The basics: what breaks down and what doesn't
Compost works because certain materials rot. Browns — dry leaves, shredded paper, cardboard, wood chips — contain carbon. Greens — grass clippings, vegetable scraps, coffee grounds, fresh plant trimmings — contain nitrogen. Microbes in the pile consume both, and the pile heats up as they work. What you add determines whether your pile becomes usable compost or turns into a smelly, slow mess.
The rule is straightforward: add things that came from plants or animals, avoid things that won't rot or will cause problems. But the details matter. A banana peel breaks down. A banana sticker does not. Grass clippings are fine; grass treated with herbicide is not. Knowing the difference saves you months of waiting and keeps your pile from attracting pests or producing odor.
Key Takeaways
- Vegetable and fruit scraps, coffee grounds, eggshells, and grass clippings are safe greens that speed decomposition.
- Dry leaves, shredded paper, cardboard, and wood chips are browns that provide structure and balance nitrogen.
- Meat, fish, dairy, oils, and diseased plants should never go in a home compost pile because they attract pests or spread infection.
- Treated wood, glossy paper, and synthetic materials do not break down and will remain in your finished compost.
- A working pile needs roughly three parts brown to one part green by volume, added in layers.
Greens: nitrogen-rich materials that break down fast
Greens are the fast-rotting half of compost. Vegetable scraps — carrot peels, celery ends, lettuce leaves, broccoli stems, onion skins — all go in. Fruit scraps work too: apple cores, banana peels, orange rinds, berry containers. Coffee grounds and tea bags (remove the staple first) add nitrogen and moisture. Crushed eggshells add calcium and break down slowly, which is fine; they do not slow the pile.
Fresh grass clippings, plant trimmings from your garden, and pulled weeds are greens. Seaweed and kelp from the beach work if you rinse off excess salt. Manure from herbivores — horse, rabbit, chicken — is a green. Do not use manure from meat-eating animals; it can harbor pathogens. Spent plant material from houseplants is fine. Leaves that are still green or freshly fallen are greens; dry leaves are browns.
Avoid greens that are diseased, moldy, or treated with pesticides. A tomato plant with late blight will spread that disease through your pile and into your garden soil. Grass clippings from a lawn treated with weed killer will kill your compost's microbes. When in doubt, leave it out — the cost of a replacement pile is higher than the cost of throwing one item away.
Browns: carbon-rich materials that provide structure
Browns are the slow-rotting half. Dry leaves are the most common and easiest to find. Shredded paper — newspaper, office paper, junk mail without plastic windows — works well. Cardboard boxes, paper bags, and paper towel tubes can go in; tear them into pieces so they break down faster. Wood chips, sawdust, and untreated wood shavings are browns. Straw and hay (not the same thing; straw is hollow stalks, hay is dried grass with seeds) both work, though hay may sprout.
Pine needles, wood ash from a fireplace, and coconut coir are browns. Shredded cotton or wool fabric breaks down. Paper napkins and paper plates without plastic coating are fine. Avoid glossy or colored paper, which contains inks and coatings that do not break down. Do not use treated wood, plywood, or particle board; they contain glues and chemicals. Sawdust from pressure-treated lumber is toxic.
The pile needs browns to stay loose and airy. Without them, greens pack down into a wet, airless mat that smells like ammonia and rots slowly. A good rule: add a layer of browns after every layer of greens, roughly three to four inches of brown for every one to two inches of green.
Never compost: meat, dairy, oil, and problem plants
Meat and fish attract rats, raccoons, and flies. They also take months to break down and smell terrible while they do. Dairy — milk, cheese, yogurt, butter — does the same. Oils and fats coat other materials and slow decomposition. Bones take years. Avoid all of these in a home pile.
Diseased plants spread infection. Weeds with seeds will germinate in your finished compost and spread through your garden. Invasive plants like bindweed or morning glory should be burned or thrown away, not composted. Walnut leaves and black walnut hulls contain juglone, a chemical that kills many plants; do not compost them if you plan to use the finished product in a vegetable garden.
