The Basic Rule: Brown and Green Materials Break Down, Everything Else Doesn't
Compost works because microorganisms eat organic matter and turn it into soil. They thrive on two types of material: greens (nitrogen-rich, usually wet) and browns (carbon-rich, usually dry). Anything that isn't plant or food waste—plastic, metal, glass, treated wood—won't break down and will sit in your pile unchanged. The fastest way to ruin a compost bin is to add something that looks like it should work but doesn't: glossy paper, "compostable" bags that need industrial heat, or meat that attracts rats.
Your job is straightforward: put in things that came from plants or animals raised for food, and keep out everything else. The rest is timing and moisture.
Key Takeaways
- Greens include fresh grass clippings, vegetable scraps, coffee grounds, and plant trimmings; browns include dry leaves, shredded paper, cardboard, and wood chips.
- Never add meat, fish, dairy, oils, or pet waste because they attract pests and create odor problems that take weeks to fix.
- Diseased plants, weeds with seeds, and treated wood belong in the trash, not the bin, because they survive composting and spread problems to your garden.
- Glossy paper, plastic bags labeled "compostable," and coal-based charcoal do not break down in home compost and should be thrown away.
- A working pile needs roughly three parts brown to one part green by volume; too much green creates a smelly, anaerobic mess.
Greens: What to Add and How Much
Greens are fresh, moist materials that feed the microorganisms doing the work. They include vegetable and fruit scraps (no oils or dressings), grass clippings, plant trimmings, coffee grounds with filters, tea bags without staples, and fresh manure from herbivores like horses, rabbits, or chickens. Crushed eggshells also go in the green pile, though they take months to break down fully.
The mistake most people make is adding too much green at once. A pile that is more than half green by volume will become waterlogged, smell like ammonia or rotten eggs, and attract flies. Add greens gradually—a handful or two at a time—and mix them with browns. If your pile already smells bad, stop adding greens and add a thick layer of dry leaves or shredded paper on top to absorb moisture and cut off oxygen to the smell-causing bacteria.
Grass clippings are free and abundant, but they mat down and trap moisture. Spread them out to dry for a day before adding them, or mix them thoroughly with browns as you layer. The same goes for kitchen scraps: chop them into small pieces so they break down faster, and bury them under a layer of brown material so they do not attract pests to the surface.
Browns: Dry Materials That Balance Your Pile
Browns are dry, carbon-rich materials that keep your pile from becoming a soggy, smelly mess. They include dry leaves, shredded newspaper or cardboard (no glossy coating), wood chips, sawdust from untreated wood, straw, and hay. These materials also create air pockets that let oxygen reach the microorganisms, which speeds decomposition.
Dry leaves are the easiest brown to find in fall. Collect them before they get wet, shred them with a mower or by hand into pieces smaller than your palm, and store them in a bag or bin so you have them on hand year-round. Cardboard and newspaper work well too—tear them into strips and wet them slightly before layering them in. Avoid anything with a glossy or plastic coating, including pizza boxes with grease stains and magazine pages.
Wood chips and sawdust are useful but come with a catch: they take longer to break down than other browns, sometimes six months to a year. Use them if you have them free from tree trimming, but do not rely on them as your main brown source. Never use sawdust or chips from treated wood, which contains chemicals that will leach into your finished compost and harm plants.
Never Add These: Meat, Dairy, Oils, and Pests
Meat, fish, bones, and dairy products break down very slowly and attract rats, raccoons, and flies while they do. The smell can last for weeks even after you bury them deep. If you want to compost animal products, you need a hot compost system that reaches 140°F or higher—a temperature most home piles never reach. For a standard backyard bin, keep all meat, fish, cheese, butter, milk, and yogurt in the trash.
Oils, grease, and fatty foods also belong in the trash. They repel water, slow decomposition, and create a slick layer that blocks air from reaching the pile. This includes salad dressing, cooking oil, nuts, seeds, and fatty meat scraps. Peanut butter and avocado are high in oil and should not go in either.
Pet waste—dog or cat feces—should never go in a home compost bin. It contains pathogens that survive the composting process and can contaminate vegetables you grow. Horse, rabbit, and chicken manure are safe because those animals eat plants, but carnivore waste is not.
