What Can't You Compost? (The Complete No-Compost List and Why)

Quick Answer: The items you can't compost depend on the type of composting system you use. In traditional outdoor bins, avoid meat, fish, dairy, cooked food, diseased plants, pet waste, and anything treated with pesticides — these attract pests, create odors, or introduce pathogens. Some of these restrictions don't apply to sealed aerobic systems designed to handle a broader range of food waste. A smaller list of items — glass, plastic, metal, synthetic materials — can never be composted in any system.

Table of Contents

• Never Compostable in Any System
• Not Recommended for Outdoor / Traditional Composting
• Which Restrictions Apply to Electric Composters?
• Items That Depend on Your System
• Quick Reference: No-Compost List
• Frequently Asked Questions
• References

Never Compostable in Any System {#never-compostable}

These items cannot break down biologically and do not belong in any composting system:

Inorganic materials:

• Glass — silica-based, does not decompose
• Metal (cans, foil, utensils) — does not decompose; aluminum foil fragments persist
• Plastic bags, packaging, wrapping — petroleum-based; does not decompose
• Synthetic fabrics (polyester, nylon, acrylic) — petroleum-based polymers
• Rubber — does not biodegrade in composting conditions
• Treated or painted wood — paint and chemical treatments persist in finished compost

Chemically problematic:

• Produce treated with persistent pesticides — some pesticide residues (notably aminopyralid and clopyralid) survive composting and can damage plants when compost is applied. Grass clippings from chemically treated lawns are the most common source.
• Glossy or coated paper — clay coating on glossy paper doesn't break down; plain paper and cardboard are fine
• Heavily inked paper — modern soy-based inks are generally fine; older oil-based inks with heavy metals are not

Not Recommended for Outdoor / Traditional Composting {#outdoor-restrictions}

These items can decompose biologically, but create practical problems in open outdoor compost piles:

Meat, Fish, and Bones

Why not: Meat and fish attract rats, raccoons, flies, and other pests. In open piles, they also create foul odors during the protein breakdown phase (hydrogen sulfide and putrescine production by anaerobic bacteria). Bones take years to break down in outdoor piles.

The alternative: Sealed systems that maintain aerobic conditions and odor filtration can handle meat and fish. Bones remain slow regardless of system.

Dairy Products (Cheese, Butter, Cream, Yogurt)

Why not: Same as meat — high fat content attracts pests, and the breakdown of dairy fats in non-sealed conditions creates odor.

The alternative: Sealed aerobic systems handle dairy without pest or odor issues.

Cooked Food and Leftover Meals

Why not: Cooked food is more attractive to pests than raw scraps, and mixed meals (with oil, sauces, protein) are harder to manage in open piles. The "no cooked food" guideline is largely practical (pest prevention) rather than biological — cooked food breaks down just fine if pest access is controlled.

The alternative: Sealed indoor composters handle cooked food without restriction.

Oily or Greasy Food

Why not: Large quantities of oil slow aerobic decomposition (oil coats organic particles, limiting microbial access and oxygen) and attract pests.

Nuance: Small amounts mixed with other materials are fine. The issue is large quantities applied concentrated.

Diseased Plants

Why not: Many plant diseases are caused by pathogens (fungi, bacteria, viruses) that can survive in compost if the pile doesn't reach adequate thermophilic temperatures (55–65°C sustained for several days). Applying compost made from diseased plant material can spread disease to your garden.

Exception: Hot composting systems that reliably reach and sustain 55°C+ for several days can kill most plant pathogens. If your pile doesn't heat adequately, do not compost diseased plant material.

Weeds That Have Gone to Seed

Why not: Similar to diseased plants — weed seeds can survive in cold or inconsistent compost piles and germinate when you apply the finished compost.

Exception: Thermophilic composting (sustained 55°C+) kills most weed seeds. If your pile heats reliably, seeded weeds can be composted.

Pet Waste (Dog and Cat Feces)

Why not: Pet waste can contain pathogens harmful to humans — including Toxoplasma gondii (cats) and various parasites (dogs). Unlike herbivore manure (chickens, cows, horses), which is commonly composted, carnivore waste carries higher pathogen risk.

Exception: Vermicomposting with specific protocols, or dedicated "pet waste composters" designed for this purpose. Do not use pet-waste compost on food gardens.

Human Waste

Why not: Same pathogen concerns as pet waste. Dedicated humanure composting systems exist with specific safety protocols; standard backyard composting is not appropriate for human waste.

Which Restrictions Apply to Electric Composters? {#electric-composter-exceptions}

Not all electric composters are the same, and some outdoor restrictions don't apply to sealed systems.

Sealed aerobic systems (like Reencle):

The reason sealed aerobic systems handle these inputs: they maintain oxygen throughout (preventing the anaerobic conditions that cause pest-attracting odors), use activated carbon filtration to contain smells during protein breakdown, and eliminate pest access by design.

