Materials That Will Ruin Your Compost
Some materials break down so slowly, attract pests so reliably, or introduce toxins that they will damage your finished compost or make the pile smell and fail. The safest rule is to compost only plant matter, food scraps, and natural materials—and to avoid anything treated with chemicals, anything that came from an animal, or anything designed not to rot.
The most common mistake is adding meat, dairy, oils, or pet waste thinking "it's organic, so it will break down." It will, but only after months of putrefaction that creates odor, attracts rats and flies, and can introduce pathogens into your finished compost. The second mistake is adding glossy paper, treated wood, or diseased plants, which either won't break down or will spread problems to your garden.
Key Takeaways
- Never add meat, fish, bones, dairy, eggs, or oils—they decompose slowly, smell terrible, and attract rodents and insects.
- Do not compost pet waste, diseased plants, or weeds with mature seeds, because they can introduce pathogens or spread problems to your garden.
- Avoid treated wood, glossy or coated paper, synthetic materials, and anything labeled with pesticides or herbicides, because they contain chemicals that don't break down.
- Coal ash, charcoal, and diseased plant material should stay out of your pile even if they seem like they should compost.
Animal Products and Oils to Keep Out
Meat, poultry, fish, and bones take months to decompose and create a smell that attracts rats, raccoons, and flies while they rot. Even small amounts—a chicken bone or a bit of leftover steak—can trigger a pest problem that spreads to your neighbors' yards. The same applies to dairy products like milk, cheese, and yogurt, which decompose slowly and smell sour as they break down.
Eggs and eggshells are often debated. Eggshells alone are fine—they are just calcium and break down slowly but harmlessly. Egg contents (the white and yolk) should stay out because they decompose like other animal proteins and attract pests. Oils, grease, and butter also belong in the trash, not the pile. They create a waterproof layer that blocks air from reaching the center of your compost, slowing decomposition and creating anaerobic (airless) conditions that smell like rotten eggs.
Plant Material That Causes Problems
Diseased plants—those with powdery mildew, blight, rust, or root rot—should never go into a home compost pile. Most home piles do not reach the sustained high temperature (above 140°F for several days) needed to kill plant pathogens. When you spread that finished compost on your garden, you are spreading the disease to healthy plants. If a plant is visibly diseased, throw it in the trash or burn it if your local rules allow.
Weeds with mature seeds are another trap. If a weed has gone to seed, those seeds will survive your compost pile and germinate in your garden beds when you spread the finished product. Weeds without seeds are safe. Diseased lawn clippings (grass with fungal spots or patches) should also be kept out. Grass clippings themselves are excellent for compost, but only if the lawn is healthy.
Treated Wood, Paper, and Synthetic Materials
Pressure-treated wood, plywood, and particle board contain glues and preservatives (often including arsenic or copper compounds in older wood) that do not break down in a compost pile. They will leach into your finished compost and eventually into your soil and plants. The same applies to wood that has been painted, stained, or varnished. Untreated, raw wood chips and sawdust are fine, but if you are unsure whether wood has been treated, leave it out.
Glossy or coated paper—including magazine pages, colored cardboard, and paper with a plastic or wax coating—breaks down very slowly or not at all. Newspaper and plain cardboard are safe. Synthetic materials like plastic, rubber, foam, and fabric do not decompose in a home compost pile and will remain as fragments in your finished compost. This includes biodegradable plastic bags, which break down too slowly for a typical pile.
Ash, Charcoal, and Treated Plants
Wood ash from a fireplace or wood stove can go into compost in small amounts (a light dusting), but coal ash and charcoal should never be added. Coal ash contains sulfur and other compounds that can harm plants, and charcoal is treated with chemicals. Even wood ash in large quantities will raise the pH of your compost too high, making it less useful for acid-loving plants like blueberries and rhododendrons.
Plants treated with pesticides or herbicides—including grass clippings from a lawn that was recently sprayed—should stay out of your pile. The chemicals do not break down during composting and will end up in your garden soil. If you use herbicide on weeds, wait at least two weeks before composting any plant material from that area, and even then, only if the plants themselves were not sprayed directly.
Pet Waste and Human Waste
Dog and cat feces contain pathogens that can survive home composting and contaminate your soil and vegetables. Even if the pile heats up, most home piles do not stay hot enough long enough to kill these organisms reliably. Cat litter, even if it is labeled "flushable" or "compostable," should not go in your pile—it often contains clay or other additives that do not break down.
Human waste (including diapers and sanitary products) belongs in the sewer or trash, never in a compost pile. The pathogens are too dangerous and too persistent. Some municipalities have industrial composting facilities that can handle these materials at high temperatures, but home piles cannot.
Frequently Asked Questions
Can I compost citrus peels and onion skins?
Yes, in moderation. Citrus peels and onion skins break down fine, but large amounts can slow decomposition and repel worms. Chop them into small pieces and mix them throughout the pile rather than dumping them in one spot. A few peels or skins per week is safe.
What about bread, pasta, and cooked food?
Cooked food, bread, and pasta should stay out of your pile. They decompose slowly, attract pests, and can introduce mold and bacteria. Raw vegetable scraps and fruit are safe; cooked versions are not.
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 will contaminate your compost. If you are unsure whether the wood was treated, do not use the sawdust.
Can I put glossy junk mail in my compost?
No. Glossy paper has a plastic or wax coating that does not break down. Plain newspaper and uncoated cardboard are fine, but anything shiny or slick should go in the recycling bin or trash.
What if I accidentally added meat to my pile?
Remove it if you can find it. If you cannot locate it, cover the pile with a layer of finished compost or leaves to reduce odor and pest attraction, and turn the pile more frequently to speed decomposition. The meat will eventually break down, but it will smell and may attract animals in the meantime.