What Should You Never Put in a Compost Bin? A Complete List of What to Avoid
Meat, dairy products, pet feces, diseased plants, treated wood, and synthetic materials should never go in a backyard compost bin. These materials either attract dangerous pests, introduce pathogens like E. coli and Salmonella that standard home composting temperatures cannot reliably eliminate, or contain chemicals that contaminate finished compost. The good news: the vast majority of your kitchen and garden waste — fruit and vegetable scraps, coffee grounds, eggshells, cardboard, dried leaves — composts safely and beautifully. This guide explains exactly what to avoid, and why the science behind each restriction matters.
Table of Contents
- Why Some Materials Don't Belong in Compost
- Meat, Fish, and Bones
- Dairy Products and Eggs (but not eggshells)
- Pet and Human Waste
- Diseased or Pest-Infested Plants
- Treated, Painted, or Pressure-Treated Wood
- Oils, Fats, and Greasy Food
- Synthetic Materials and Coated Papers
- What You CAN Compost: A Quick Contrast
- Quick Reference Checklist
- Frequently Asked Questions
- References
Why Some Materials Don't Belong in Compost
Composting is fundamentally a biological process. Microorganisms — bacteria, fungi, and actinomycetes — break down organic matter under specific conditions: the right moisture, aeration, carbon-to-nitrogen balance, and temperature. When you add materials that disrupt those conditions, or introduce biological hazards the process can't safely neutralize, you create problems ranging from bad smells and pest infestations to genuinely dangerous compost contaminated with human pathogens.
The U.S. EPA's composting guidelines distinguish clearly between what is safe for standard backyard composting and what requires industrial-scale high-temperature processing to render safe [U.S. EPA, 2023]. The restrictions below are based on that science, not arbitrary rules.
Meat, Fish, and Bones
Never add to a backyard compost bin. This is the most important rule for most home composters.
Meat and fish scraps decompose, but they do so in ways that cause two serious problems:
1. Pathogen risk. Raw meat and fish harbor Salmonella, Listeria, E. coli, and other harmful bacteria. Standard home compost piles often do not sustain the temperatures needed (55–65°C / 131–149°F, held for several days) throughout the entire pile to ensure pathogen destruction. The outer layers of a passive pile almost never reach thermophilic temperatures [Cornell Composting, Cornell University].
2. Pest attraction. The odor compounds released as meat decomposes — cadaverine, putrescine, and fatty acids — are powerful attractants for rats, raccoons, foxes, dogs, and flies. A single addition of cooked chicken can bring pests that are difficult to dislodge for weeks.
Bones decompose extremely slowly — taking years in a home pile — and should be kept out for that reason alone, in addition to the pest concern.
Exception: Industrial composting facilities operating at sustained high temperatures (55–70°C throughout) can safely process meat and fish. Some electric home composters, including the Reencle, use controlled temperature and microbial environments that can handle a broader range of food waste — always check your specific unit's guidelines.
Dairy Products and Eggs (but not eggshells)
Avoid: milk, cheese, butter, yogurt, cream, and whole eggs. Allow: clean, dry eggshells.
Dairy products create the same two problems as meat — pathogen risk (particularly Listeria monocytogenes, which is associated with soft cheeses and can survive in moist environments) and powerful pest-attracting odors as fats go rancid.
The chemistry is straightforward: dairy fats are high in triglycerides that break down into free fatty acids during decomposition. Those fatty acids are both smelly and slow to fully mineralize in a typical aerobic pile.
Eggshells are fine. They are composed almost entirely of calcium carbonate and contribute calcium to finished compost. Rinse them lightly, let them dry, and crush them before adding — they'll break down faster.
Pet and Human Waste
Never add dog feces, cat feces, or human sewage to home compost.
Pet waste is categorically different from farm animal manure (which can be composted under the right conditions). The key distinction is diet. Dogs and cats are carnivores; their digestive tracts routinely harbor:
- Toxocara canis (dog roundworm) and Toxocara cati (cat roundworm) — eggs that survive in soil for years and can infect humans, particularly children.
- Toxoplasma gondii — a protozoan parasite shed in cat feces, dangerous to pregnant people and immunocompromised individuals.
- Campylobacter, Salmonella, and E. coli O157:H7 — bacterial pathogens present in both dog and cat waste [U.S. EPA, 2023].
The U.S. EPA explicitly advises against composting pet waste at home, as home composting systems cannot reliably achieve and maintain the temperatures necessary (55°C+ held for 15 days or longer) to destroy these organisms throughout the entire pile [U.S. EPA, 2023].
Cat litter — even if labeled "natural" or "biodegradable" — should also be excluded. Clay litters are not biodegradable; plant-based litters may be compostable in theory but the fecal content makes them unsafe for home systems.
Diseased or Pest-Infested Plants
Do not compost: plants showing active fungal disease (powdery mildew, late blight, damping-off), bacterial disease, or plants that are heavily infested with persistent pests (clubroot-affected brassicas, plants with scale or root-knot nematodes).
Most pathogens and pest eggs can be killed in a properly managed hot compost pile that reaches 55–65°C throughout. The problem is that most backyard piles don't achieve those temperatures uniformly. Outer layers and corners often stay at ambient or mesophilic temperatures [Haug, R.T., 1993].
If diseased material survives composting and is applied to garden beds, you reintroduce the pathogen — sometimes in higher concentration than if you had simply disposed of it.
Safe to compost: lightly wilted or spent plants with no signs of disease, healthy prunings, vegetable tops (carrot tops, bean plants after harvest).
Best practice: When in doubt, bag it and send it to a municipal green waste program, which uses industrial hot composting.
