Materials That Will Ruin Your Compost or Attract Pests

Meat, fish, dairy, oils, and fats break down slowly and create odors that draw rats, raccoons, and flies to your pile. Even small amounts—a chicken bone, a spoonful of butter, spoiled milk—will cause problems. These materials also tend to create anaerobic pockets (areas without oxygen) that smell like rotting eggs and slow the entire decomposition process.

Pet waste from dogs and cats should never go in a home compost pile. It carries parasites and pathogens that survive the composting process and can contaminate your finished compost. The same applies to human waste and soiled diapers. If you want to compost pet bedding, use only bedding from herbivores like rabbits or guinea pigs, and only if the animals are healthy.

Key Takeaways

  • Meat, fish, dairy, oils, and fats attract rodents and create foul odors that will persist for weeks.
  • Pet waste and diseased plant material introduce pathogens that survive composting and contaminate your finished product.
  • Glossy or coated paper, treated wood, and synthetic materials do not break down and will remain in your compost indefinitely.
  • Weeds with mature seeds and invasive plants like bindweed will spread throughout your garden when you use the finished compost.
  • Sawdust from treated lumber and charcoal ash contain chemicals that kill beneficial microbes and harm plants.

Diseased Plants and Weeds With Seeds

If a plant shows signs of disease—powdery mildew, blight, rust, or root rot—do not compost it. Most home compost piles do not reach temperatures high enough to kill fungal spores and bacterial pathogens. When you spread that compost on your garden beds, you are spreading the disease to healthy plants. Throw diseased material in the trash instead.

Weeds that have already gone to seed should also stay out. Dandelions, crabgrass, lamb's quarters, and other prolific seeders will survive composting and germinate in your garden beds when you use the finished compost. If you pull weeds before they flower and set seed, they are safe to compost. The same rule applies to invasive plants like bindweed, morning glory, and bermuda grass—even small root fragments can regenerate, so these belong in the trash.

Paper, Cardboard, and Wood Products

Glossy or coated paper (magazines, colored cardboard, takeout containers, paper plates with plastic coating) will not break down. The coating is designed to resist moisture and decomposition. Shred plain cardboard and newspaper into small pieces before adding them—they break down faster that way. Avoid cardboard that has been taped or labeled with stickers.

Treated wood and sawdust from pressure-treated lumber contain copper, arsenic, or other preservatives that kill the microbes your compost needs. Sawdust from untreated wood is fine, but if you are not certain the wood was untreated, leave it out. Charcoal ash (from a grill or fireplace) also contains compounds that harm decomposition. Wood ash from a wood stove is acceptable in small amounts, but charcoal ash is not.

Synthetic Materials and Plastics

Anything labeled "compostable" or "biodegradable" that is not certified for home composting will sit in your pile for years. Even certified compostable plastics require industrial composting facilities that reach much higher temperatures than a backyard pile. Regular plastic bags, plastic-lined coffee filters, and synthetic tea bags do not belong in compost. Remove tea bags from their packaging and compost only the leaves inside.

Rubber, synthetic fabrics, and treated textiles will not decompose. This includes old clothing, synthetic rope, and rubber bands. If you want to compost natural fibers like cotton or wool, cut them into small pieces and use them sparingly—they take much longer to break down than plant material.

Citrus Peels, Onions, and Garlic in Large Amounts

Small quantities of citrus peels, onion skins, and garlic are fine, but large amounts can slow decomposition and repel earthworms. The acidity and sulfur compounds in these materials are useful in moderation but become problematic when they make up more than 10 percent of your pile. If you have a lot of citrus peels from canning or cooking, add them gradually over several weeks rather than all at once.

The same caution applies to cruciferous vegetables like broccoli and cabbage. A few leaves are harmless, but a whole head of cabbage will create a dense, slow-to-decompose mass. Chop these materials into small pieces and distribute them throughout the pile rather than layering them together.

Grass Clippings From Treated Lawns and Coal or Charcoal Ash

If your lawn has been treated with herbicides, pesticides, or synthetic fertilizers, do not compost the clippings for at least two weeks after process. These chemicals persist in the grass and will end up in your finished compost, where they can harm plants or contaminate vegetables you grow. Check the label on any lawn treatment product for a waiting period before composting is safe. Even untreated grass clippings should be added in thin layers mixed with brown material (leaves, straw, shredded paper). A thick layer of fresh grass will mat down, exclude oxygen, and create a slimy, anaerobic mess that smells terrible. Spread clippings no more than 2 to 3 inches thick and alternate with brown material.

Coal ash contains sulfur and heavy metals that accumulate in compost and eventually in your soil. Charcoal briquettes used in grills often contain lighter fluid or other additives that are toxic to plants and soil organisms. Wood ash from a wood stove or fireplace is different—it is alkaline and can be added in small amounts (no more than 10 percent of your pile by volume) to balance acidic materials. But if you are uncertain whether the ash came from treated wood or charcoal, leave it out.

Frequently Asked Questions

Can I compost bread, pasta, and grains?

Bread, pasta, rice, and cereal are technically compostable, but they attract rodents just like meat and dairy do. If you want to compost these foods, bury them deep in the center of a hot, active pile where temperatures stay above 140°F. For most home composters, it is safer to throw them in the trash.

What about eggshells and coffee grounds?

Both are excellent for compost. Eggshells add calcium and take several months to break down completely, but that is fine. Coffee grounds and filters (if unbleached) decompose quickly and add nitrogen. Wet coffee grounds can mat if added in large clumps, so spread them out or mix them with brown material.

Is it okay to compost pine needles and conifer leaves?

Pine needles and conifer leaves are very acidic and break down slowly, but small amounts are harmless. They can be useful for mulching acid-loving plants like blueberries. If your pile is already acidic (from lots of fruit scraps or grass clippings), avoid adding more conifers. Large quantities will slow decomposition significantly.

Can I compost hair, nail clippings, and feathers?

Hair, nail clippings, and feathers are made of protein and will eventually decompose, but they take a very long time—often six months to a year. They are safe to add in small amounts, but do not expect them to break down quickly. Shred them into small pieces to speed the process.

What if I accidentally put something in my pile that should not be there?

If you added meat, dairy, or pet waste, remove it as soon as you notice. If the pile smells bad, turn it frequently to add oxygen and speed decomposition. If you added diseased plants or mature weed seeds, you can still use the finished compost on ornamental plants and lawns, but avoid using it on vegetable gardens for one season.