The basics: what breaks down and what doesn't

Your compost pile works because certain materials rot predictably while others don't rot at all or take so long that they'll outlast your bin. The rule is straightforward: put in things that were once alive and will return to soil. Leave out anything synthetic, anything treated with chemicals, and anything that will attract pests or create odor problems.

The materials that work fall into two groups. Brown materials (carbon-rich) include dry leaves, shredded paper, cardboard, and wood chips. Green materials (nitrogen-rich) include grass clippings, vegetable scraps, coffee grounds, and plant trimmings. A working pile needs both—roughly three parts brown to one part green by volume—because the microbes that do the breaking down need that balance to function.

What doesn't work includes anything plastic, anything treated with pesticides or wood stain, meat and fish, dairy, oils, and diseased plant material. These either won't break down, will kill the microbes doing the work, will create a smell that brings rats, or will spread disease to your garden.

Key Takeaways

  • Vegetable and fruit scraps, coffee grounds, crushed eggshells, and grass clippings are safe to compost and speed up the process.
  • Dry leaves, shredded paper, cardboard, and wood chips provide the carbon structure your pile needs to stay balanced and avoid odor.
  • Meat, fish, dairy, oils, diseased plants, and anything treated with chemicals should never go in because they attract pests, create smell, or kill the microbes breaking things down.
  • Glossy paper, pet waste, and treated wood will not break down or will contaminate your finished compost with toxins.
  • Layering brown and green materials as you add them keeps your pile working faster than dumping everything at once.

Vegetable and fruit scraps that work well

Almost all vegetable and fruit scraps break down in a compost pile. Carrot peels, apple cores, banana peels, lettuce leaves, broccoli stems, onion skins, and potato peels all rot without problems. Citrus peels take longer than softer scraps—sometimes two to three months—but they do break down. Chop larger pieces into smaller chunks so they decompose faster.

The one exception is avocado pits and peels. The pit takes so long to break down that it's not worth including. Avocado peels are fine, but the pit will still be recognizable when your compost is done. Tomatoes, including the seeds, are safe and actually speed up decomposition because they're acidic.

Cooked vegetables are technically safe to compost, but they attract pests more readily than raw scraps, so most home composters skip them. If you do add cooked food, bury it deep in the pile under brown material so it's not visible to rats or insects.

Paper, cardboard, and wood materials

Shredded newspaper, plain cardboard, and kraft paper (the brown paper bags use) all break down in a compost pile. Tear or shred cardboard into pieces roughly the size of your hand so it decomposes faster. Corrugated cardboard works fine—the glue holding it together is water-soluble and breaks down. Avoid cardboard that has a plastic or waxy coating, which won't rot.

Glossy paper, like the kind used in magazines or color newspaper inserts, should not go in because the coating doesn't break down and can leave residue in your finished compost. Newspaper printed with soy-based ink is safe; most modern newspapers use it, but if you're unsure, stick with plain kraft paper.

Wood chips and sawdust from untreated wood are excellent brown material. Sawdust from pressure-treated lumber, plywood, or anything stained or painted must stay out—the chemicals won't break down and will contaminate your soil. If you're not certain the wood was untreated, don't use it.

Grass clippings, leaves, and plant trimmings

Fresh grass clippings are nitrogen-rich and speed up decomposition, but they mat together and can create odor if you add too much at once. Mix grass clippings with brown material as you layer—never dump a whole bag of clippings on top of your pile. If your lawn was treated with herbicide or pesticide, let the grass dry for a week before composting it, or skip it entirely if you're unsure about the chemical used.

Dry leaves are the easiest brown material to find in fall. Shred them with a mower or by running over them with a leaf blower to speed decomposition. Whole leaves pack down and exclude air, which slows the process. Leaves from diseased trees (like those with oak wilt or Dutch elm disease) should not be composted because the disease can survive and spread when you use the finished compost.

