What Can You Compost? The Complete Yes, No and Maybe List

How good compost actually works

Before sorting anything, it helps to understand why some materials work better than others. Organic material falls roughly into two groups: green, nitrogen-rich material and brown, carbon-rich material. A healthy pile needs both to turn into crumbly, dark compost instead of a sour, matted mess.

Green material is moist and breaks down quickly — think food scraps, grass clippings and fresh plant trimmings. It supplies nitrogen and moisture, but too much of it without something dry mixed in leaves the pile airless, dense and prone to smelling like ammonia or rot.

Brown material is dry and breaks down slowly — dry leaves, cardboard, straw and egg cartons are typical examples. It adds structure, holds air pockets and soaks up excess moisture from the green side. Too much brown material with no green nearly stalls the whole process, since there isn't enough moisture or nitrogen to feed the microbes.

Green vs. brown material at a glance

You don't need to weigh anything precisely. A simple rule of thumb: for every bucket of food scraps or grass clippings you add, have a roughly equal amount of dry, brown material ready to mix in at the same time or shortly after. If the pile looks wet and dense, add more brown. If it's breaking down too slowly and feels dry, add more green material or a light watering.

The breakdown itself is handled by bacteria, fungi and worms that are already present in the soil and find their own way into the pile. A well-run heap gets noticeably warm in the middle during summer, often much warmer than the air around it, because the microbes are working hard. That's also the real difference between hot and cold composting: an insulated, enclosed bin holds heat and works faster, while an open backyard pile runs on its own schedule and simply takes longer.

What you have the most of also shifts with the season. Summer tends to bring grass clippings, food scraps and spent flowers — mostly green. Autumn swings the other way, with dry leaves and woody stems dominating — mostly brown. It's worth bagging up a sack of dry autumn leaves or shredded cardboard and keeping it near the bin, so you always have brown material on hand once green waste starts piling up again in spring.

Good to know. Keep a bag of dry leaves or torn-up cardboard near the compost bin through autumn. That way you'll always have brown material ready to mix in when food scraps and grass clippings dominate through spring and summer.

What kitchen scraps can you compost?

Most of what's left over from cooking and eating can go straight in the bin, as long as it doesn't contain meat, fish or dairy. Here's what works well:

• Fruit and vegetable scraps. Peels, cores, stems and trimmings break down quickly and add good moisture.
• Eggshells. Crush them if you can — whole shells break down far more slowly, though they won't harm the pile as they are.
• Coffee grounds and plain paper filters. One of the best nitrogen-rich materials you'll have in the kitchen, and most paper filters compost right along with them.
• Tea leaves and plastic-free tea bags. Check the bag first — many modern tea bags contain a nearly invisible plastic mesh that never breaks down.
• Bread, in small amounts. It composts fine, but larger quantities left uncovered attract pests fast.
• Potato and root vegetable peels. Completely ordinary compostable material.
• Wilted cut flowers from the kitchen table. Once they've faded, they go in just like any other plant material.
• Nut shells, such as walnut or hazelnut. Crush them if possible — whole shells can take years to break down.
• Shellfish shells, like shrimp or crab. A good nitrogen source, but they can attract gulls and other scavengers outdoors, so bury them well.

Coffee grounds deserve a special mention. They're one of the most nitrogen-dense materials you'll typically have on hand, and the coarse texture helps keep the pile airy while still feeding it. Eggshells break down slowly but add calcium to the finished compost, which many vegetables — tomatoes included — appreciate.

Citrus peel and onions are the two ingredients that come up most in composting debates. Both are fine in ordinary household amounts in a regular backyard pile. The myth that they're harmful mostly comes from worm bins specifically, where concentrated acids and sulphur compounds can bother the worms — it doesn't apply to a standard garden compost heap.

Good to know. Cut citrus peel into smaller pieces before composting. Whole peels have a waxy layer that makes them break down noticeably slower than most other kitchen scraps.

What yard waste can you compost?

Yard waste often fills the bin fastest during the growing season. Most healthy plant material works, but size and cleanliness affect how quickly it actually breaks down.

• Grass clippings. Add them in thin layers and mix in dry material right away — a thick mat of clippings compacts into an airless, smelly layer instead of breaking down.
• Leaves. Shredded leaves break down faster than whole ones. Large autumn hauls are also worth composting separately into leaf mould.
• Weeds without mature seed heads. Pull weeds before they've gone to seed and they compost like any other green material.
• Spent flowers and old stems. Cut back perennials in autumn and add them directly, chopped up if the stems are woody.
• Twigs and branches, chipped or cut small. Whole, thick branches take a very long time — chip or chop them into short pieces so they keep pace with the rest of the pile.
• Pine needles, in modest amounts. They acidify the soil slightly, but mixed in with other material in normal quantities that's not an issue.

