Almost everything that was recently alive and came from a plant can go in a home compost heap. The genuine exclusions are short: meat, fish, dairy, oily cooked food, pet waste from cats and dogs, diseased plants, weeds carrying seed, and anything chemically treated. Nearly everything else people worry about — citrus, onions, coffee grounds, bread, paper — is either fine or fine in moderation.
The table sorts it out, and the section after it corrects the myths that cause most of the unnecessary caution.
The quick reference
| Goes in freely | In moderation | Keep out |
|---|---|---|
| Fruit and vegetable scraps | Citrus peel | Meat and fish |
| Coffee grounds and filters | Onion and garlic skins | Dairy and cheese |
| Tea leaves | Bread and cooked grains | Oily or greasy food |
| Crushed eggshells | Grass clippings (thick layers mat) | Cat and dog waste |
| Dry leaves | Wood ash | Diseased plants |
| Shredded cardboard and paper | Sawdust (untreated only) | Weeds gone to seed |
| Straw and hay | Manure from herbivores | Treated or painted wood |
| Prunings and dead plants | Paper towel | Glossy or coated paper |
| Untreated wood chips | Cut flowers | “Compostable” plastics |
Why the exclusions are excluded
Meat, fish, dairy and oily food do compost — municipal facilities take them routinely, because they run hot enough and are managed. A backyard heap is neither. These materials break down slowly, go rancid, and reliably attract rodents. If you want to compost them at home, a bokashi system ferments them in a sealed bucket instead.
Cat and dog waste can carry pathogens — Toxoplasma, roundworm — that a home pile does not get hot enough to destroy. Never use it on food crops. Manure from herbivores (horse, cow, chicken, rabbit) is a different matter and is excellent.
Diseased plants should stay out because home piles rarely sustain the temperatures needed to kill fungal spores and bacteria, and you would be distributing the problem across your garden with the finished compost.
Weeds with mature seed heads for the same reason: the seeds survive and you sow them wherever the compost goes. Weeds pulled before seeding are fine — good nitrogen, in fact. Perennial roots like bindweed and couch grass need drowning in a bucket of water for a few weeks first, or they simply regrow in the heap.
“Compostable” plastics and bioplastic liners are the one that catches people out. Most are certified for industrial composting — sustained high temperature over a set period — and simply do not break down in a garden heap. They come out looking much as they went in.
The myths worth correcting
“Citrus kills the worms.” Citrus is acidic and breaks down slowly, so large volumes of peel are genuinely a problem for a worm bin. In an ordinary compost heap, moderate amounts are entirely fine. Chop it and spread it through rather than dumping a bag of peel in one spot.
“Onions ruin compost.” Same story. Fine in a normal heap, unwelcome in quantity in a worm bin.
“Eggshells add calcium quickly.” They add calcium eventually — whole shells can still be identifiable after a year. Crush them finely or accept them as a slow-release ingredient rather than a fix for anything.
“Coffee grounds are acidic and will lower pH.” Used grounds are close to neutral, most of the acid having gone into the coffee. They are a good nitrogen source and worms are drawn to them.
“You need a compost activator or starter.” You need nitrogen, moisture and air. A layer of grass clippings, some manure, or a few shovelfuls of soil or finished compost does the same job as a purchased product.
“Bread attracts rats so it can’t go in.” Bread composts perfectly well. Bury it in the centre of the pile rather than leaving it on the surface, and the rodent concern largely disappears.
The ratio matters more than the list
Nearly every heap that smells, stalls, or turns to slime has a ratio problem rather than a contamination problem. Compost microbes need roughly two to three parts carbon-rich “browns” to one part nitrogen-rich “greens” by volume.
Kitchen scraps are almost all greens and they arrive daily. Browns — dry leaves, cardboard, paper — arrive seasonally or not at all. So most home piles drift nitrogen-heavy, which is what causes the classic wet, smelly, ammonia-scented failure. Keeping a sack of shredded cardboard beside the bin and covering every food deposit with a couple of handfuls fixes the problem structurally.
Our beginner’s guide covers the ratio and the first month in detail, and the DIY bin guide has five builds if you still need somewhere to put it.
Two habits that prevent most problems
Chop things smaller. Surface area drives decomposition speed. A whole cabbage stalk takes a year; the same stalk chopped takes weeks.
Bury fresh food scraps in the centre rather than leaving them on top. It controls smell, deters flies and rodents, and puts the material where the pile is warmest and most active.