What NOT to Compost: 15 Surprising No-Nos

Almost everything from the kitchen and garden can be composted, but the exceptions matter more than people think. A few well-known no-nos — meat, dairy, diseased plants — get all the attention, while a handful of surprising items quietly cause smells, attract pests, spread weeds, or even contaminate your finished compost with persistent chemicals. Knowing what to keep out is half of making good compost.

Here are 15 things you should not put in a typical backyard compost pile, why each one is a problem, and what to do with them instead. This list assumes a standard home bin; municipal compost programs often accept more.

The 15 No-Nos

1. Meat, Bones, and Fish

They rot with a powerful odor, draw flies, raccoons, and dogs, and take a long time to break down in a cool pile. Cooked or raw, they do not belong in a backyard bin. Option: municipal organics pickup, or a bokashi system that handles them indoors.

2. Dairy Products

Milk, cheese, yogurt, and butter go rancid, smell terrible, and attract pests just like meat. Keep them out of the pile. A few teaspoons of yogurt are fine; a whole carton is not.

3. Fats, Oils, and Grease

Grease coats compost particles, seals out oxygen, and turns the pile anaerobic and slimy. A smear on a paper towel is harmless; bottles of oil are not. Put cooking oil in the trash or recycle it where your city allows.

4. Diseased Plants

Compost bins rarely sustain the sustained high heat needed to kill pathogens like tomato blight or powdery mildew spores. Tossing infected foliage back in the garden means recycling the disease. Option: dispose of badly diseased plants in the trash, or solarize them.

5. Weeds That Have Gone to Seed

Even hot piles do not reliably kill every seed. A cool pile full of seed heads becomes a weed factory when you spread the compost. Pull weeds before they flower, or leave seed heads to dry in the sun — and keep the most aggressive weeds, like bindweed and bermudagrass, out entirely. See our weed suppression guide for safer disposal options.

6. Cat and Dog Waste

Pet feces can carry parasites and pathogens that survive composting and are dangerous near food crops. Even with a separate pet-waste bin, the finished product should never touch edible gardens.

7. Treated or Painted Wood Scraps

Sawdust and chips from pressure-treated, painted, or varnished wood can leach chemicals into your compost and then into your soil. Use only untreated wood waste, in small amounts mixed with plenty of browns.

8. Glossy or Coated Paper

Magazines, shiny catalogs, and coated paper break down slowly and can contain clay coatings and inks. Plain paper, cardboard, and newspaper are fine — the glossy stuff is not.

9. Coal Ash

Unlike wood ash, coal and charcoal briquette ash can contain heavy metals and sulfur compounds that harm plants. Only wood ash in moderation belongs in compost.

10. Walnut Leaves and Husks

Black walnut contains juglone, a natural chemical that inhibits many plants — tomatoes and peppers are especially sensitive. Walnut debris in compost can carry juglone into your beds for months. Keep it out.

11. Synthetic Fabrics and “Compostable” Plastics

Polyester, nylon, and most “biodegradable” plastics either never break down in a home pile or require industrial composting. Only certified compostable products with clear labels belong in a bin, and even then many home piles are too cool to process them.

12. Large Branches and Logs

Thick wood takes years to break down and will clog your turning efforts. Chip or shred branches first, or set them aside for a dedicated wood pile. What you can chip, you can compost — what you cannot, leave out.

13. Lawn Clippings Treated with Weed Killer

Herbicide residues on grass clippings can persist through composting and damage vegetables later. If you treat your lawn with weed-and-feed products, keep those clippings off the pile for several mowings.

14. Ashes from Treated Lumber or Charcoal

Any ash from painted, glued, or manufactured wood products can carry chemical residues. Wood ash from clean, untreated firewood is fine in small amounts; everything else goes in the trash.

15. Citrus Peels in Bulk (For Worm Bins)

Citrus is fine in a hot outdoor pile, but it is a real problem in vermicomposting: the acidity and peel oils irritate red wigglers and attract fruit flies. If you keep a worm bin, add citrus sparingly, in small chopped pieces, and only occasionally. Understanding greens vs. browns helps you judge where each item belongs.

What to Do Instead

• For meat, dairy, and oils: municipal organics programs, bokashi bins, or the trash.
• For diseased plants and seed heads: trash or hot-sun solarization; keep them out of cool piles.
• For wood waste: chip it for mulch, or burn clean wood for ash and use sparingly.
• For paper and cardboard: shred plain, uncoated types and add as browns; recycle the glossy stuff.

FAQ

Can I compost citrus peels and onions?

In an outdoor pile, yes — in moderation, chopped small, and mixed in so they do not sit in a sour clump. Worms and compost tea makers are the exception: citrus and onions are hard on red wigglers and can taint tea brews. Keep those two out of vermicomposting systems.

Is it safe to compost bread and cooked pasta?

Stale bread, rice, and plain pasta are fine in small amounts — they are carbon-rich greens/browns hybrids. The risk is pests: rodents love grains. Bury them deep under browns or skip them if you have a rodent problem. Avoid anything coated in oil or sauce.

What about eggshells?

Eggshells are one of the best additions — rinse them, crush them, and add them. They provide calcium, improve structure, and help buffer pH. They do not belong on the no list; the only rule is to crush them so they break down within a season instead of a year.

How hot does a pile need to be to kill weed seeds and pathogens?

Sustained temperatures of roughly 130–150°F for several days, with regular turning so every particle gets exposed, are what it takes to reliably kill most seeds and pathogens. Most backyard piles never reach that consistently — one more reason to keep the riskiest materials out rather than gamble on heat. Building a proper hot pile from the start is the best way to get there.

Bottom Line

The golden rule of composting is simple: when in doubt, leave it out. Meat, dairy, oils, diseased plants, seeded weeds, pet waste, and chemically treated materials are the categories that cause the worst problems, while the surprising items — walnut leaves, coal ash, glossy paper, synthetic fabrics — quietly degrade your finished compost. Compost the good stuff, divert the bad, and your pile stays sweet, hot, and productive. What you feed the pile is exactly what your garden will get back, so feed it well.

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