Plant trimmings from your garden—stems, spent flowers, vegetable plant tops—all break down. Diseased plant material should go in the trash, not the pile. If a plant had powdery mildew, blight, or any visible fungal infection, composting won't kill the disease; it will survive in your finished compost and infect next year's garden.

Coffee grounds, tea, and food byproducts

Used coffee grounds and tea bags (remove the staple first) are nitrogen-rich and decompose quickly. Coffee filters are fine too. Crushed eggshells add calcium and break down slowly, which is fine—they'll be mostly gone by the time your compost is finished. Nut shells, including walnut shells, can go in, though they take longer to break down than softer materials.

Hair and nail clippings technically break down, but they decompose very slowly and can mat together. Most home composters skip them because the benefit is small and they take up space. If you do add them, chop them finely and mix them in rather than layering them.

Stale bread, cooked grains, and pasta can go in, but like cooked vegetables, they attract pests. Bury them deep under brown material if you add them. Moldy food is fine—the mold is already breaking it down, which actually speeds the process.

What absolutely cannot go in your compost

Meat, fish, bones, and any animal products except eggshells and coffee grounds will attract rats, raccoons, and flies. The smell will be strong enough that neighbors will notice. Even if you bury meat deep in the pile, rodents will dig it out. Don't compost it.

Dairy products—milk, cheese, yogurt, butter—create odor and attract pests. Oils and fats, including cooking oil and grease, slow decomposition and create a water-resistant barrier that prevents air from reaching the pile. Both should go in the trash.

Pet waste (dog or cat feces) can contain pathogens that survive composting and contaminate your soil. Human urine is sometimes composted in specialized systems, but human feces should never go in a home pile. Diseased plant material, as mentioned above, spreads disease to next year's garden. Treated wood, plastic, synthetic materials, and anything with a chemical coating will not break down or will leave toxins in your finished compost.

Weeds with seeds should not go in unless your pile reaches a temperature of at least 140°F (60°C) for several weeks—most home piles don't get that hot. If you're unsure whether your pile heats up, leave out weeds that have gone to seed. Perennial weeds like bindweed and quackgrass can regrow from root fragments, so avoid those too.

How to layer materials for faster breakdown

The fastest compost piles are built in layers, not filled gradually. Start with a 4-inch layer of brown material (dry leaves or shredded cardboard), then add a 1-inch layer of green material (grass clippings or vegetable scraps), then brown again, then green, until the pile is 3 to 4 feet tall. Water each layer lightly as you build so everything is damp but not soggy.

If you're adding to your pile gradually over weeks, mix each new addition with brown material right away rather than dumping greens on top. A pile that's mostly green on the surface will smell and attract pests. A pile that's mostly brown will decompose slowly. Mixing as you go keeps the balance right.

Turn the pile every two to three weeks by moving material from the outside to the center. This speeds decomposition because it brings cooler material into the hot center where the microbes are most active. If you don't turn it, decomposition still happens—it just takes two to three times longer.

Frequently Asked Questions

Can I compost citrus peels and onion skins?

Yes. Citrus peels take longer than other fruit scraps—two to three months—but they break down completely. Onion skins decompose quickly. Both are safe and won't create odor or pest problems.

What about shredded paper from a paper shredder?

Plain white paper from a shredder is fine. Avoid shredded paper with ink, colored dyes, or a glossy coating. If the paper came from a document you're unsure about, stick with newspaper or kraft paper instead.

Can I put in weeds from my garden?

Annual weeds without seeds are safe. Weeds that have gone to seed or perennial weeds like bindweed can survive composting and regrow in your garden when you use the finished compost. If you're unsure, throw them in the trash.

Is it okay to compost sawdust from my workshop?

Only if the wood was untreated. Sawdust from pressure-treated lumber, plywood, or stained wood contains chemicals that won't break down and will contaminate your soil. If you don't know the source, don't use it.

Why can't I compost meat or fish?

Meat and fish attract rats and raccoons, which will dig into your pile and scatter it across your yard. The smell is strong enough to bother neighbors. Even buried deep, rodents will find it. Composting meat at home creates a pest problem that's difficult to solve.