Browns and Greens: The Two Categories That Matter

A working compost heap needs two types of material: browns (dry, carbon-rich) and greens (wet, nitrogen-rich). Browns break down slowly and create air pockets; greens heat up the pile and speed decomposition. The pile works best when you have roughly three parts brown to one part green by volume, though exact ratios are forgiving — your compost will still form if you are off.

Browns include dry leaves, shredded paper, cardboard, straw, wood chips, and sawdust. Greens include grass clippings, vegetable scraps, coffee grounds, and fresh plant trimmings. If your pile smells like ammonia or looks slimy, you have too much green; add more brown. If it is not heating up or breaking down, you likely need more green.

Key Takeaways

  • Browns (leaves, cardboard, straw) and greens (grass, vegetable scraps, coffee grounds) must both go in — a pile of only one type will not decompose properly.
  • Never add meat, dairy, oils, pet waste, or diseased plants, as these attract pests, spread illness, or create odour problems.
  • Grass clippings and plant material from treated lawns can introduce herbicides into your finished compost.
  • Shredded paper and cardboard work well as browns, but glossy or coated paper should stay out.
  • Cooked food, bread, and pasta belong in compost only if your pile reaches high enough temperature to kill pathogens — most home heaps do not.

Vegetable and Fruit Scraps: What Breaks Down and What Does Not

Raw vegetable and fruit scraps are ideal for compost: carrot peels, apple cores, lettuce leaves, onion skins, and melon rinds all decompose readily. Bury them under a layer of brown material to keep flies away. Citrus peels, avocado skins, and banana peels take longer than softer scraps but will eventually break down — do not avoid them, just expect them to sit in the pile for several months.

Cooked vegetables, bread, pasta, and rice should not go in a home compost heap. These foods attract rats and other rodents, and home piles rarely reach the sustained high temperature needed to kill the bacteria that grow on cooked food. If you have a municipal compost program that accepts food waste, those facilities are designed to handle cooked food safely; your backyard pile is not.

Meat, Dairy, Oils, and Other Foods to Exclude

Never add meat, fish, bones, or any animal product to a backyard compost heap. These materials attract rats, raccoons, and other pests that will tear open your pile and scatter it across your yard. They also create strong odours as they decompose. The same rule applies to dairy products — milk, cheese, yogurt, and butter all draw pests and smell foul.

Cooking oils, grease, and fat should stay out as well. They slow decomposition, create water-resistant barriers that prevent air from reaching the centre of the pile, and can attract pests. If you have a municipal food-waste program, check whether they accept oils; most do not, and you may need to dispose of them separately.

Grass Clippings, Leaves, and Plant Material

Fresh grass clippings are a valuable green and heat up your pile quickly. Spread them in a thin layer and mix them with brown material so they do not mat down and become slimy. If your lawn has been treated with herbicides or pesticides, do not add the clippings — those chemicals will persist in your finished compost and can harm plants you grow with it.

Fallen leaves are one of the best browns you can use. Shred them with a mower or leaf shredder if you can; whole leaves pack down and decompose slowly. Diseased plant material — leaves with fungal spots, plants killed by blight, or anything visibly infected — should go in the trash or a separate pile that reaches very high temperature. A home heap may not get hot enough to kill disease spores, and you risk spreading the problem to your garden.

Paper, Cardboard, and Wood Products

Shredded newspaper, plain cardboard, and kraft paper are excellent browns. Tear or shred them into pieces so they break down faster. Avoid glossy paper, coated cardboard, and anything printed with coloured inks — the coatings and dyes can contain substances you do not want in your soil. Wax-coated cardboard (like takeout containers) also belongs in the trash.

Sawdust and wood chips work as browns, but use them sparingly. Untreated wood is fine; pressure-treated lumber and plywood contain chemicals that will contaminate your compost. If you are unsure whether wood has been treated, leave it out. Pine needles and conifer branches decompose very slowly and can make your pile acidic — they are not harmful, but they will sit there for years.

Coffee Grounds, Tea, and Garden Waste

Coffee grounds and tea bags (remove the staple first) are excellent greens and add nitrogen to your pile. Used grounds are already wet and break down fast. Coffee filters are fine to add as well. Eggshells take months to decompose because they are mostly calcium, but they do eventually break down and add minerals — crushing them first speeds the process.

Weeds are safe to add if they have not gone to seed; if the seed heads are mature, either burn them or throw them away. Pulled weeds with roots intact will sometimes regrow in your compost pile if it does not get hot enough, so bury them deep. Plant trimmings from the garden are fine as long as they are not diseased.

What to Keep Out: Pet Waste, Treated Materials, and Problem Items

Dog and cat waste should never go in compost. These contain pathogens that can survive in a home pile and contaminate your vegetables. The same applies to human waste and soiled paper products. Glossy magazines, treated wood, synthetic materials, and plastics have no place in compost — they do not break down or they release harmful substances into your soil.

Charcoal ash from a grill or fireplace should stay out; it contains substances that can harm plants. Wood ash from a fireplace is acceptable in small amounts (it raises pH), but do not add it if your soil is already alkaline. Coal ash and anything from a treated fire log should go in the trash. Synthetic fertilizers, pesticides, and herbicides do not belong in compost — they do not break down and will persist in your finished product.

Frequently Asked Questions

Can I put onions and garlic in compost?

Yes. Raw onion and garlic scraps break down normally. Some people avoid them because they worry about smell or pests, but if you bury them under brown material and maintain a good brown-to-green ratio, they decompose without problems.

What about moldy food or fruit?

Mold is actually a sign that decomposition is working. Moldy fruit and vegetables are fine to add — the mold helps break them down faster. Do not add mold-covered bread or cooked food, as those attract pests.

Can I compost paper towels and napkins?

Plain paper towels and napkins are fine. Avoid ones with bleach or heavy dyes. If they have food residue on them, bury them under brown material so they do not attract pests.

Is it safe to use compost made from treated grass clippings?

No. Herbicides and pesticides persist through the composting process and can damage or kill plants grown in that soil. If you are unsure whether your lawn was treated, contact whoever maintains it or assume it was and skip the clippings.

How long does it take for bones or shells to break down?

Bones, shells, and eggshells take years to decompose in a home pile. Since bones also attract pests, it is better to leave them out entirely. Crushed eggshells break down in a few months and are worth adding.