Materials That Will Ruin Your Compost

Your compost pile breaks down organic matter through heat and microbial activity, but some materials either won't decompose, will attract pests, or will introduce chemicals and pathogens that make the finished compost unsafe for food gardens. The most common mistake is adding meat, dairy, oils, and diseased plants—items that either create odor problems and rodent infestations or spread plant diseases directly to your garden beds.

Knowing what to exclude saves you months of troubleshooting and prevents your pile from becoming a pest magnet or a source of contamination. The rule is straightforward: if it came from an animal, if it's treated with chemicals, or if it's diseased, it does not belong in your pile.

Key Takeaways

  • Never add meat, fish, bones, dairy, eggs, or oils—these attract rats and raccoons and create foul odors as they decompose.
  • Do not compost diseased plants, weeds with seeds, or grass clippings treated with herbicides, because they will spread disease or chemicals to your garden.
  • Avoid glossy or colored paper, treated wood, coal ash, and pet waste, all of which contain toxins or pathogens that persist in finished compost.
  • Citrus peels, onions, and garlic can slow decomposition and repel worms, so use them sparingly or in small amounts only.
  • Wood chips and sawdust from treated lumber will leach arsenic and other preservatives into your soil over years.

Animal Products and Oils That Attract Pests

Meat, poultry, fish, bones, and any scraps with fat or oil are the fastest way to turn your compost into a rodent and raccoon feeding station. These materials decompose slowly in a home pile and create a strong smell that travels far. Rats will tunnel into your pile, and raccoons will tear it apart nightly. Even small amounts—a chicken bone, a spoonful of leftover gravy—are enough to trigger pest activity.

Dairy products including milk, cheese, yogurt, and butter belong in the same category. They decompose slowly, smell bad, and attract the same animals. Eggs and eggshells are often debated: eggshells alone (rinsed and crushed) are fine, but the egg white and yolk inside will attract pests, so crack them fully and rinse them before adding the shell only.

Cooking oils, grease, and salad dressings coat other materials and slow decomposition. They also attract pests and can create a waterproof layer that prevents air from reaching the pile's center. If you have a small amount of oil on a plate, wipe it with a paper towel and compost the towel instead of rinsing it into your pile.

Diseased Plants and Weeds With Seeds

A home compost pile typically reaches 130 to 150 degrees Fahrenheit at its hottest point, which is not hot enough to kill many plant diseases or weed seeds. If you add tomato plants with early blight, squash with powdery mildew, or any plant showing spots, wilting, or unusual discoloration, you are moving that disease directly into your finished compost and then spreading it across your garden beds the following season.

Weeds that have already gone to seed will germinate in your pile and then again in your garden. Dandelions, crabgrass, and chickweed are common culprits. If you pull weeds before they flower, they are safe to compost. If they are already flowering or have seed heads, throw them in the trash instead. The same rule applies to grass clippings from a lawn treated with herbicides or fungicides—those chemicals persist through decomposition and will damage or kill vegetables and flowers when you use the finished compost.

Diseased wood (wood with cankers, rot, or fungal growth) should also stay out. Burning it is better than composting it, because the disease can survive in your pile and spread to healthy plants later.

Treated Wood, Coal Ash, and Glossy Paper

Pressure-treated lumber, plywood, and particle board contain arsenic, copper, or other preservatives that leach into compost over months and years. If you use that compost on a vegetable garden, those metals accumulate in the soil and eventually in the plants you eat. Sawdust and wood chips from treated lumber carry the same risk. Untreated wood chips and sawdust are fine, but if you are not certain whether the wood was treated, leave it out.

Coal ash and charcoal ash differ from wood ash. Wood ash from a fireplace or wood stove is safe in small amounts (it raises pH and adds potassium), but coal ash contains heavy metals and sulfur compounds that damage soil and plants. If you burn coal, do not compost the ash. Charcoal briquettes used in grills also contain additives and should not go in your pile.

Glossy or colored paper, including magazine pages, coated cardboard, and printed labels, contains inks and coatings that do not break down. Plain newsprint and uncoated cardboard are fine. If the paper has a shiny finish or bright colors, it stays out of the pile.

Pet Waste and Human Waste

Dog and cat feces contain pathogens that survive home composting temperatures and can persist in finished compost for years. These pathogens cause serious illness if they contaminate vegetables or fruit. Even if you never eat raw produce from your garden, the pathogens can leach into groundwater or be tracked into your home on shoes and hands. The same applies to human waste from composting toilets or septic systems—it requires industrial-scale heat and processing to be safe.

Used cat litter, even if it is made from plant material, should not go in your pile because it is contaminated with cat feces. Throw it in the trash instead. If you have a dog, pick up waste from your yard and dispose of it separately.

Slow-Decomposing Materials and Pest Repellents

Citrus peels (orange, lemon, lime), onion skins, and garlic cloves decompose very slowly and can repel earthworms and other decomposers that speed up the process. A single orange peel will sit in your pile for months. If you want to add them, chop them into small pieces and use only a handful at a time, mixed well into the pile. Better yet, save them for a separate pile or bury them directly in garden beds where they will break down in place.

Walnut leaves and black walnut hulls contain juglone, a chemical that inhibits plant growth and also slows decomposition. Do not add them to your pile. Eucalyptus leaves have similar properties and should also be avoided.

Glossy or waxed produce stickers and labels will not decompose. Peel them off before composting the fruit or vegetable. Bread and baked goods are safe to compost, but they can attract pests if your pile is not well-managed, so bury them deep in the center of the pile rather than leaving them on top.

Synthetic Materials and Treated Textiles

Plastic, rubber, synthetic fabrics, and polyester do not decompose in a home compost pile. Even "compostable" bags labeled for industrial composting will not break down in your backyard pile—they require the heat and processing of a commercial facility. If a label says "industrial compost only," it does not belong in your home pile.

Cotton and wool fabric are natural and will decompose, but only if they are undyed and untreated. Dyed or printed clothing, synthetic blends, and elastic should stay out. If you have old cotton rags or wool sweaters with no dyes or finishes, you can add them, but cut them into small pieces first so they break down faster.

Dryer lint is mostly synthetic fibers and should not go in your pile. Lint from natural fabrics only (which is rare) is acceptable in small amounts, but most dryer lint is a mix of polyester and other synthetics.

Frequently Asked Questions

Can I compost paper towels and napkins?

Yes, plain white paper towels and napkins are safe to compost. If they have food residue on them, that is fine—just make sure the food is not meat, dairy, or oil. Avoid colored or printed napkins, which may contain inks that do not break down.

What about coffee grounds and tea bags?

Coffee grounds are excellent for compost and add nitrogen. Tea bags are safe only if they are made from paper or plant material—many commercial tea bags contain plastic mesh or synthetic fibers. Remove the tea from the bag and compost the leaves, or use loose-leaf tea and compost the whole thing.

Is it okay to add a small amount of meat or fish to my pile?

No. Even a small amount will attract rodents and create odor problems. If you want to compost fish scraps or meat bones, use a separate enclosed bin designed for that purpose, or bury them deep in your garden bed where they will decompose in place without attracting pests to your main pile.

Can I compost sawdust from my workshop?

Only if the wood is untreated. Sawdust from pressure-treated lumber, plywood, or particle board contains preservatives that will contaminate your compost. If you are unsure whether the wood was treated, do not use the sawdust.

What should I do with diseased plants I pull from my garden?

Throw them in the trash or burn them if local regulations allow. Do not add them to your compost pile, because the disease will survive and spread to your garden beds when you use the finished compost the following season.