The Basic Rule: Plant Matter Yes, Animal Products and Treated Materials No

Compost breaks down plant material through decomposition. Browns (dry leaves, straw, shredded paper) and greens (grass clippings, vegetable scraps, coffee grounds) rot down into soil-like material over months. The rule is straightforward: put in what came from a plant and will rot. Keep out meat, dairy, oils, diseased plants, and anything treated with chemicals.

The reason matters. Meat and dairy attract rats and create odor problems. Oils slow decomposition. Diseased plants spread infection to your garden soil. Treated wood and glossy paper contain chemicals that don't break down and will end up in your finished compost. Understanding what breaks down and what doesn't prevents a pile that smells, attracts pests, or produces unusable compost.

Key Takeaways

  • Vegetable and fruit scraps, grass clippings, leaves, and shredded paper are safe to compost and break down reliably.
  • Meat, fish, dairy, oils, and fats should never go in a home compost bin because they attract rodents and create odor.
  • Diseased plants, weeds with seeds, and plants treated with pesticides will contaminate your finished compost.
  • Glossy or colored paper, treated wood, and synthetic materials do not break down and should be thrown away or recycled instead.
  • Pet waste and human waste belong in the trash or sewer, never in compost meant for food gardens.

Greens You Can Compost

Greens are nitrogen-rich materials that heat up a pile and speed decomposition. Vegetable and fruit scraps are the backbone of most home compost: carrot peels, apple cores, lettuce leaves, onion skins, banana peels, and melon rinds all break down quickly. Coffee grounds and tea bags (remove the staple from tea bags first) add nitrogen and moisture. Grass clippings work well if they have not been treated with herbicide or pesticide.

Plant-based kitchen waste is safe, but avoid putting in large quantities of citrus peels or onions at once—they can slow decomposition if they dominate the pile. Crushed eggshells are fine and add calcium. Fresh manure from herbivores (horse, cow, chicken) can go in if you are willing to wait longer for the pile to finish, because it needs time to cool and kill pathogens; never use manure from meat-eating animals.

Browns You Can Compost

Browns are carbon-rich materials that provide structure and balance nitrogen. Dry leaves, straw, hay, and shredded newspaper or cardboard are standard. Sawdust from untreated wood works, as does wood chips from trees without disease. Paper towel tubes, plain cardboard boxes (remove tape and labels), and shredded office paper all decompose. Pine needles take longer but will eventually break down.

Avoid glossy or colored paper, which contains inks and coatings that do not decompose. Cardboard with plastic coating (like takeout boxes) should not go in. Sawdust from pressure-treated wood or plywood is off-limits because it contains chemicals. If you are unsure whether paper or cardboard was treated, leave it out—it is easier to throw it away than to contaminate a whole pile.

What Never Goes in Home Compost

Meat, fish, bones, and seafood shells attract rats, raccoons, and flies. They also create strong odors as they rot. Dairy products (milk, cheese, yogurt, butter) have the same problem—they draw pests and smell bad. Oils, grease, and fatty foods slow decomposition and can make the pile anaerobic (oxygen-starved), which creates a foul smell. Avoid these entirely in a home bin.

Pet waste from dogs and cats should never go in compost meant for food gardens, because it can carry pathogens harmful to humans. Human waste belongs in the sewer, not a compost pile. Diseased plants—those with blight, mildew, or fungal infection—will spread disease to your garden soil when you use the finished compost. Weeds that have gone to seed will germinate in your garden later. Plants treated with herbicide or pesticide should be thrown away, not composted, because the chemicals persist through decomposition.

Treated and Synthetic Materials to Avoid

Pressure-treated wood, plywood, and composite wood contain arsenic or other preservatives that do not break down. Glossy magazines, colored paper, and paper with plastic coating contain inks and polymers that persist. Synthetic materials—plastic, rubber, metal, glass—do not decompose and will end up as contamination in your finished compost. Charcoal and coal ash contain substances that inhibit plant growth; use only wood ash from untreated wood, and use it sparingly.

Treated sawdust, wood stain, paint, and varnish are chemical products that belong in hazardous waste, not compost. If you are uncertain whether something is treated, assume it is and throw it away. A small amount of contamination in a large pile may not ruin the batch, but it is not worth the risk to your garden.

How to Handle Borderline Materials

Some materials work in commercial or hot compost but not in home bins. Bread, pasta, and cooked grains can go in if your pile gets hot enough (above 140°F) to kill pathogens, but in a cool home pile they attract pests. Nuts and seeds may not break down fully and can sprout later. If you have a hot, well-managed pile, you can include them; if you are unsure, leave them out.

Glossy or waxed cardboard (like milk cartons) is sometimes compostable in industrial facilities but not reliably in home bins. Sawdust from treated wood is never safe. When in doubt, the safest choice is to throw it away or recycle it rather than risk contaminating your compost.

Signs Your Compost Has a Problem

A strong ammonia or rotten smell means you have too much nitrogen (too many greens) or something that should not be there—usually meat or dairy. Stop adding greens, mix in more browns, and turn the pile. A sour or vinegary smell indicates anaerobic conditions, usually from too much moisture or too little air; turn the pile and add dry material.

If you see maggots or fruit flies, you likely have meat, dairy, or cooked food in the pile. Remove what you can, bury new scraps deeper under browns, and cover the top with a layer of soil or finished compost. Rodent activity means meat or dairy is present; stop adding it and consider moving to a bin with a solid bottom and lid. These problems are preventable by following the rules about what goes in.

Frequently Asked Questions

Can I compost bread, pasta, and rice?

Cooked bread and pasta can attract pests in a home pile, so it is safer to avoid them. Raw grains and uncooked pasta are better, but they may not break down fully. If your pile stays above 140°F and you turn it regularly, you can include them; otherwise, throw them away.

What about citrus peels and onions?

Both are safe to compost, but in large amounts they can slow decomposition and repel worms. Chop them into small pieces and mix them throughout the pile rather than dumping them in one spot. A handful at a time is fine.

Can I compost paper with ink on it?

Plain black ink on newspaper or office paper is safe. Glossy magazines, colored inks, and coated paper should not go in because the inks and coatings do not break down. When in doubt, recycle the paper instead.

Is wood ash from my fireplace okay to add?

Wood ash from untreated wood is safe in small amounts and adds potassium, but use it sparingly—too much raises pH and can inhibit plant growth. Never use ash from charcoal, coal, or treated wood.

What if I accidentally put meat in the pile?

Remove it if you can. If it is already buried and decomposing, stop adding food scraps, turn the pile to speed decomposition, and add more browns. The pile will eventually finish, but it may smell and attract pests in the meantime. Next time, keep a separate container for meat scraps and throw them away instead.