What belongs on your compost heap
A compost heap takes brown materials (carbon-rich) and green materials (nitrogen-rich) and turns them into dark, crumbly soil you can use in gardens and pots. Browns include dry leaves, shredded paper, cardboard, and wood chips. Greens include grass clippings, vegetable scraps, coffee grounds, and plant trimmings. The heap works best when you layer them — roughly three parts brown to one part green — and keep it damp like a wrung-out sponge.
Kitchen scraps that break down well are vegetable and fruit peels, cores, seeds, and flesh; crushed eggshells; tea bags and loose tea leaves; and coffee grounds with their filters. Garden waste that composts quickly includes grass clippings (if the lawn has not been treated with herbicide), plant stems and leaves, deadheaded flowers, and hedge trimmings chopped into short pieces. Paper and cardboard work too — shredded newspaper, cardboard boxes torn into strips, and paper bags all decompose, though glossy or coloured paper should stay out.
Key Takeaways
- Greens (grass, vegetable scraps, coffee grounds) and browns (leaves, cardboard, wood chips) need to be layered roughly three to one for the heap to heat up and break down.
- Meat, fish, dairy, oils, and cooked food attract rats and create smell, so keep them out entirely.
- Diseased plants, weeds with seeds, and grass treated with herbicide or pesticide can survive composting and spread problems to your garden.
- Wood ash from a fireplace can go on in small amounts, but coal ash and charcoal ash should never be added.
- Compost heaps take three months to two years depending on size, moisture, layering, and whether you turn the pile.
Never compost meat, fish, dairy, or cooked food
These materials attract rats, foxes, and other animals that will dig into your heap and scatter it across the garden. They also create strong odours as they rot. Meat and fish take months to break down and can harbour harmful bacteria during that time. Dairy products like milk, cheese, and yoghurt also draw pests and smell as they decompose.
Cooked food — even vegetable scraps that have been heated — should stay off the heap. The combination of fats, oils, and proteins in cooked meals creates the same pest and smell problems as raw meat. If you want to compost food waste, stick to raw vegetable and fruit scraps only.
Keep out diseased plants and weeds with seeds
A home compost heap does not get hot enough to kill all plant diseases and weed seeds. If you add a plant infected with powdery mildew, rust, or blight, those spores can survive the composting process and end up back in your garden soil when you use the finished compost. The same applies to weeds that have already set seed — the seeds often survive and germinate in your beds and pots.
If a plant is visibly diseased or a weed is flowering or has seed heads, bin it instead. Healthy plant material and weeds you pull before they flower are safe to add. If you are unsure whether a plant is diseased, err on the side of caution and throw it away.
Avoid treated grass, leaves, and wood
Grass clippings are excellent compost material — but only if the lawn has not been treated with herbicide or pesticide in the last few weeks. These chemicals can survive composting and end up in your soil, where they may harm plants or linger in vegetables you grow. If you use a lawn treatment, ask the product label how long to wait before composting clippings, or assume at least four weeks.
The same rule applies to leaves raked from under treated trees or shrubs. Wood chips and sawdust are fine if they come from untreated wood, but avoid wood that has been painted, stained, or pressure-treated — these contain chemicals that do not break down in a home heap and can contaminate your soil.
Wood ash is okay in small amounts; coal and charcoal ash are not
Wood ash from a fireplace or wood stove can go on your compost heap in thin layers — sprinkle it as you would sprinkle brown material, rather than dumping it in. Wood ash adds potassium and other minerals that plants need. However, do not add it if your soil is already alkaline, because ash raises pH further.
Coal ash and charcoal ash must never go on a compost heap. Coal ash contains heavy metals and sulphur compounds that do not break down and can poison soil. Charcoal briquettes used in barbecues are treated with chemicals and should be kept off the heap entirely.
Glossy paper, synthetic materials, and pet waste do not belong
Newspaper, cardboard, and plain paper break down in a compost heap, but glossy or colour-printed paper should stay out — the inks and coatings do not decompose and can leave residue in your finished compost. Magazines, junk mail with colour printing, and laminated cardboard all fall into this category. Shredded office paper is fine as long as it is plain black and white.
Synthetic materials like plastic, rubber, and treated fabrics will not break down in a home heap, no matter how long you leave them. Pet waste — including cat litter and dog faeces — should never be composted because it can carry parasites and pathogens that survive the composting process and contaminate your soil and any food you grow.
How long compost takes and how to speed it up
A compost heap left alone takes anywhere from one to two years to become finished compost. A well-maintained heap — one that is layered correctly, kept damp, and turned every few weeks — can be ready in three to six months. Turning the pile mixes the materials, introduces oxygen, and speeds decomposition. If you do not turn it, decomposition happens more slowly but still happens.
Size matters too. A heap smaller than one cubic metre (roughly three feet by three feet by three feet) does not generate enough heat to decompose quickly. A larger heap heats up faster and breaks down material more efficiently. Chopping or shredding materials before adding them also speeds the process — smaller pieces have more surface area for microbes to work on.
Frequently Asked Questions
Can I compost citrus peels and onion skins?
Yes, citrus peels, onion skins, and garlic skins all compost fine. They break down slowly because they are tough, but they will decompose eventually. Chop them into smaller pieces to speed the process. Some gardeners avoid large amounts of citrus because the acidity can slow decomposition slightly, but normal kitchen quantities are not a problem.
What about bread, pasta, and rice?
Cooked bread, pasta, and rice should not go on the heap because they attract rats and can go mouldy before decomposing. Raw flour and uncooked grains are also best avoided — they draw pests. If you want to compost grain-based food waste, it is safer to bury it deep in the centre of a large, well-maintained heap where pests cannot easily reach it.
Can I put sawdust from my workshop on the compost?
Sawdust from untreated wood is fine and counts as a brown material. Sawdust from treated, painted, or stained wood should stay off. If you are unsure whether the wood was treated, do not add it. Sawdust is very carbon-rich, so use it sparingly — too much will slow decomposition because there is not enough nitrogen to balance it.
Is it okay to add weeds I have pulled up?
Yes, as long as they have not flowered or set seed. Once a weed has flowers or seed heads, the seeds can survive composting and spread back into your garden. Pull weeds before they reach that stage and they are safe to add. Young, tender weeds break down faster than mature ones.
Can I compost pine needles and conifer clippings?
Pine needles and conifer trimmings decompose very slowly because they are tough and acidic. They can go on the heap, but they will take longer to break down than other materials. If you have a lot of them, it is better to use them as mulch around acid-loving plants like rhododendrons, or bag them for garden waste collection.