What Not to Compost: Materials That Should Never Go in Your Pile

Knowing what to exclude from a home compost pile is as important as knowing what to include. Some materials are excluded because they attract pests. Others carry pathogens that a home pile cannot reliably neutralize. Others contain synthetic compounds that do not biodegrade and contaminate finished compost. Understanding the reason behind each exclusion helps you apply the rules correctly to unfamiliar materials.

Meat, Fish, and Seafood

Raw and cooked meat, fish, and seafood are among the most pest-attractive materials that can go in a compost pile. They also carry a range of pathogens including Salmonella, Listeria, and E. coli. While a properly managed hot pile at sustained temperatures above 160°F can destroy these pathogens, most home piles do not reliably achieve or maintain these temperatures throughout the pile.

The smell of decomposing meat is also a powerful attractant for rats, raccoons, foxes, and neighborhood cats. In urban and suburban settings, this is a practical problem that makes meat composting in a standard open or enclosed bin impractical regardless of the temperature question.

Do not add meat, fish, poultry scraps, bones, skin, or fat to a standard home compost pile.

Dairy Products

Dairy products, including milk, cheese, butter, cream, and yogurt, create the same odor and pest-attraction problem as meat. They also decompose in a way that produces strong sour or putrid smells if they go anaerobic in a pile. Exclude all dairy from a home compost pile.

Oils and Fats

Cooking oils and solid fats coat organic material in the pile and create a water-repellent layer that slows decomposition and limits oxygen exchange. Large amounts of oil in a pile cause it to go anaerobic. Small traces of oil (salad dressing residue on vegetable scraps, minor fat residue on paper towels) are not a practical concern, but poured-off cooking oil, fat drippings, and lard should not go in the pile.

Diseased Plant Material

Plant material showing signs of disease should not go in a home compost pile unless you can guarantee sustained thermophilic temperatures throughout the entire pile mass. Common plant diseases caused by fungal pathogens (including club root, late blight in tomatoes, and rose black spot) can survive a cold or inadequately managed pile and be returned to the garden in the finished compost. A hot pile that consistently reaches 130°F or above is generally considered safe for diseased plant material. A cold pile is not.

Perennial weeds with extensive root systems and seed heads from annual weeds are in the same category: cold piles will not kill weed seeds, and using finished compost containing viable weed seeds introduces a weed problem across every application site.

Pet Waste (Cats and Dogs)

Cat and dog feces should not go in a home compost pile used for garden application. The pathogen risk is covered in detail in the dog poop in compost guide.

Synthetic Materials and Non-Biodegradables

Glass, metal, plastic, and all materials that do not biodegrade have no place in a compost pile. This includes: plastic bags, plastic packaging, synthetic fabric, rubber, polystyrene, and any material that cannot be broken down by microbial activity.

Chemically treated wood products, including pressure-treated lumber, plywood, and particleboard containing formaldehyde-based binders, should not be composted as they can contribute toxic compounds to the finished product.

Materials Treated with Persistent Herbicides

Grass clippings, plant material, and manure from animals that have grazed on land treated with aminopyralid or clopyralid (persistent herbicides used in some pasture management programs) can cause serious damage to compost and to the plants grown in it. These herbicides are not broken down by composting and persist in finished compost, causing stunting and deformity in plants grown in the treated material. If you source grass clippings or animal manure from unknown sources, this is a risk worth being aware of.

Coal Ash

Wood ash in small quantities is a borderline input (covered in the wood ash in compost guide), but coal and charcoal ash should never go in a home compost pile. Coal ash contains heavy metals and sulfur compounds in concentrations that are toxic to plants and soil organisms. This exclusion applies to coal ash from fireplaces and stoves, and to charcoal ash from grills unless the charcoal is pure lump hardwood charcoal.