What Should I Never Put in a Compost Bin?

Most organic matter will break down in a compost pile, but that does not mean everything belongs there. Some materials attract pests, spread disease, introduce toxins, or simply make the composting process much harder to manage. Knowing what to leave out is just as important as knowing what to add.

Meat, Fish, and Dairy Products

Cooked or raw meat, fish, bones, and dairy products like cheese and butter should never go into a standard open compost bin. These materials break down very slowly and create powerful, unpleasant odours as they do. More importantly, they attract rats, foxes, and other animals that will dig into the pile searching for food. Once you have a pest problem, it is very difficult to get rid of. If you want to compost cooked food and meat products, a sealed bokashi system is the right tool — it uses fermentation rather than decomposition, so smells are contained and pests cannot access the material.

Diseased Plant Material

Plants showing fungal infections, bacterial diseases, or viral symptoms should not go into a home compost pile. Most domestic piles do not reach the sustained temperatures needed to kill pathogens reliably. Adding diseased material risks spreading that disease back into the garden when you use the finished compost. Common problem materials include tomato plants with blight, roses with black spot, and alliums with white rot. Dispose of these in your council green waste bin instead — commercial composting reaches much higher temperatures. The same logic applies to club root-infected brassica roots, which can survive for years in soil.

Weeds That Have Set Seed

Healthy annual weeds without seeds are fine to compost. The problem comes when weeds have already flowered and set seed. Many weed seeds survive composting at home because the pile never gets hot enough to destroy them. When you spread the finished compost, those seeds germinate across your beds. Perennial weeds with spreading root systems — bindweed, couch grass, ground elder — are equally risky. Their roots can survive composting and regenerate when spread. Either dry these out in the sun until completely dead before composting, or dispose of them separately.

Treated Wood and Synthetic Materials

Sawdust or wood chips from treated timber, MDF, or plywood can contain preservatives, formaldehyde, or other chemicals that harm the micro-organisms doing the composting work and contaminate the finished product. Stick to sawdust from untreated softwood only. Similarly, synthetic materials like plastic bags, rubber bands, synthetic fabrics, and anything with a plastic lining do not belong in the pile. They do not break down and will end up spread across your garden in the finished compost. Always check that paper products are uncoated before adding them.

Pet Waste and Coal Ash

Dog and cat faeces can carry harmful pathogens including E. coli and toxoplasmosis that are not reliably destroyed at domestic composting temperatures. Keep these out of any pile that will be used on food crops. Coal ash — as opposed to wood ash — contains high levels of sulphur and other compounds that are harmful to soil life and should never go in the pile. Similarly, avoid adding large quantities of citrus peel at once; in moderation it composts fine, but heavy amounts can make the pile too acidic and discourage the worms that help break down material.