Every "what can I compost" list on the internet is an invitation list. This one is the bouncer. What not to put in compost is the more useful question, because the cost of a wrong item is higher than the benefit of a right one. One diseased tomato plant can infect a garden for years. One bag of dog waste can contaminate compost you planned to put on food crops. Get the exclusions right and almost everything else is fair game.
The never list
Meat, fish, and bones. They attract rats, raccoons, and flies, smell awful, and can harbor bacteria a home pile never gets hot enough to kill. The pest problem alone is disqualifying.
Dairy products. Milk, cheese, yogurt, sour cream: same story as meat. Foul odors while decomposing and a dinner bell for pests.
Fats, grease, and oils. They do not break down like other food scraps, coat materials and slow decomposition, and can take years to finish. Small incidental amounts on food are fine; the fryer oil is not.
Cat and dog waste (and cat litter). Pet waste from carnivores can carry parasites, bacteria, and pathogens harmful to humans, including toxoplasma and roundworms. Herbivore manures (rabbit, chicken, cow, horse) are fine and nitrogen-rich. Cat and dog waste never is.
Diseased or insect-infested plants. Most home piles do not sustain the heat needed to kill pathogens and pest eggs. You are not composting the disease away; you are giving it a lift back into the garden. Bag it and trash it.
Weeds that have gone to seed. Many weed seeds survive home composting temperatures. You will find out when they germinate by the thousand wherever you spread the compost.
Charcoal ash. Unlike plain wood ash, which is fine in small amounts, charcoal ash contains chemical residues from the manufacturing process.
Chemically treated yard waste. Clippings from lawns treated with weed killers or pesticides can kill the beneficial organisms your pile depends on, and residues may persist into the finished compost. Know what was sprayed before you add it.
Thermal paper receipts and shipping labels. Paper is compostable, but receipt paper is thermal paper containing chemicals like BPA. Shredded newspaper and plain cardboard are fine; receipts are not.
The surprising yes list
The never list gets all the attention, but the border items are where people waste good material:
Paper towels and napkins. Perfectly fine as long as they have no chemical cleaners on them. They are a carbon source, like shredded newspaper.
Cardboard (plain, uncoated). An excellent brown. Remove tape and labels, and avoid wax-coated boxes.
Coffee filters and tea bags. Coffee grounds are a nitrogen source; the filters are carbon. Both go in.
Dryer lint. Fine in small amounts, though it is worth noting that synthetic fabric lint adds microplastics, so lint from natural fibers is the better choice.
Hair and fur. Slow to break down, but nitrogen-rich and perfectly safe.
Eggshells. A calcium source. Crush them first or they will outlast everything else in the pile.
Woodstove ash. Fine in small quantities for its mineral content. Charcoal ash is not, which trips people up.
The it-depends list
A few items live in the middle. Large amounts of citrus, onion, or garlic are widely believed to repel earthworms at high concentrations, so go easy in a worm bin even if the outdoor pile handles them. Bread and pasta are compostable but nitrogen-heavy and prone to mold; small amounts mixed well are fine. Black walnut material contains juglone, a compound toxic to many plants, so finished compost containing it should not go on sensitive garden beds. And sawdust from treated or painted wood stays out, while sawdust from untreated wood is a fine brown.
My rule of thumb
If you have to talk yourself into an item, leave it out. The upside of composting one borderline thing is a slightly faster pile; the downside is disease, pests, or contaminants in compost that touches your food. The never list above covers nearly every real disaster I have seen in beginner piles. Master the exclusions and the inclusions take care of themselves, and you can check the math on everything you do add with the ratio tool below.
What should you never put in compost?
In a home pile: meat, fish, dairy, fats and oils, cat or dog waste, diseased plants, seeded weeds, charcoal ash, chemically treated yard waste, and thermal paper receipts.
Can you compost paper towels?
Yes, as long as they have no chemical cleaners on them. They are a carbon source like shredded newspaper and cardboard.
Why can't you compost meat and dairy at home?
They attract pests, create foul odors, and can harbor pathogens a home pile never gets hot enough to kill. Commercial facilities run hot enough to handle them, which is why curbside programs allow what backyard bins should not.
Can you compost diseased plants?
Not in a typical home pile. Pathogens and pest eggs survive most home composting, and the finished compost reintroduces the disease to your garden. Bag diseased material and trash it.
Want more of these? I write one practical guide a week on gardening, composting, and the tools that decode them. Subscribe free here.