What You Can and Can’t Compost: The Greens, Browns, and Never-Add List Most Guides Skip
The full greens, browns, and never-compost list — plus the invisible risk (persistent herbicides) most compost guides never mention.
Most “what to compost” lists are binary: an item is either fine or it isn’t. In practice, a good third of what gardeners ask about — citrus, dog waste, wood ash, “compostable” takeout containers — doesn’t fit either box. The honest answer is “depends on your pile,” and almost nothing tells you which pile qualifies.
This list sorts every common item into three tiers: always safe, safe only under specific conditions, and never. It also covers the one contaminant that ruins more home compost than anything on a typical avoid list — and it isn’t meat or dairy.
The Three Tiers That Actually Matter
Whether an item belongs in your bin depends less on what it is than on what your pile can do to it. A pile that never gets hot can’t sterilize anything; a pile that regularly hits 140°F (60°C) or higher can process things a cold heap never should touch. That distinction drives every “it depends” answer below — if you haven’t settled on hot, cold, or tumbler composting yet, our composting guide walks through choosing a method before you commit to a bin.
Always Compost: Greens (Nitrogen-Rich)
Greens supply the nitrogen bacteria need to reproduce and generate heat. Add these in thin layers — a solid mat of wet greens goes anaerobic and smells like ammonia.
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| Item | Note |
|---|---|
| Fruit and vegetable scraps | Chop large pieces (melon rinds, whole squash) to speed breakdown |
| Coffee grounds + paper filter | Filter composts along with the grounds; no need to separate |
| Grass clippings | Add in thin layers only — thick mats compact and go anaerobic |
| Crushed eggshells | Technically neutral, not a green, but pairs well here — adds calcium, not much nitrogen |
| Fresh plant trimmings, deadheaded flowers | Skip anything showing disease symptoms — see the Never list |
| Manure (herbivore: cow, horse, chicken, rabbit) | Check it isn’t from hay or pasture treated with persistent herbicide — see below |
| Tea leaves and paper tea bags | Remove any bag with a plastic mesh or staple first |
| Seaweed and kelp | Rinse off excess salt if harvested from a beach |
Always Compost: Browns (Carbon-Rich)
Browns provide the carbon that fuels bacterial energy and keeps a pile from turning into a slimy, smelly mass. Most home piles run short on browns, not greens — keep a bag of dry leaves on hand year-round to balance fresh kitchen scraps.
| Item | Note |
|---|---|
| Dry leaves | Shred first — whole leaves mat down and block airflow |
| Cardboard and non-glossy paper | Remove tape, labels, and glossy inserts; shred for faster breakdown |
| Straw and hay | Verify the source wasn’t treated with a persistent herbicide before adding |
| Untreated wood chips and sawdust | Only from untreated, unpainted wood |
| Pine needles | Slow to break down; use in moderation, chopped if possible |
| Twigs and chipped branches | Run through a shredder or chipper first — whole branches take years |
| Natural-fiber dryer lint (cotton, wool) | Skip lint from synthetic fabrics — see the Never list |
| Nutshells (not walnut) | Break into smaller pieces; they decompose slowly whole |
Aim for roughly equal parts browns to greens by volume for a balanced, low-odor pile, per the EPA’s home composting guidance[1].
The “Depends on Your Pile” Tier
These are the items that generate the most confused advice online, because the correct answer genuinely varies.
Citrus and onions. The old warning that citrus and onions harm compost microbes doesn’t hold up for a traditional outdoor pile or bin — a Cooperative Extension Master Gardener response[2] confirms citrus peels are fine for backyard composting, with negligible effect on soil or compost acidity. The real exception is vermicomposting: Iowa State University Extension[3] specifically advises against feeding worms onion, garlic, peppers, or citrus — the acidity concentrates in a bin in a way it doesn’t in an open pile, and alliums decompose so slowly they rot and smell before the worms get to them. Keep both out of a worm bin specifically, even though they’re fine in a regular pile.
“Compostable” packaging and bioplastics. A compostable-labeled cup, fork, or produce bag is almost never home-compostable, whatever the label implies. According to the EPA[4], products certified compostable are generally tested for industrial facilities that run hotter and longer than a backyard bin ever will — there’s currently no ASTM standard test for home-compost performance at all. Unless the packaging specifically says “home compostable,” treat it as trash, not a green.
Wood ash and high-tannin leaves (oak, cottonwood). UF/IFAS Extension[5] lists wood ash as conditional rather than a flat yes — a light dusting is fine, but more than that spikes soil pH fast enough to hurt acid-loving plants down the line. Oak and cottonwood leaves are the same story: their tannins slow decomposition, so chop them well and use them as a minority ingredient, not the bulk of your browns.