The material that needs extra thought is anything that spreads by root fragments, like couch grass or ground elder, along with weeds that have already gone to seed. Both can survive an ordinary compost pile and sprout again once you spread the finished compost. More on that under what to avoid completely, further down.

Season matters too. Cutting back perennials and shrubs in autumn usually produces woodier, drier trimmings than what you prune during summer, which makes them more of a brown material than a green one. Needles from a Christmas tree or hedge trimmings work as well, but acidify the soil slightly — mix them in with other material rather than piling on a thick layer of pure needles.

Paper, cardboard and other household items

Quite a lot of what you'd otherwise put in the recycling or the bin actually works well as brown material in the compost.

• Plain, unbleached paper and newspaper. Tear it into pieces so it breaks down faster and mixes in more easily.
• Egg cartons and corrugated cardboard. Remove any plastic tape or printed plastic labels first.
• Paper bags and unscented paper towels. Plain brown paper is fine; skip heavily scented or chemically treated varieties.
• Cotton and other natural-fibre fabric, cut into pieces. Pure cotton or linen breaks down eventually; fabric blends with synthetic fibres don't.
• Hair and nail clippings. Unusual, but they break down and actually add nitrogen.
• Sawdust and untreated wood shavings. Good brown material in modest amounts, mixed well so it doesn't clump.
• Straw and hay. Classic structural material that keeps a pile airy.
• Ash from clean wood fires, in small amounts. One small handful at a time, spread thinly — too much raises the pH sharply.

Paper and cardboard are especially useful to keep on hand because they're dry, easy to tear up, and almost always available at home. They give the pile structure and airflow in a way that's hard to achieve with kitchen and yard waste alone. Skip thermal-paper receipts — they're often coated with chemicals that have no place in growing soil, even though they look like ordinary paper.

If you keep rabbits, guinea pigs or other plant-eating small pets, their bedding and droppings compost without any issue, since it's essentially just hay, straw and broken-down plant material. That's a completely different situation from dog and cat waste, which comes from meat-eating animals and can carry parasites that survive a standard backyard pile. The difference comes down to what the animal eats, not the fact that it's animal waste in general.

The grey zone: what to compost with caution

These materials work, but only in the right amount or handled a certain way. It's worth thinking these through rather than applying a blanket rule.

Grey-zone materials — what to keep in mind

Pizza boxes are a classic grey-zone case. The clean, dry parts of the box compost just like any other corrugated cardboard, but sections soaked through with cheese and oil should stay out. Tear off and bin the greasy part, and compost only what's still dry and clean.

Plant-based milk cartons — oat or soy milk, for instance — don't belong here at all, even though the contents themselves are compostable. The carton is lined with a thin layer of plastic that never breaks down, so the packaging should go in with regular cardboard recycling, not the compost.

Common mistake. Leaving food scraps — cooked or raw — sitting on top of the pile instead of burying them. No matter how harmless the contents are, visible food attracts rats and other pests fast. Always dig it into the pile.

What should you never compost?

Some things don't belong in an ordinary backyard compost pile no matter how much brown material you mix in. They either smell, attract pests, or risk spreading disease and contaminants into soil you'll later grow food in.

Materials that should never go in home compost

Some of the materials on this list, like plastic-labelled "compostable" bags, are genuinely compostable — just not at home. They need an industrial facility that holds a steady temperature above roughly 60°C (140°F) for several days straight. A backyard pile rarely gets that hot or that consistent, which is the real difference between what a municipal composting plant can handle and what your own heap can manage.

If you've had tomato plants affected by blight, it's especially important not to compost them — the spores can lie dormant in soil and infect next year's plants as soon as you spread the compost. Burn the material or take it to your local waste facility instead.

The finished compost is otherwise one of the best ways to improve garden soil, fill a raised bed, or mix into the growing medium for vegetables grown in containers — which is exactly why it matters that nothing harmful goes in to begin with.

Why does compost smell or stop breaking down?

Most compost problems come down to a poor green-to-brown balance, or not enough air getting in. Here's how to spot the cause and fix it.

Does it smell sour, sharp or like ammonia? You likely have too much wet, green material and not enough brown. Mix in dry leaves, cardboard or straw and turn the pile so air can get back in.

Is the pile dry and nothing seems to be breaking down? Probably too much brown material, or just too dry overall. Add more green material and water it lightly — compost should feel like a damp, wrung-out sponge, not bone dry or soaking wet.

