The basic rule: plant matter yes, animal products and treated materials no
Compost breaks down because microbes and fungi eat organic material. They thrive on plant scraps and yard waste, but they struggle with or actively avoid meat, dairy, oils, and anything treated with chemicals. The fastest way to know what works is to remember that anything that came from a plant and isn't coated, treated, or mixed with animal fat will compost. Everything else either takes years, attracts pests, or creates odor problems that make your pile unusable.
What you put in determines whether your compost becomes dark, crumbly, earth-smelling material in months or a smelly, pest-filled mess that your neighbors notice. The difference is not effort—it is knowing the actual list of what microbes will eat.
Key Takeaways
- Fruit and vegetable scraps, coffee grounds, tea bags, grass clippings, leaves, and shredded paper all break down reliably in a home compost pile.
- Meat, fish, bones, dairy, oils, fats, and pet waste never belong in compost because they attract rodents and create odor problems.
- Diseased plants and weeds with seeds should stay out because your pile may not get hot enough to kill them.
- Glossy or colored paper, treated wood, and anything sprayed with pesticides will not break down or will introduce chemicals into your finished compost.
- Cooked food, even if it contains only plant ingredients, can attract pests and should be avoided unless your pile reaches 140°F or higher.
What composts reliably: the green and brown categories
Green materials (nitrogen-rich) include fruit and vegetable scraps, coffee grounds and filters, tea bags without staples, fresh grass clippings, plant trimmings, and manure from herbivores like horses, rabbits, and chickens. These materials break down quickly and heat up your pile.
Brown materials (carbon-rich) include dry leaves, shredded newspaper and cardboard, straw, wood chips, sawdust from untreated wood, and paper bags. These break down more slowly but create the air pockets your pile needs. A working pile needs both: roughly three parts brown to one part green by volume.
Eggshells, crushed or whole, will eventually break down but take months. Nut shells (except black walnut, which is toxic to plants) decompose very slowly. If you add them, expect them to still be visible in your finished compost. Shredded cardboard and newspaper work well, but remove tape, staples, and plastic windows first.
What never goes in: meat, dairy, oils, and animal waste
Meat, fish, bones, and any animal protein attract rats, raccoons, and flies. Even if your pile gets hot enough to kill pathogens, the smell will draw pests before decomposition happens. The same applies to dairy products, grease, cooking oils, and butter. These materials also create a waterlogged, anaerobic layer that smells like rotten eggs.
Pet waste—dog or cat feces—should never go in a home compost pile. It can harbor parasites and pathogens that survive the composting process and contaminate your vegetables. Horse and chicken manure are safe because those animals eat plants; their waste does not carry the same human pathogens.
Cooked food is a gray area. If it contains only plant ingredients (cooked vegetables, grains, pasta), it will compost, but it attracts pests just like raw meat does. Unless your pile reaches and maintains 140°F or higher—which requires active management and a thermometer—leave cooked food out. Raw plant scraps are safer and faster.
Plants and seeds: when to exclude them
Diseased plants should not go in a home pile. If your tomato plant had blight or your squash had powdery mildew, the spores may survive your pile's temperature and spread the disease to next year's garden. Commercial facilities reach 160°F or higher; home piles usually peak at 120–140°F. When in doubt, throw it away.
Weeds with mature seeds will survive composting and spread when you use the finished product. Dandelions, crabgrass, and any weed that has gone to seed should go in the trash. Weeds without seeds are fine. If you are unsure whether a weed has set seed, err on the side of excluding it.
Black walnut leaves and twigs contain juglone, a compound toxic to many plants. Do not compost them. Oak leaves are safe despite their tannins—they just take longer to break down.
Paper, cardboard, and treated materials
Plain cardboard and newspaper shred easily and compost well. Glossy or colored paper, including most magazines and junk mail, may contain inks and coatings that do not break down or introduce heavy metals into your pile. Shredded office paper is usually safe, but avoid anything with a plastic coating or lamination.
Treated wood, plywood, and particle board contain glues and chemicals that do not compost. Sawdust from treated lumber should stay out. Untreated wood chips and sawdust from a local arborist or woodworker are fine, though they decompose slowly.
Paper bags with plastic windows or plastic-lined bottoms will not break down. Wax-coated paper (like milk cartons) takes years and leaves wax residue. Stick to plain cardboard and uncoated paper.
Yard waste: grass, leaves, and wood chips
Fresh grass clippings are excellent compost material, but they mat down and create a slimy, anaerobic layer if you add too much at once. Mix them with brown materials or spread them in thin layers. If your lawn was treated with herbicide or pesticide, wait at least one mowing cycle before composting the clippings—or skip them entirely if you cannot confirm the lawn is untreated.
Dry leaves are ideal brown material. Shred them with a mower if you have a large pile; whole leaves take longer to break down. Pine needles and conifer needles are acidic and decompose slowly, but they will eventually compost.
Wood chips from an arborist are free or cheap and add bulk to your pile. They take a year or more to break down fully, so do not expect to use them in finished compost for at least 18 months. Avoid chips from diseased trees or trees treated with fungicide.
Synthetic materials and chemicals
Anything plastic—bags, ties, labels, or microplastics in "biodegradable" products—does not belong in compost. Synthetic fertilizers, pesticides, and herbicides will not break down and will contaminate your finished product. If you spray your garden, do not compost the treated plants.
Coal and charcoal ash contain substances that do not decompose. Wood ash from an untreated wood fire is safe in small amounts (it raises pH), but do not add more than 5 to 10 percent of your pile's volume. Glossy or colored paper, plastic-coated cardboard, and anything with a waterproof coating should all stay out.
Frequently Asked Questions
Can I compost citrus peels and onions?
Yes, citrus peels and onion skins compost fine. They break down slowly because of their tough outer layer, but they will not harm your pile. Chop them into smaller pieces to speed decomposition. Some gardeners avoid large amounts of citrus because the acidity can slow microbial activity, but normal kitchen scraps are not a problem.
What about sawdust from my workshop?
Sawdust from untreated wood is safe and composts well. Sawdust from treated lumber, plywood, or particle board should not go in your pile because it contains glues and preservatives. If you are unsure whether the wood was treated, leave it out. Untreated hardwood sawdust takes longer to break down than softwood but works fine.
Can I compost glossy or colored paper?
Glossy and colored papers may contain inks and coatings that do not break down or introduce heavy metals. Plain newspaper, cardboard, and uncoated office paper are safer choices. When in doubt, use plain materials instead—they compost just as well and you avoid the risk.
Is it okay to add weeds to my compost pile?
Weeds without seeds are fine. Weeds with mature seeds will survive your pile and spread when you use the compost. If a weed has gone to seed, throw it away instead. Young weeds pulled before flowering are safe to compost.
Can I compost bread, pasta, and grains?
Raw bread and pasta will compost, but they attract pests just like cooked food does. Unless your pile reaches 140°F or higher, leave them out. Cooked versions are even more attractive to rodents and insects. Stick to raw fruit and vegetable scraps, which are safer and faster to break down.