Herbicide-treated grass clippings and hay — the one nobody warns you about. This is the gap in almost every composting list: clippings or hay from a lawn or pasture treated with clopyralid, aminopyralid, aminocyclopyrachlor, or picloram can carry active herbicide residue straight into your finished compost. NC State Extension[6] documents these residues persisting in compost and soil for a year or longer, causing twisted, cupped leaves and stunted, misshapen fruit in broadleaf vegetables at concentrations as low as 3 parts per billion — grasses are unaffected, which is exactly why the contamination goes unnoticed until tomatoes and beans start failing. If you can’t confirm the herbicide history of clippings, hay, or manure from animals fed treated forage, run a simple bioassay before trusting it near vegetables: mix a sample 1:1 with potting soil, sow pea or bean seeds next to a clean-soil control, and compare growth after two to three weeks. I ran this test on a batch of “aged manure” from a neighbor’s horse and watched the bean seedlings curl within ten days — the untreated control grew straight. That one test saved a season’s worth of tomatoes.
Never Compost These
| Item | Why |
|---|---|
| Meat, fish, bones | Attracts rodents and other pests; doesn’t fully break down in most backyard piles |
| Dairy, fats, oils, grease | Same pest and odor problems as meat |
| Pet waste (dog, cat) and cat litter | Can carry pathogens transmissible to humans; cold piles don’t reach a temperature that neutralizes them |
| Diseased or pest-infested plants | Most home piles don’t sustain the heat needed to kill every pathogen — see our guide on disposing of diseased plants for what to do instead |
| Weeds gone to seed (especially bindweed, morning glory) | Some seeds survive even sustained heat — see the mechanism below |
| Treated, painted, or pressure-treated wood | Chemical preservatives and pigments persist through composting |
| Glossy or colored magazine paper, produce stickers | Inks and coatings don’t break down cleanly |
| Synthetic dryer lint (polyester, nylon, spandex) | Sheds microplastic fibers instead of decomposing |
| Charcoal briquette ash | Contains additives that can harm soil microbes (plain wood ash, in small amounts, is different) |
| Black walnut leaves, twigs, and hulls | Juglone remains toxic to sensitive plants like tomatoes even after composting |
Most of what lands on this table isn’t really trash — it’s just wrong for a backyard bin. Meat scraps, dairy, glossy packaging, and “compostable” bioplastics can often go into a municipal curbside organics program or a commercial composting facility, both of which run hot enough and long enough to process them safely. Check with your local waste service before defaulting straight to the regular garbage can.
Why Heat — Not Time — Decides What Survives
The reason cold piles and hot piles have different rules isn’t tradition — it’s microbiology. Weed seeds and plant pathogens don’t die because compost sits for months; they die because a pile gets hot enough, for long enough. Research summarized by eOrganic[7] found that most weed seeds lose 90% viability in under three hours at 140°F, and reach complete kill in under an hour at 158°F. Field bindweed is the outlier: it needs seven straight days at 180°F to fully eliminate, which is why it tops nearly every “never compost” list regardless of pile type. Pathogens fare similarly — in one frequently cited study, only 2 of 17 plant pathogens tested survived a properly managed static pile.
A cold pile, by design, never reaches those temperatures, which is exactly why weed seeds, diseased material, and pet waste stay off the list even for patient gardeners. If sanitizing tricky material is the goal, hot composting is the method built for it — not a cold heap left to sit longer.
Frequently Asked Questions
Can I compost citrus and onions?
Yes, in a traditional outdoor pile or enclosed bin. Keep both out of worm bins specifically — the oils and sulfur compounds bother earthworms.
Is “compostable” packaging safe for my home bin?
Usually not, unless the label specifically says “home compostable.” Most certified-compostable products are rated for industrial facilities, not backyard piles.
How do I know if manure or hay has herbicide residue?
Ask the source directly, or run the bioassay: mix a sample with potting soil, plant beans or peas beside an untreated control, and watch for twisted or stunted growth over two to three weeks.
Will a little wood ash hurt my compost?
A light dusting is fine. Regular, heavy additions raise pH quickly enough to cause problems for acid-loving plants once the compost is applied.
Skip the cold, slimy compost pile.
Enter your brown and green materials — get a balanced C:N recipe and temperature targets that activate hot composting.
→ Build My Compost RecipeI already added something from the Never list — now what?
One mistake batch isn’t a disaster. Pull out anything still recognizable, like bones, plastic, or wood scraps. If you’re running a hot pile, let it finish a full heat cycle before using the compost. If the pile stays cold, or you suspect herbicide-contaminated material went in, run the bioassay above before spreading that batch anywhere near vegetables.
Sources
- Composting At Home — US EPA. epa.gov/recycle/composting-home
- Citrus peels in backyard compost — Ask Extension (Cooperative Extension System). ask.extension.org
- How to Create and Use Vermicompost — Iowa State University Extension. yardandgarden.extension.iastate.edu
- Frequently Asked Questions about Plastic Recycling and Composting — US EPA. epa.gov/trash-free-waters
- Can I Compost It? — UF/IFAS Extension, Sarasota County. sfyl.ifas.ufl.edu
- Manage Compost and Soil Contaminated with Broadleaf Herbicides — NC State Extension. content.ces.ncsu.edu
- Composting to Reduce Weed Seeds and Plant Pathogens — eOrganic. eorganic.org