Plants and Seeds: What Looks Safe but Isn't
Not all plant material is safe to compost. Diseased plants—those with powdery mildew, blight, or fungal spots—should go in the trash. Composting does not kill most plant diseases, so you will spread them back to your garden when you use the finished compost. The same goes for weeds that have gone to seed: the seeds survive composting and germinate in your garden beds.
Invasive plants like bindweed, bermuda grass, and morning glory can regrow from tiny root fragments. If you have them, throw them away rather than compost them. Black walnut leaves and twigs contain a chemical called juglone that kills many plants, so keep them out of the pile if you plan to use the compost on a vegetable garden.
Citrus peels, onion skins, and garlic are safe to compost but break down slowly. They are fine to add, but do not expect them to disappear quickly. Avoid adding large quantities of any single item—a bucket of orange peels will sit in the pile for months—and mix them with other materials instead.
Paper, Cardboard, and Materials That Fool You
Plain cardboard and newspaper compost well, but many paper products do not. Glossy paper, coated cardboard, and magazines have a plastic or wax coating that does not break down. Shredded office paper is fine if it is not colored or coated. Paper with ink is safe—newspaper ink is soy-based and breaks down—but avoid anything that feels slick or shiny.
Bags labeled "compostable" or "biodegradable" are a common mistake. Most require industrial composting facilities that reach 140°F or higher and stay there for weeks. Your home pile will not get hot enough, and the bag will sit intact for years. Throw these in the trash or recycling, not the compost. The same goes for "compostable" food containers and utensils unless your local facility specifically accepts them.
Charcoal briquettes and coal-based charcoal do not belong in compost. They are treated with chemicals and do not break down. Wood ash from a fireplace is fine in small amounts—it adds minerals—but charcoal is not.
Treated Wood, Synthetic Materials, and Everything Else
Treated wood, pressure-treated lumber, and plywood contain chemicals designed to resist rot and insects. These chemicals leach into compost and contaminate soil. Use only untreated wood chips or sawdust. The same rule applies to anything synthetic: plastic, rubber, synthetic fabrics, and foam do not break down in a home compost pile and should go in the trash.
Glass, metal, and ceramics never break down and will stay in your finished compost as fragments. Pick them out before you use the compost, or better yet, do not add them in the first place. Aluminum foil, tin cans, and bottle caps belong in recycling or trash.
Lint from a dryer is mostly synthetic fiber and should not go in compost. Natural fiber lint from cotton clothing can go in, but it is not worth sorting. Vacuum cleaner bags, used tissues, and paper towels with food on them are fine in small amounts, but do not add them in bulk.
Frequently Asked Questions
Can I compost bread, pasta, and cooked food?
Bread and pasta can go in compost, but they attract pests and mold quickly. Bury them deep under browns and add them sparingly. Cooked food with oils, sauces, or meat should not go in because the oil slows decomposition and the meat attracts rats. Plain cooked vegetables with no seasoning or oil are okay in small amounts.
What about sawdust from my workshop?
Sawdust from untreated wood is a good brown material. Sawdust from treated wood, plywood, or particle board contains chemicals and should not go in. If you are unsure whether the wood was treated, do not add it. Sawdust from sanding also contains fine particles that can compact the pile, so mix it well with other materials.
Can I put in leaves that have been sitting wet for weeks?
Wet, matted leaves will pack down and slow air flow. Spread them out to dry for a few days if you can, or break them up and mix them thoroughly with greens and other materials. If they have started to mold or smell, they are already decomposing and will work fine in the pile—just mix them in well.
Is it okay to add a little bit of meat or fish?
No. Even a small amount of meat or fish will attract pests and create odor problems that take weeks to resolve. The smell can spread to neighbors' yards and make your compost bin unusable. Keep all meat and fish in the trash.
What if I accidentally added something I should not have?
If you added plastic, glass, or metal, dig it out as soon as you notice it. If you added meat or dairy and smell develops, add a thick layer of dry leaves or wood chips on top to cut off oxygen and reduce odor. Stop adding greens until the smell goes away. If the pile becomes a persistent problem, you can turn it out onto a tarp, remove the problem material, and rebuild the pile.