Glossy paper, plastic-coated cardboard, and synthetic materials do not break down. They will sit in your pile for years and end up in your finished compost as fragments. Treated wood, pressure-treated lumber, and plywood contain chemicals that persist. Pet waste — dog or cat feces — can harbor parasites harmful to humans; do not compost it. Diseased lawn clippings or plant material spread fungal and bacterial infections.
Materials that work but need care
Citrus peels, onions, and garlic break down fine, but in large amounts they can slow the pile or repel worms. Add them in moderation. Bread, pasta, rice, and cooked food scraps attract pests and should be buried deep in the pile, not left on top. Nuts and seeds may not break down fully and can sprout. Glossy or colored paper from magazines contains inks; plain newsprint is safer.
Sawdust is a brown, but only from untreated wood. Sawdust from a hardware store or construction site may come from treated lumber. If you do not know the source, do not use it. Leaves from black walnut trees contain juglone; if you are composting for a vegetable garden, skip them. Leaves from diseased trees — oak wilt, Dutch elm disease — should not go in a home pile.
The ratio that works: how much brown and green to add
A compost pile works best when you layer materials. Start with a base of coarse browns — wood chips or straw — about four inches thick. This lets air flow underneath. Add greens next, about two inches. Then browns again, four inches. Keep layering until the pile is three to four feet tall. A pile smaller than three feet does not heat up enough to break down material quickly.
By volume, aim for roughly three parts brown to one part green. This ratio keeps the pile from getting soggy or smelly. If your pile smells like ammonia, add more browns. If it is not heating up and breaking down, add more greens. If it is wet and matted, add browns and turn it to let air in.
A working pile will heat up in the center within a few days if you have the ratio right and the pile is large enough. You can feel this by pushing a stick into the middle. If it is hot, the pile is working. If it is cool and nothing is breaking down after two weeks, you likely have too much brown or not enough moisture.
What happens to problem materials
If you accidentally add meat or dairy, the pile will smell bad and attract pests. Remove what you can and bury the rest deep under new layers of browns and greens. Turn the pile frequently to speed breakdown and reduce odor. Next time, keep a separate container for scraps you cannot compost and throw them in the trash.
Plastic, treated wood, and synthetic materials will not break down. They will sit in your pile for years and end up in your finished compost as fragments. If you notice them, remove them before you use the compost in your garden. Glossy paper and cardboard with plastic coating will separate into layers and take years to break down; shred them finely or skip them.
Diseased plants or weeds with seeds will spread problems to your garden. If you realize you added them, do not use that compost on vegetables or ornamentals. Use it only on areas where you do not mind the disease or weeds spreading, or let it sit for a full year before using it, which may kill some pathogens.
Frequently Asked Questions
Can I compost cooked food?
Cooked food scraps — rice, pasta, vegetables — can go in, but bury them deep in the pile, not on top. They attract pests and smell if exposed. Cooked meat and fish should not go in at all. Cooked vegetables are fine if buried and the pile is large enough to heat up and break them down quickly.
What about paper and cardboard with printing on them?
Plain newsprint and office paper are safe. Glossy paper, colored cardboard, and paper with plastic coating should not go in; the inks and coatings do not break down. Cardboard boxes are fine if they are not waxed or plastic-lined. Tear them into pieces so they break down faster.
Can I compost grass clippings from a treated lawn?
No. Herbicides and pesticides persist in the clippings and will kill the microbes in your compost pile or contaminate your finished compost. Only use clippings from lawns you know have not been treated. If you are unsure, leave them out.
How long does it take for compost to be ready?
A hot pile that you turn regularly can produce finished compost in two to three months. A pile you leave alone takes six months to a year. Finished compost is dark brown, crumbly, and smells like soil. You should not see the original materials anymore.
Can I compost sawdust from my workshop?
Only if it is from untreated wood. Sawdust from pressure-treated lumber, plywood, or particle board contains chemicals that do not break down and will contaminate your compost. If you do not know the source, do not use it.