What not to put in a home compost heap
Most guides to composting focus on what to add. This one is about what to leave out, because a handful of common mistakes account for most of the pest problems, smells, and stalled heaps that people run into.
Quick fix
- Keep meat, fish, dairy, and cooking oil out entirely — they’re the biggest cause of rats and flies.
- Skip cooked food and pet waste from cats and dogs.
- Bin, don’t compost, diseased plants and weeds that have gone to seed.
Meat, fish, dairy, and oils
These break down fine in an industrial composting facility that runs hot enough to kill off problems fast, but a backyard heap usually doesn’t get anywhere near that hot, consistently enough, to handle them safely. Left in a home pile, they smell strongly as they decompose and are one of the most reliable ways to attract rats, flies, and other scavengers. Cooking oil and fat cause a similar problem — they don’t break down cleanly, they coat other material and slow it down, and they smell rank as they go rancid.
If you want to deal with food scraps that include meat and dairy, a sealed bokashi bin is worth mentioning — it ferments rather than composts, using a different process entirely, and is designed to handle this kind of waste safely. It’s a separate system from an open heap, though, not something to mix in.
Cooked food
Even without meat or dairy in it, cooked food behaves differently to raw scraps once it starts breaking down — it tends to go slimy and smelly faster, and like meat, it’s a strong pest attractant. Raw vegetable peelings, fruit scraps, and coffee grounds are fine; leftover pasta, rice, or cooked vegetables are best going elsewhere, ideally a council food-waste collection if you have one.
Pet waste
Waste from cats and dogs can carry pathogens that a home compost heap won’t reliably kill, since backyard piles rarely sustain the sustained high temperatures needed to neutralise them. It’s best kept out of a heap that will eventually go on food-growing beds. Herbivore manure is a different matter and is a genuinely useful addition — see the browns-and-greens reference guide for more on that.
Diseased plants and weeds gone to seed
Plant material affected by disease or fungal problems can survive the composting process in a typical home heap and reintroduce the same problem when you spread the finished compost. The same goes for weeds that have already set seed: rather than being killed off, the seeds often just get redistributed around your garden along with the compost. Both are best sent to a council green-waste collection, which usually processes material at higher, more consistent temperatures than a garden heap can manage. A genuinely hot, well-managed pile can kill off some seeds and pathogens, but it’s hard to guarantee that consistently at home, so treating it as a maybe rather than a certainty is the safer call.
Glossy paper and large woody material
Glossy or heavily coated paper — magazines, glossy flyers — often has coatings that resist breaking down and can leave unwanted residue in the finished compost. Plain cardboard and newspaper are fine. Large branches, thick prunings, or big offcuts of wood take a very long time to break down whole and can sit in a heap for years without much changing. If you want to use woody material, shred or chip it first so there’s more surface area for the process to work on; whole branches are better sent to a green-waste collection or used elsewhere, like as edging or kindling.
What this leaves you with
Cutting these out doesn’t shrink your options much: raw fruit and vegetable scraps, coffee grounds and tea leaves, eggshells, garden trimmings, dry leaves, cardboard, and shredded paper cover the large majority of everyday household and garden waste, and none of them carry the same pest or pathogen risk. When in doubt about something specific, the safer choice is usually to leave it out and find another route for it rather than risk it in the heap.