Items That Will Ruin Your Compost or Attract Pests
Meat, fish, dairy, oils, and fats break down slowly and create odours that draw rats and raccoons to your bin. Avoid putting any animal products in your compost unless you are using a hot composting method that reaches 140°F (60°C) consistently — most home bins do not. Even then, the smell during decomposition can be a problem in residential areas.
Pet waste and diseased plants also belong in the trash. Cat and dog faeces contain parasites that survive composting and can contaminate vegetables you grow later. Plants infected with fungal diseases, powdery mildew, or blight will spread those diseases to your finished compost and then to your garden beds.
Key Takeaways
- Meat, fish, dairy, oils, and fats attract rodents and create strong odours that neighbours will notice.
- Pet waste and diseased plants introduce parasites and pathogens that survive the composting process.
- Glossy or coated paper, synthetic materials, and treated wood release chemicals into your compost and soil.
- Weeds with seeds and invasive plants like bindweed will survive composting and spread throughout your garden.
- Coal and charcoal ash contain substances harmful to plants, while wood ash from treated wood releases toxins.
Paper and Cardboard You Should Not Compost
Glossy or coated paper — including magazine pages, takeout containers, and pizza boxes with grease stains — does not break down well and can introduce chemicals into your soil. The coating is designed to repel moisture, which is exactly what slows decomposition. Newspaper and plain cardboard are fine, but anything with a shiny finish should go in recycling or the bin.
Synthetic materials like plastic, rubber, and polyester will not decompose in a home compost pile. This includes biodegradable plastic bags, which require industrial composting facilities to break down properly. If you use compost bags, choose paper ones or add the contents directly to your bin without the bag.
Treated Wood, Coal, and Charcoal Ash
Pressure-treated wood, plywood, and wood that has been painted or stained contain arsenic, chromium, or other preservatives that leach into compost and then into your vegetables. Sawdust from treated wood has the same problem. Untreated wood chips and sawdust are safe, but always check the source before adding them.
Coal ash and charcoal ash are not the same as wood ash. Coal ash contains heavy metals and should never go in compost. Charcoal briquettes used in barbecues are treated with chemicals and lighter fluid — throw the ash away. Wood ash from a fireplace or untreated wood fire is acceptable in small amounts, but ash from treated wood is toxic.
Weeds and Invasive Plants
Weeds with mature seeds will survive your compost pile and spread throughout your garden when you use the finished product. Dandelions, thistles, and plantain are common culprits. If you are unsure whether a plant has gone to seed, it is safer to throw it in the trash.
Invasive plants like bindweed, ground ivy, and Japanese knotweed should never enter your compost, even without seeds. These plants can regenerate from tiny root fragments that survive the composting process. If you have invasive plants, bag them and put them in the trash or contact your local waste service about disposal options.
Citrus Peels, Onions, and Garlic in Large Amounts
Small amounts of citrus peels, onion skins, and garlic are fine, but large quantities can slow decomposition and repel worms. Citrus oils are antimicrobial and can kill the microorganisms that break down compost. If you peel several pounds of oranges or onions at once, spread them out over weeks or bury them deep in the pile rather than adding them all at once.
The same applies to other strong-smelling kitchen scraps like pineapple tops and avocado pits. Avocado pits take months to break down and do not add much value to finished compost, so composting them is more trouble than it is worth.
Glossy or Coloured Paper and Magazines
Magazines, junk mail with colour printing, and glossy brochures contain inks and coatings that do not belong in compost destined for food gardens. The chemicals used in colour printing can accumulate in soil over time. Black-and-white newspaper is safe because the ink is soy-based and breaks down readily.
Shredded office paper is acceptable if it is plain white or black-and-white printed. Coloured paper from a home printer is also safe in small amounts, but avoid large quantities of brightly coloured paper, which may contain dyes that are not food-safe.
Sawdust and Wood Chips From Unknown Sources
Sawdust from a lumber mill or untreated wood is valuable for compost, but sawdust from a furniture maker, cabinet shop, or construction site may come from treated, stained, or varnished wood. If you cannot confirm the source, do not use it. The same rule applies to wood chips — ask whether they came from untreated wood or from tree trimming of unknown origin.
Arborist chips are usually safe because they come from freshly cut trees, but always ask the company whether they trim trees that have been treated with pesticides or fungicides. Some tree services spray trees before removal, and that residue stays in the chips.
Frequently Asked Questions
Can I compost bread, pasta, and cooked grains?
Cooked food of any kind can attract rodents and should be avoided in outdoor bins. If you have a sealed, rodent-proof bin or a hot compost system, small amounts of bread and cooked grains are acceptable. Raw grains and uncooked pasta are safer because they do not smell as strong, but they still attract pests if your bin is not well-sealed.
What about eggshells and coffee grounds?
Both are safe and valuable for compost. Eggshells add calcium and break down slowly, which is fine. Coffee grounds add nitrogen and break down quickly. Unbleached coffee filters are also safe. Tea bags are acceptable if they are made of natural fibres, but remove the staple first.
Is it okay to compost grass clippings from a lawn that was treated with weedkiller?
No. Herbicides and pesticides survive the composting process and will contaminate your finished compost. Wait at least two to three weeks after a chemical treatment before adding grass clippings, and check the product label for how long to wait. If you are unsure when the lawn was last treated, compost the clippings separately or throw them away.
Can I put dryer lint in my compost?
Lint from natural fibres like cotton is safe in small amounts, but synthetic fibres like polyester will not break down. If you cannot tell what your clothes are made of, it is safer to throw the lint away. Lint from wool dryers is safe and breaks down quickly.
What should I do with meat and fish scraps?
Throw them in the trash or consider a separate composting method like bokashi or a sealed hot bin designed for meat. If you have a municipal composting program, check whether they accept meat — many industrial facilities do, but home bins should not.