The Basic Rule: Browns and Greens, Nothing Else

Your compost bin breaks down two types of material: greens (nitrogen-rich, wet things like food scraps and grass clippings) and browns (carbon-rich, dry things like leaves and shredded paper). Almost everything that rots belongs in one of those two categories. Almost everything else does not.

The reason is straightforward: compost works because bacteria and fungi eat the material and turn it into soil. They can only eat organic matter—things that were once alive. Plastic, metal, glass, and treated wood will sit in your bin for years unchanged. Meat, dairy, and oils attract rats and create smell. Diseased plants and weeds with seeds spread problems to your garden instead of solving them.

The difference between what works and what does not comes down to whether it will rot, whether it will attract pests, and whether what comes out will actually help your soil.

Key Takeaways

  • Greens include fruit and vegetable scraps, coffee grounds, grass clippings, and fresh plant trimmings; browns include dry leaves, shredded paper, cardboard, and wood chips.
  • Never add meat, fish, dairy, oils, fats, or cooked food because they attract rodents and create odor problems.
  • Do not compost diseased plants, weeds with seeds, or plants treated with pesticides, as they will survive the process and spread problems to your garden.
  • Avoid glossy paper, treated wood, coal ash, pet waste, and anything synthetic, as these either do not break down or introduce toxins into your finished compost.
  • The finished compost works best when you maintain a rough balance of greens to browns—roughly three parts brown to one part green by volume.

Greens: What to Add

Greens are the wet, nitrogen-rich materials that speed up decomposition. Fruit and vegetable scraps are the most common: apple cores, banana peels, carrot tops, lettuce leaves, onion skins, and potato peels all break down quickly. Citrus peels take longer but will eventually rot. Coffee grounds and tea bags (remove the staple from the bag first) add nitrogen and break down fast.

Fresh plant material also counts as green: grass clippings from an unmowed lawn, plant trimmings from pruning, and pulled weeds (as long as they have not gone to seed). Seaweed and kelp, if you live near the coast, are excellent greens. Even hair and nail clippings will compost, though they take months.

The catch with greens is that they pack down and can create a wet, airless layer that smells like ammonia. Add them in thin layers and mix them with browns to keep air flowing through the pile.

Browns: What to Add

Browns are the dry, carbon-rich materials that provide structure and let air move through the pile. Dry leaves are the most obvious—rake them up in fall and save them in bags. Shredded paper and cardboard (plain, uncoated, no tape or labels) break down well. Newspaper works too, though glossy inserts do not. Wood chips and sawdust from untreated wood are excellent browns, though sawdust can pack down if you use too much.

Straw and hay add bulk and air space—use straw rather than hay if you can, because hay often contains seeds. Pine needles, wood ash from a fireplace (not charcoal or coal ash), and shredded junk mail all work. Even cardboard egg cartons, paper towel tubes, and the cardboard sleeves from paper towels can go in.

The key is that the brown material should be dry and untreated. Pressure-treated wood, plywood, and painted wood release chemicals into your compost and should never go in.

What Never Goes In: Meat, Dairy, and Oils

Meat, fish, bones, and any animal product except manure will attract rats, raccoons, and other pests to your bin. They also create a strong rotting smell that your neighbors will notice. The same goes for dairy products—milk, cheese, yogurt, butter—and cooking oils or grease. These materials do eventually break down in a commercial composting facility with high heat, but a home bin does not get hot enough, and the smell and pests arrive long before decomposition finishes.

Cooked food of any kind should stay out, even if it contains no meat or dairy. Cooked rice, pasta, bread, and baked goods attract pests and can create mold problems. Raw vegetables and fruit are fine; cooked versions are not.

What Never Goes In: Disease, Seeds, and Chemicals

Do not add plants that show signs of disease—powdery mildew, blight, rust, or any visible fungal growth. Home compost piles do not reach temperatures high enough to kill plant diseases. When you spread that finished compost in your garden, you spread the disease with it. The same rule applies to weeds that have gone to seed: the seeds survive composting and you end up planting weeds instead of preventing them.

Plants treated with pesticides or herbicides should also stay out. Grass clippings from a lawn that was recently sprayed, plants from a commercial nursery that may have been treated, or anything from a yard that used weed killer will introduce those chemicals into your compost and eventually into your soil and food.

What Never Goes In: Synthetics and Treated Materials

Plastic, metal, glass, and rubber do not break down in a compost bin—they will still be there when you dig out the finished compost years later. The same goes for synthetic fabrics, even if they are labeled "natural" or "biodegradable." Glossy or coated paper (like magazine pages, takeout containers, or wax-coated cardboard) does not break down and can leach chemicals. Coal ash and charcoal ash should not go in; wood ash from a fireplace is fine in small amounts, but coal and charcoal contain substances that harm soil.

Pet waste—dog or cat feces—should never be composted at home. It can contain pathogens that survive the process and contaminate your vegetables. Treated wood, plywood, particle board, and anything painted or stained will release toxins as they break down.

The Balance: How Much Green and Brown

A working compost pile needs both greens and browns. A rough target is three parts brown to one part green by volume—that is, for every bucket of grass clippings or food scraps, add three buckets of dry leaves or shredded paper. This ratio keeps the pile from becoming a wet, smelly sludge while still providing enough nitrogen to speed decomposition.

If your pile smells like ammonia or looks slimy, you have too much green. Add more brown material and mix it in. If your pile is not heating up and seems to be sitting unchanged, you may have too much brown or not enough moisture. Add some greens and water it lightly.

The exact ratio varies depending on what materials you have and your climate. The point is to keep the pile from tipping too far in either direction.

Frequently Asked Questions

Can I put sawdust in my compost?

Yes, but use it sparingly. Sawdust from untreated wood breaks down, but it packs down tightly and can block air flow. Mix it with coarser browns like leaves or wood chips. Never use sawdust from treated, painted, or stained wood.

What about paper with ink on it?

Plain newspaper and junk mail with standard black ink are fine. Glossy magazine pages, colored inserts, and anything with heavy coating should stay out. The coating does not break down and can contain chemicals.

Can I compost weeds if I pull them before they flower?

Yes, as long as they have not set seed. Once a weed has flowered or produced seed heads, the seeds can survive composting. If you are not certain whether seeds have formed, leave the weeds out to be safe.

Is it okay to add a little bit of meat or fish?

No. Even small amounts attract rodents and create smell. The smell arrives before the material breaks down, and rats can smell meat from a distance. Keep all meat, fish, and bones out entirely.

Can I compost plants from my garden if they looked sick?

No. Home compost piles do not reach the temperature needed to kill plant diseases. If you compost a diseased plant, you spread that disease to every garden bed where you use the finished compost. Throw diseased plants in the trash instead.