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 below sorts it out. The sections after it explain the reasoning behind each exclusion, correct the myths that cause most of the unnecessary caution, and cover the “compostable” packaging that confuses almost everyone.
What can you put in compost?
| 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 (check bags for plastic) | Bread and cooked grains | Oily or greasy food |
| Crushed eggshells | Grass clippings (thick layers mat) | Cat and dog waste |
| Dry leaves | Wood ash (light dusting) | 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 |
| Hair and pet fur | Black walnut leaves | Coal or charcoal ash |
“In moderation” generally means the material is fine mixed through the pile but causes problems in bulk: it mats, changes pH, breaks down slowly, or tips the balance of carbon and nitrogen. Grass clippings are the classic example: a thin layer is excellent nitrogen, but a thick fresh layer compacts into a slimy, airless mat. Spread clippings thinly and mix them with dry leaves, or let them wilt for a day first. The same thinking applies to bread and cooked grains, which are fine buried in small amounts but attract pests when dumped on the surface in bulk.
What should you never put in compost?
According to the US EPA, home composters should leave out dairy, fats and oils, meat and fish scraps, pet wastes, diseased or insect-ridden plants, coal or charcoal ash, and yard trimmings treated with chemical pesticides. University of Minnesota Extension gives a similar list and the reasons: meat, bones, grease, whole eggs and dairy attract rodents and other animals, and pet feces can transmit disease. Here’s why each matters.
Meat, fish, dairy and oily food do compost. Municipal facilities take them routinely, because their piles run hot and are professionally managed. A backyard heap is neither. These materials break down slowly, go rancid, and reliably attract rodents. If you want to deal with them at home, a bokashi system ferments them in a sealed bucket instead.
Cat and dog waste can carry pathogens, including Toxoplasma and 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. The same goes for plants heavily infested with insects.
Weeds with mature seed heads stay out for the same reason: the seeds survive and you sow them wherever the compost goes. Weeds pulled before seeding are fine, and 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.
Coal and charcoal ash is on the EPA’s list because it can contain substances harmful to plants. Ash from untreated firewood is different: it’s fine as a light dusting, but it is strongly alkaline, so add it sparingly rather than by the bucketful.
Pesticide-treated yard trimmings can carry residues into the finished compost. If a lawn service sprays your grass, leave those clippings on the lawn instead.
Black walnut leaves and twigs appear on the EPA’s exclusion list because they contain juglone, a compound that inhibits the growth of some plants. Oregon State University Extension takes a more relaxed view: juglone breaks down in a well-managed pile with adequate moisture and a diverse mix of materials. A few leaves mixed through an active heap are a modest risk; a whole season of walnut leaves composted on their own is not worth it if you grow tomatoes.
Are compostable plastics really compostable at home?
Mostly, no. Bioplastic bags, cups, cutlery and coffee pods are usually certified for industrial composting, which holds material at sustained high temperatures for a set period. A garden heap doesn’t do that, and the items often come out looking much as they went in.
Even the home-certified versions are unreliable. In University College London’s Big Compost Experiment, published in Frontiers in Sustainability in 2022, 902 UK households tracked biodegradable and compostable packaging in their own home compost. The researchers found that most items did not fully disintegrate, including 60 percent of those certified “home compostable.” They also found that labeling was confusing: in a sample of 50 item images, 46 percent showed no identifiable home-composting certification. Their conclusion was that home composting is not an effective way to dispose of compostable packaging.
The practical rule: keep all plastics out of the heap, including anything sold as compostable, and send them where your local authority directs. Plain paper bags and uncoated cardboard are the safe alternatives.
Can you compost citrus, onions and coffee grounds?
Yes, all three. The worries come from worm bins and get misapplied to ordinary heaps. These are 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.
“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. Paper filters can go in with them.
“Eggshells add calcium quickly.” They add calcium eventually, but 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.
“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 center of the pile rather than leaving it on the surface, and the rodent concern largely disappears.
Why does my compost smell?
Nearly every heap that smells, stalls, or turns to slime has a ratio or moisture problem rather than a contamination problem. Oregon State University Extension traces the two classic smells to different causes:
| Symptom | Likely cause | Fix |
|---|---|---|
| Ammonia smell | Too much nitrogen (greens) | Turn and add sawdust, leaves or another carbon source |
| Rotten-egg smell | Too wet, not enough oxygen | Turn to aerate and add dry material such as straw or wood chips |
| Soggy, slimy texture | Too wet | Turn and mix in dry, absorbent browns |
| Dry, nothing happening | Too little water or nitrogen | Water, add greens, and turn |
The underlying chemistry is the carbon-to-nitrogen ratio. According to Cornell Waste Management Institute, the ideal is about 30 parts carbon to 1 part nitrogen by weight. Food waste sits around 15:1 and grass clippings around 19:1, while leaves run 40–80:1 and paper about 170:1. In practice that means roughly two to three parts browns to one part greens by volume.
Kitchen scraps are almost all greens and they arrive daily. Browns arrive seasonally or not at all. So most home piles drift nitrogen-heavy, which causes the 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 twelve weeks in detail.
Three habits that prevent most problems
Chop things smaller. Surface area drives decomposition speed. A whole cabbage stalk can take a year; the same stalk chopped breaks down far faster.
Bury fresh food scraps in the center 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.
Use a closed bin if you have rodents nearby. A lid, a mesh floor and the no-meat rule together solve most pest visits. Our DIY compost bin guide compares five builds, including the most rodent-resistant designs.