a community plot, a shared heap is one of the best things a group of growers can set up together — everyone contributes waste, everyone takes away compost.

Greens and browns: the balance that does the work

Composting comes down to two ingredients. Greens are soft, wet and nitrogen-rich. Browns are dry, fibrous and carbon-rich. Microbes need both, and the trick is to aim for roughly two to three parts browns by volume to one part greens. Most failed heaps fail because they are drowning in greens.

• Greens: vegetable and fruit peelings, tea leaves and coffee grounds, grass clippings, annual weeds before they set seed, spent flowers, comfrey leaves, manure from vegetarian pets.
• Browns: torn cardboard and brown paper, newspaper, egg boxes, straw, dried autumn leaves, twiggy prunings, woodchip in small amounts, untreated sawdust, the cardboard tubes inside toilet rolls.
• Keep out: cooked food, meat, fish and dairy, perennial roots such as bindweed and couch grass, diseased plant material, dog or cat faeces, glossy magazines, and coal ash.

A note on teabags: many UK teabags still contain a plastic mesh that will not break down. Tear the bag open and compost the leaves, or switch to a plastic-free brand.

Choosing a spot and a container

A heap needs contact with the soil so worms and other creatures can move in. If you can, site your bin on bare earth in a level, partly shaded spot that is easy to reach in the rain. Full sun will dry it out.

You do not need anything expensive. A simple plastic bin with a lid and a hatch at the base works well in a small garden. If you have more room, a three-bay system built from pallets lets you turn material from one bay to the next as it matures — one bay filling, one cooking, one ready to use. For kitchen waste, keep a small lidded caddy by the sink. Line it with a couple of sheets of newspaper or a compostable liner to make emptying cleaner.

Air and moisture: the things most people forget

Once your heap is built, two conditions decide how quickly it breaks down. The first is air. Composting is an aerobic process, which means the microbes doing the work need oxygen. Add scrunched cardboard or a handful of twiggy prunings in layers rather than letting everything slump into a solid mass, and fork the pile over every two or three weeks if you have the energy.

The second is moisture. Squeeze a handful of the middle of the heap: it should feel like a wrung-out sponge, damp but not dripping. If it is bone dry, water it as you turn it. If it is sodden, mix in more browns and cover it with a lid or an old piece of carpet.

Get this right and a heap one metre cubed — the minimum size for serious heat — can reach 60°C or more in the middle. That is the sweet spot where weed seeds and many pests are killed off. A cooler heap still works perfectly well; it just takes longer, often six to twelve months rather than a few weeks.

When things go wrong

• Slimy and smelly, like rotten eggs: too wet and too many greens. Turn it, add browns, and leave the lid off for a day or two.
• Nothing is happening: too dry, or lacking greens. Add grass clippings or peelings and water as you mix.
• Clouds of tiny flies: bury kitchen waste under a layer of browns rather than leaving it on the surface.
• Rats or mice: usually a sign of cooked food or a bin with a broken base. Stick to raw peelings and keep the lid closed.
• Big woody lumps that never rot: chop prunings smaller, or leave them out and use them as a mulch under shrubs instead.

Knowing when it is ready — and what to do with it

Your compost is finished when it looks like dark, crumbly soil with an earthy, woodland smell and you can no longer recognise what went in. If you can still see eggshells or twigs, that is fine — sieve them out and throw them back into the next heap.

Use it as a mulch around shrubs and perennials, spread it on vegetable beds in autumn, or mix it into the soil when planting. For seed sowing, sieve it and blend it with loam and grit so it is not too rich. If you garden on a community plot, agree a simple rota for turning and emptying the bays — shared heaps work best when everyone knows what goes in and what comes out.