Have you noticed rats or flies around the bin? Food scraps have likely been left visible near the surface. Always bury new scraps a bit into the pile and cover them with a layer of brown material.

Nothing seems to be breaking down at all, no matter what you do? Check the particle size. Whole apple cores, unchipped branches and whole cabbage leaves take far longer than chopped material. Cut or tear up anything large before it goes in.

Getting fruit flies or wasps around the pile? That's usually sweet food scraps, like fruit or melon, left sitting on the surface. Bury them right in the centre and always cover with a layer of brown material on top.

Seeing ants in and around the compost? Ants like dry, warm conditions, and they're often a sign the pile is simply too dry. Water it lightly and mix in more moist, green material at the same time.

Good to know. A well-run pile should feel noticeably warm in the middle during summer if it's large enough and reasonably well managed. A pile that stays cold and wet all the time is the first sign the green-to-brown balance needs adjusting.

How do you know when compost is finished?

Finished compost should be dark, crumbly, and smell like forest soil rather than garbage or anything rotten. Most of the original materials — food scraps, leaves, grass clippings — are no longer recognisable as individual pieces. Coarser items like twigs or avocado pits might still be visible even once everything else is done; sift them out and toss them back into a new batch rather than waiting for absolutely everything to disappear.

Expect roughly 6 to 12 months in an ordinary, open pile that gets basic upkeep, and considerably less time in an insulated hot bin that's turned regularly. If you're not sure it's fully ready, let it sit an extra week or two before using it near delicate seedlings — half-finished compost can still pull heat and nitrogen from the surrounding soil instead of feeding it.

Common composting mistakes

• Only food scraps, no brown material. Produces a dense, wet, smelly heap instead of airy, crumbly compost.
• Whole citrus peels and large chunks left uncut. Takes unnecessarily long to break down and stays visible in the finished compost.
• Composting weeds that have already gone to seed. Spreads fresh weed seeds across the whole garden once you use the compost.
• Assuming everything "natural" is fine. Diseased plant material, sprayed cut flowers and spreading root weeds are all natural, but none of them belong in the pile.
• Adding whole, large branches. Chip or cut them down first, or they'll sit there almost unchanged for years.
• Never turning or aerating the pile. Without occasional turning, compost compacts and goes airless in the middle even with a good green-to-brown balance otherwise. A couple of times per season is plenty for most home piles.

Frequently asked questions

Can you compost citrus peel and onions?
Yes, in normal household amounts they're both fine in an ordinary backyard pile. The myth that they're harmful comes mainly from worm bins, where concentrated acids and sulphur compounds can bother the worms specifically. In a regular compost heap it's not an issue, though cutting citrus peel into smaller pieces helps it break down faster.

Can you compost cooked food?
Cooked food without meat, fish, dairy or oil can be composted, but in smaller amounts and always buried in the centre of the pile. Left visible on the surface, it attracts rats, mice and flies quickly, even though the food itself is harmless to the composting process.

Can you compost diseased tomato plants?
No, not if they had blight or another fungal disease. The spores can survive in soil for years and infect next season's plants as soon as you spread the finished compost. Burn the material or take it to your local waste facility instead.

How much grass clippings can you add to compost at once?
Add them in thin layers, a couple of centimetres at a time, mixing in dry material like leaves or cardboard right away. A thick mat of clippings compacts quickly into a dense, airless layer that smells bad rather than breaking down properly.

Can you compost printed paper?
Ordinary black-and-white newspaper is fine. Avoid glossy, colour-printed paper like flyers and magazines in large quantities, since the ink can contain substances that shouldn't end up in growing soil. The occasional piece won't make much difference, but don't make it a habit.

Do you need exactly equal amounts of green and brown material?
No, it doesn't need to be precise. A simple rule of thumb works: for every bucket of food scraps or grass clippings, keep a roughly equal amount of dry material ready to mix in. If it smells bad, add more brown. If it's breaking down too slowly and feels dry, add more green material or water it lightly.

Can you compost weeds?
Weeds without mature seed heads compost fine, just like any other green yard waste. Skip weeds that have already gone to seed, and skip spreading root weeds like couch grass or bindweed — both can survive composting and sprout again once you spread the finished material.

Can you compost wood ash?
Ash from clean wood fires is fine in small amounts, one handful at a time spread thinly, since it raises pH sharply. Never compost ash from coal, briquettes or treated wood — it contains chemicals and heavy metals that don't belong in growing soil. If you're unsure of the source, it's safest to skip the ash altogether.

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