What sealed electric composters still cannot handle:

• Glass, metal, plastic, synthetic materials (same as any system)
• Large bones (too slow for any home system)
• Pesticide-treated material (chemical issue, not biological)
• Entire large quantities of pure oil or grease at once

Items That Depend on Your System {#depends-on-system}

These items are commonly asked about and the answer depends on specifics:

Citrus peels: Fine in outdoor composting in normal amounts. They decompose like any fruit peel. In large quantities, their acidity and limonene content can be inhibitory, but this is rarely a practical issue. Fully accepted in electric composters.

Onions and garlic: Fine in composting. The "no onions" advice is outdated and not supported by research. The strong smell can attract pests if left on the surface, but buried in an active pile, they decompose normally.

Tea bags: Depends on the bag material. Paper tea bags = compostable. Plastic mesh (nylon) tea bags = not compostable. Check your brand — many have switched to compostable bags but not all.

Coffee filters: Unbleached and oxygen-bleached paper filters = compostable. Plastic mesh filters = not compostable. See our full guide to composting coffee filters.

Bread and grains: Compostable, but attract pests in outdoor piles. Bury deeply or use a sealed system.

Eggshells: Fully compostable in any system. They decompose slowly (calcium carbonate takes time) but are beneficial for pH balance. Crushing them speeds breakdown.

Newspaper and plain paper: Yes — soy-based and water-soluble inks are safe. Shred first. Avoid glossy coated paper.

Cardboard: Yes — an excellent carbon source. Remove tape and staples. Tear into pieces; large flat sheets can mat together and block airflow.

Wood ash: Small amounts are fine and can help adjust pH. Large amounts raise pH significantly — avoid if your soil is already alkaline.

Dryer lint: Depends on what clothes are made of. Lint from synthetic fabrics = microplastics, not compostable. Lint from 100% natural fiber clothing (cotton, wool) = compostable but hard to verify composition.

Quick Reference: No-Compost List {#quick-reference}

Never in any system:

• Glass, metal, plastic, rubber
• Synthetic fabrics
• Treated/painted wood
• Glossy coated paper
• Produce with persistent pesticide residues
• Large bones

Not for outdoor bins (sealed systems may accept):

• Meat and fish
• Dairy products
• Cooked food and meals
• Oily/greasy food in quantity
• Diseased plants (unless pile heats reliably)
• Weeds that have gone to seed (unless pile heats reliably)
• Pet waste
• Bread and grains (pest attraction risk)

Use caution or in small amounts:

• Wood ash (pH impact)
• Dryer lint (check fiber content)
• Tea bags (check bag material)

Frequently Asked Questions {#faq}

Q: Can I compost moldy food?

A: Yes — mold is a sign of biological breakdown already underway. Moldy food is fine in any composting system. Some people avoid adding large quantities of heavily molded food to worm bins (worms can be sensitive to certain molds in large amounts), but outdoor bins and electric composters handle it without issue.

Q: What about meat if I bury it deep in the pile?

A: Burying meat reduces (but doesn't eliminate) pest attraction in outdoor piles. If you have a very hot, active pile that you maintain carefully, small amounts of buried meat can work. In practice, most home composters don't maintain piles hot enough consistently enough to do this reliably. A sealed system is the more practical solution.

Q: I accidentally added something I shouldn't have — will it ruin my compost?

A: Rarely. Single additions of "forbidden" items usually don't ruin a pile. Remove what you can, monitor for odor or pest activity, and add carbon material to neutralize. Composting is forgiving.

Q: Can I compost paper towels and tissues?

A: Plain paper towels used with water or food scraps: yes. Paper towels used with cleaning chemicals or bleach: avoid or use sparingly. Tissues: generally yes if not heavily contaminated with cleaning products.

Q: What about citrus — I've heard it kills worms?

A: Small to moderate amounts of citrus are fine in most composting systems, including worm bins. The "citrus kills worms" concern is exaggerated — worms will avoid a heavily citrus-loaded area of a bin temporarily, but normal amounts mixed with other materials are not an issue. In outdoor piles and electric composters, citrus is fully accepted.

References {#references}

1. U.S. Environmental Protection Agency. Composting At Home. https://www.epa.gov/recycle/composting-home
2. Cornell Waste Management Institute. Backyard Composting. https://cwmi.css.cornell.edu/
3. Rynk, R. (Ed.). (1992). On-Farm Composting Handbook. NRAES-54. Cornell University.
4. University of California Cooperative Extension. Worm Composting (Vermicomposting). https://ucanr.edu/
5. Bateman, M. (2020). Home composting: What goes in and what doesn't. Journal of Environmental Extension, 14(2).