Materials That Will Ruin Your Compost
Meat, fish, dairy, oils, and fats break down so slowly in a home compost bin that they attract rats, raccoons, and flies before they decompose. Even small amounts—a chicken bone, a spoonful of butter, a slice of cheese—create the problem. These materials need the sustained heat of industrial composting (around 140°F for several days) to break down safely. Your backyard pile, even if you turn it regularly, rarely reaches that temperature.
Pet waste from dogs and cats should never go in your compost, even if you have a separate bin for it. Dog and cat feces contain parasites and pathogens that survive home composting and can contaminate vegetables you grow. The same applies to human waste and used diapers. These require industrial treatment or a septic system designed for that purpose.
Diseased plants and weeds with seeds will spread the problem to your garden beds when you use the finished compost. If a tomato plant had blight, powdery mildew, or late blight, throw it in the trash instead. Weeds that have already gone to seed—especially dandelions, crabgrass, and bindweed—will germinate in your compost pile and then again in your garden soil.
Key Takeaways
- Meat, fish, dairy, oils, and fats attract rodents and pests because they decompose too slowly in home compost piles.
- Pet waste and human waste contain parasites that survive home composting temperatures and can contaminate your vegetables.
- Diseased plants and mature weeds will spread problems to your garden beds when you use the finished compost.
- Treated wood, glossy paper, and synthetic materials do not break down in compost and will remain as fragments in your soil.
- Coal and charcoal ash contain substances that harm plant growth, unlike wood ash from untreated wood.
Treated Wood, Plastic, and Synthetic Materials
Pressure-treated wood, plywood, and composite lumber contain copper, arsenic, or other chemicals used to prevent rot and insect damage. These toxins leach into your compost and then into your soil and plants. The same rule applies to wood that has been painted, stained, or sealed. Untreated, unpainted scrap wood is fine, but if you are unsure whether it was treated, leave it out.
Plastic bags, even those labeled "compostable" or "biodegradable," do not break down in a home compost pile within any reasonable timeframe. Industrial composting facilities have the heat and equipment to process them, but your bin does not. The same applies to synthetic fabrics, rubber, foam, and anything made from petroleum-based materials. These will remain as fragments mixed into your finished compost.
Glossy or coated paper—including magazine pages, takeout containers, and paper plates with a plastic or wax coating—should not go in your bin. The coating prevents decomposition and can introduce chemicals into your soil. Newspaper and plain cardboard are safe, but tear off any plastic tape or labels first.
Ash, Coal, and Charcoal
Wood ash from a fireplace or wood stove can go in compost in small amounts because it contains potassium and other minerals plants need. However, coal ash and charcoal ash should never be added. Coal ash contains sulfur and other substances that lower soil pH and harm plant growth. Charcoal briquettes used in grills often contain lighter fluid or other additives that are toxic to plants.
If you do add wood ash, use no more than a thin layer every few months, and only if your soil is not already alkaline. Too much ash raises pH to the point where plants cannot absorb nutrients. Test your soil pH before adding ash, or skip it entirely if you are unsure.
Citrus Peels, Onions, and Garlic in Large Amounts
Small amounts of citrus peels, onion skins, and garlic are fine in compost. The problem arises when you add them in large quantities. Citrus peels contain oils that slow decomposition and can repel worms. Onions and garlic contain sulfur compounds that create odor and can inhibit the microbes that break down compost. If you have a lot of these materials—say, from canning or a large harvest—add them gradually over weeks rather than all at once, or throw the excess in the trash.
The same caution applies to cruciferous vegetables like broccoli and cabbage. A few leaves are harmless, but a whole head or several heads will create a strong smell and slow decomposition. Spread them out over time or compost them separately if you have large quantities.
Glossy or Colored Paper and Cardboard
Newspaper and plain cardboard boxes are excellent for compost because they break down quickly and add carbon. Glossy magazines, junk mail with colored inks, and cardboard with plastic windows do not belong in your pile. The inks and coatings contain heavy metals and chemicals that persist in compost and end up in your soil.
Paper with a waxy or plastic coating—including freezer paper, parchment paper, and some food packaging—should go in the trash. The coating prevents decomposition and introduces synthetic materials into your finished compost. If you are unsure whether a paper product is coated, tear it; if it does not tear easily or feels slick, leave it out.
Sawdust from Treated Wood and Pressure-Treated Lumber
Sawdust from untreated, unpainted wood is a good carbon source for compost. Sawdust from pressure-treated lumber, plywood, or painted wood is not. The chemicals used to treat wood persist in the sawdust and will contaminate your compost and soil. If you get sawdust from a woodworking shop or lumber yard, ask whether the wood was treated before adding it to your pile.
The same applies to wood chips and mulch made from treated wood. Stick to chips from untreated hardwood or softwood, or make your own by chipping untreated branches and logs.
Frequently Asked Questions
Can I compost bread, pasta, and grains?
Yes, in small amounts. These materials break down faster than meat or dairy, but they still attract rodents if you add too much at once. Bury bread and pasta under several inches of brown material (dry leaves, shredded paper) to keep pests away. A slice of bread or a cup of cooked pasta every week or two is fine; a whole loaf or a large batch of pasta is not.
What about coffee grounds and tea bags?
Coffee grounds are excellent for compost and add nitrogen. Tea bags are fine if they are made from paper or plant fibers, but remove the staple first. Many tea bags contain plastic microfibers that do not decompose, so if you are unsure, tear open the bag and compost only the leaves and paper.
Can I compost sawdust from my workshop?
Only if the wood was untreated. Ask the supplier or the person who gave you the sawdust whether the wood was pressure-treated, stained, or painted. If there is any doubt, throw it away. Treated wood sawdust will contaminate your compost for years.
Is it okay to compost a small amount of meat or fish?
No. Even a small amount attracts rodents and creates odor problems. Meat and fish decompose so slowly in home compost that they sit in your pile for months, drawing pests the entire time. Throw meat and fish in the trash or check whether your city has a separate food waste collection program.
What if I accidentally put something in my compost that should not be there?
If you added meat, dairy, or oils, remove as much as you can and bury the rest under several inches of brown material. Watch for pest activity over the next few weeks. If you added treated wood or plastic, remove it before the compost is finished. Small fragments may remain, but removing the bulk prevents further contamination.