The Basic Rule: Browns and Greens

A compost bin breaks down two types of material: greens (nitrogen-rich, wet things) and browns (carbon-rich, dry things). The bin works fastest when you layer them roughly equally—aim for about one part green to two parts brown by volume. Greens heat up the pile and speed decomposition. Browns keep it from becoming a soggy, smelly mess and give microbes the carbon they need to work.

The materials that work best are things that came from plants or animals and will rot. Avoid anything treated with chemicals, anything diseased, or anything that will attract rats and raccoons to your bin. If you are unsure whether something belongs in the bin, the safest choice is to leave it out—a wrong addition can stop the whole pile from breaking down properly.

Key Takeaways

  • Greens include fruit and vegetable scraps, coffee grounds, fresh grass clippings, and plant trimmings; browns include dry leaves, shredded paper, cardboard, and wood chips.
  • Never add meat, fish, dairy, oils, pet waste, diseased plants, or anything treated with pesticides or herbicides.
  • Cooked food, bread, and grains can go in a bin, but they attract rodents faster than raw vegetables, so bury them deep under browns.
  • Grass clippings and leaves from a lawn treated with chemicals will poison your finished compost and should not be added.
  • The pile breaks down fastest when you chop large items into smaller pieces and keep the moisture level similar to a wrung-out sponge.

Greens: Nitrogen-Rich Materials That Speed Breakdown

Fruit and vegetable scraps are the most common green. Add apple cores, banana peels, carrot tops, lettuce leaves, onion skins, and the trimmings from any produce you cut. Citrus peels and pineapple tops work too, though some people avoid large amounts of citrus because it takes longer to break down. Avoid avocado pits and shells—they decompose very slowly and can jam up a bin.

Coffee grounds and tea bags (remove the staple from the bag first) are excellent greens and add nitrogen without taking up much space. Fresh grass clippings work well, but only if the lawn was not treated with herbicides or pesticides in the past few months—chemicals persist in the clippings and will end up in your finished compost. Plant trimmings from indoors or outdoors, including dead flowers and houseplant leaves, belong in the bin. Seaweed and kelp are strong greens if you live near the coast and have access to them.

Manure from herbivores—horse, cow, chicken, or rabbit—is a powerful green that heats up a pile quickly. Never add dog or cat waste, which can carry pathogens harmful to humans. If you keep chickens or rabbits, their bedding (if it is straw or wood shavings, not cedar or pine) plus their droppings can go straight in.

Browns: Carbon-Rich Materials That Keep the Pile Balanced

Dry leaves are the easiest brown to find in fall. Shred or chop them into smaller pieces so they pack down less and break down faster—a leaf shredder or running a lawn mower over a pile of leaves works well. Shredded paper, cardboard, and newspaper are excellent browns and a good way to recycle paper waste. Remove plastic windows from envelopes and avoid glossy or colored inks, which may contain heavy metals. Plain cardboard boxes (with tape and labels removed) break down in a few months.

Wood chips, sawdust, and untreated wood shavings add carbon and improve air flow through the pile. Avoid treated wood, which contains chemicals that will not break down. Straw and hay are strong browns—straw is better because it has fewer seeds that might sprout in your finished compost. Pine needles and conifer bark take much longer to decompose than other browns, so use them sparingly or skip them if your bin is small.

Shredded junk mail, paper towel tubes, and paper bags all work. Avoid anything with a plastic coating or laminate. Wood ash from a fireplace or wood stove can go in, but use it sparingly—too much raises the pH and can slow decomposition. Never add charcoal ash, which contains chemicals.

Foods You Can Add (With Caution)

Cooked food, bread, grains, and pasta can go into a compost bin, but they attract rodents much faster than raw vegetables do. If you add them, bury them at least eight inches deep under a thick layer of browns. This keeps rats and raccoons from smelling them and digging into your bin. Bury each addition separately rather than creating a pocket of cooked food in one spot.

Nuts and seeds will sprout if they are not fully broken down before you use the compost, so chop them into small pieces or skip them. Oils, butter, and grease should not go in—they slow decomposition, create odor, and attract pests. Dairy products like cheese, milk, and yogurt also attract rodents and smell bad as they break down, so leave them out unless your bin is in a sealed, rodent-proof container.

What Never Goes in a Compost Bin

Meat, fish, bones, and any animal products except manure and plant-based food scraps will rot slowly, smell terrible, and draw rats and raccoons. Do not add them. Pet waste (dog or cat feces) carries pathogens that survive the composting process and can contaminate your finished compost. Diseased plants, plants infested with pests, and weeds with mature seeds should stay out—the pile may not get hot enough to kill them, and you will spread the problem to your garden.

Anything treated with pesticides, herbicides, or fungicides will end up in your finished compost and defeat the purpose of making it. This includes grass clippings and leaves from a treated lawn, treated wood, and plants from a commercial nursery (unless you know they were grown without chemicals). Glossy or colored paper, plastic, metal, and glass do not belong in a compost bin. Synthetic materials will not break down, and you will have to pick them out of your finished compost later.

Coal ash, charcoal ash, and ash from treated wood all contain chemicals or heavy metals. Sawdust from treated wood, plywood, or particle board contains glues and resins that do not decompose. If you are unsure whether wood was treated, leave it out.

How to Prepare Materials Before Adding Them

Chop or shred large items into pieces roughly the size of your thumb or smaller. Smaller pieces have more surface area for microbes to work on, so they break down two to three times faster than whole items. A kitchen knife works for soft vegetables. A leaf shredder, lawn mower, or food processor handles leaves, paper, and cardboard. A hatchet or pruning saw works for woody stems and small branches.

Wet greens before adding them if they are very dry—a compost pile needs moisture similar to a wrung-out sponge. If your bin is in a dry climate or indoors, you may need to mist the pile every few days. If it is soggy and smells like ammonia, add more browns and turn the pile to add air. A pile that is too wet will rot instead of compost and will smell bad.

Frequently Asked Questions

Can I put moldy food in the compost bin?

Yes. Mold is a sign that decomposition has already started, so moldy fruit, bread, and vegetables break down faster than fresh ones. Add them the same way you would add fresh food scraps—bury them under browns if they are cooked or if you are worried about rodents.

What about eggshells and nut shells?

Eggshells break down slowly but do belong in the bin—crush them into small pieces first. Nut shells (walnut, almond, peanut) take much longer and are harder to crush, so skip them unless you have a lot of time or a very large bin. Avoid black walnut shells, which contain a chemical that kills plants.

Can I add weeds to the compost bin?

Yes, but only if the weeds have not gone to seed. If they have mature seed heads, leave them out—most home compost piles do not get hot enough to kill seeds, and you will spread weeds to your garden. Roots and stems without seeds are fine.

Is it okay to add glossy or colored paper?

No. Glossy paper and colored inks may contain heavy metals or chemicals that do not break down. Stick to plain cardboard, newspaper, and office paper without colored ink or plastic coating.

How long does it take for things to break down?

It depends on the size of your bin, how often you turn it, and what you add. A small bin with mostly large items can take six months to a year. A large bin that you turn weekly and keep balanced with chopped greens and browns can produce finished compost in two to three months. Most home bins take three to six months.