Treated, Painted, or Pressure-Treated Wood
Never compost: pressure-treated lumber (often labeled CCA — chromated copper arsenate, or newer ACQ — alkaline copper quaternary), painted or varnished wood, or plywood and particleboard (which contain formaldehyde-based adhesives).
Pressure-treated wood contains biocides by design — chemicals specifically chosen to resist microbial and insect breakdown. These do not simply disappear in a compost pile; they accumulate in the finished material and can be transferred to soil and plants [Rynk, R. (Ed.), 1992].
Safe wood products: untreated cardboard, plain newsprint (most modern inks are soy-based and safe), paper bags, plain brown paper. These are excellent carbon-rich "brown" materials for balancing compost.
Oils, Fats, and Greasy Food
Avoid large quantities: cooking oil, lard, butter, heavily oiled or fried foods.
Small amounts of vegetable oil will break down in an active compost pile, but larger quantities cause problems. Oils coat organic particles and create a hydrophobic layer that blocks water infiltration and oxygen movement — both essential for aerobic decomposition. An oil-coated pile becomes anaerobic more quickly, slowing decomposition dramatically and creating foul odors [Cooperband, L., 2002].
A teaspoon of residual oil on a piece of cardboard is fine. A cup of used frying oil poured into a bin is not.
Synthetic Materials and Coated Papers
Never add: plastic bags (even "compostable" plastics require industrial composting conditions), synthetic fabrics, rubber bands, metal ties, glossy magazine paper, waxed cardboard (like produce boxes lined with wax).
Waxed cardboard is a common mistake. It looks like regular cardboard, but the wax coating prevents microbial access to the cellulose fibers beneath. Plain corrugated cardboard — the brown kind, with the fluting visible — is an excellent compost material when torn into pieces.
"Compostable" or "biodegradable" plastics labeled as such typically require industrial composting temperatures (58°C+ for 10+ days) to break down within a reasonable timeframe. In a home pile, they behave like regular plastic [U.S. EPA, 2023].
What You CAN Compost: A Quick Contrast
The list of what to avoid is shorter than you might think. The vast majority of kitchen and garden waste composts safely:
| Safe to Compost (Green / Nitrogen) | Safe to Compost (Brown / Carbon) |
|---|---|
| Fruit and vegetable scraps | Corrugated cardboard (plain) |
| Coffee grounds and paper filters | Brown paper bags |
| Tea leaves and paper tea bags | Dried leaves |
| Fresh grass clippings | Straw |
| Eggshells | Paper (non-glossy) |
| Fresh plant material | Sawdust (untreated wood only) |
| Vegetable peelings | Dried corn stalks |
For a complete guide to green and brown materials, see our post on What food scraps are best for composting?
Quick Reference Checklist
Never put these in a home compost bin:
- [ ] Meat and fish (raw or cooked)
- [ ] Bones
- [ ] Dairy products (milk, cheese, butter, yogurt)
- [ ] Dog or cat feces
- [ ] Cat litter (any type)
- [ ] Diseased plants or plants with persistent pest infestations
- [ ] Pressure-treated, painted, or varnished wood
- [ ] Waxed cardboard or glossy paper
- [ ] Large quantities of cooking oil or grease
- [ ] Synthetic or plastic materials (including "compostable" plastics)
Frequently Asked Questions
Q: Can I compost bread, pasta, or cooked rice? A: These are technically compostable, but they attract pests (particularly rodents) and can create odors when they ferment in a moist pile. Small amounts buried deep in an active pile are generally fine. Large quantities should be avoided in open bin systems. Electric composters with sealed units handle cooked grains much more safely.
Q: Can I compost citrus peels? A: Yes, absolutely. The old concern about citrus peels acidifying compost or harming worm bins has been largely debunked for standard composting. In a large compost pile, citrus peels break down normally. In worm bins (vermicomposting), limit citrus to small amounts, as worms prefer to avoid very acidic material. For pile composting, chop citrus into pieces to speed decomposition.
Q: What about onion and garlic scraps? I've heard they're bad for compost. A: Onion and garlic scraps are compostable and do not harm a standard compost pile. The concern about them is primarily for worm bins — worms avoid strong alliums. In a regular compost pile, they break down fine, though they may temporarily emit a strong odor. Chop them up and bury them in the pile to minimize smells.
Q: Can I put dryer lint in compost? A: Only if your clothing is made entirely from natural fibers (cotton, wool, linen). Most modern clothing is a synthetic blend, and lint from synthetic fabrics deposits microplastics in your compost and, eventually, your soil. When in doubt, don't add dryer lint.
Q: Are "compostable" food service containers (PLA cups, corn starch cutlery) safe to add? A: Not for home composting. These products are certified compostable under industrial conditions (ASTM D6400 or EN 13432), meaning they need 58°C+ for extended periods. In a home pile, they degrade slowly and unpredictably, leaving fragments. They belong in municipal composting programs that have the necessary conditions to process them properly.
References
U.S. Environmental Protection Agency. (2023). Composting at Home. https://www.epa.gov/recycle/composting-home
Cornell Waste Management Institute. Cornell Composting: Science and Engineering. https://compost.css.cornell.edu/
Haug, R.T. (1993). The Practical Handbook of Compost Engineering. Lewis Publishers.
Rynk, R. (Ed.). (1992). On-Farm Composting Handbook (NRAES-54). Northeast Regional Agricultural Engineering Service.
Cooperband, L. (2002). The Art and Science of Composting. University of Wisconsin-Madison Extension. https://learningstore.extension.wisc.edu/
USDA Natural Resources Conservation Service. Composting. https://www.nrcs.usda.gov/
Related Posts:
- What food scraps are best for composting? A complete guide to green and brown materials
- What is the carbon-to-nitrogen ratio in composting, and why does it matter?
- What role do microorganisms play in composting? The biology explained simply
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