Materials That Slow Decomposition or Attract Pests

Some materials break down so slowly or create such strong odors that they belong in the trash, not the compost bin. Meat, fish, bones, and dairy products rot rather than decompose, drawing rats and raccoons to your bin and creating a smell your neighbors will notice. Oils, fats, and grease coat other materials and prevent air from reaching the microbes that do the actual decomposing work.

Pet waste—including dog and cat feces and used cat litter—contains pathogens that compost temperatures rarely reach high enough to kill. Even in a hot bin, these organisms can survive and contaminate your finished compost. Diseased plant material from your garden should also stay out; if your tomato plant had blight or your roses had powdery mildew, composting spreads those diseases to whatever soil you use the compost in later.

Key Takeaways

  • Meat, fish, bones, dairy, oils, and grease attract rodents and create odor problems that make neighbors complain.
  • Pet waste and diseased plants introduce pathogens and fungi that survive composting and contaminate your finished product.
  • Glossy or coated paper, treated wood, and synthetic materials do not break down and leave plastic or chemical residue in your compost.
  • Weeds with seeds and invasive plants like bindweed or bermuda grass survive the composting process and spread when you use the compost.
  • Sawdust from pressure-treated lumber and coal or charcoal ash contain toxins that accumulate in soil over time.

Paper, Cardboard, and Wood Products to Avoid

Not all paper belongs in compost. Glossy or coated paper—including magazine pages, colored cardboard boxes, and paper with a plastic-like finish—does not break down. The coating is designed to resist moisture, which is exactly what you do not want in a compost bin. Shredded newspaper is fine, but avoid anything printed with colored inks or laminated.

Sawdust from pressure-treated lumber contains copper and arsenic compounds used to prevent rot and insect damage. These toxins accumulate in soil and can persist for years. Sawdust from untreated wood is safe, but if you are not certain about the source, leave it out. The same rule applies to wood ash: ash from a wood stove or fireplace is acceptable in small amounts, but never add charcoal or coal ash, which contain substances that harm soil structure and plant growth.

Synthetic Materials and Treated Textiles

Synthetic fabrics, plastic-based products, and treated textiles do not decompose and will remain in your finished compost as fragments. This includes polyester clothing, nylon, acrylic yarn, and anything labeled "synthetic blend." Even natural fibers like cotton are problematic if they have been dyed with synthetic dyes or treated with flame retardants or water-resistant coatings.

Dryer lint looks harmless but often contains microplastics from synthetic clothing fibers. Rubber bands, twist ties with plastic cores, and stickers all stay intact through the composting process. If you want to compost natural fiber scraps, stick to untreated wool, undyed cotton, and linen—and even then, tear them into small pieces so they break down faster.

Weeds, Seeds, and Invasive Plants

Weeds with mature seeds should never go in a home compost bin. Most home composting systems do not reach temperatures high enough to kill weed seeds reliably. A compost pile typically reaches 130 to 150 degrees Fahrenheit, but seeds from dandelions, crabgrass, and many other weeds survive those temperatures. When you spread your finished compost in the garden, you are spreading thousands of weed seeds.

Invasive plants like bindweed, bermuda grass, and Japanese knotweed are even more problematic because they can regrow from tiny root fragments. A piece of bindweed root the size of a grain of rice can generate a new plant. If your compost does not kill it, you have just created a way to spread the infestation across your entire garden. When in doubt, throw invasive plants in the trash.

Citrus Peels, Onions, and Certain Food Scraps

Citrus peels—orange, lemon, lime, and grapefruit—break down very slowly and can repel worms if you are using a worm bin. The oils in the peel are antimicrobial, which is why citrus is used in cleaning products. A small amount mixed into a large outdoor pile is usually fine, but do not add handfuls of citrus peels to a small bin or worm bin.

Onions and garlic also slow decomposition and can repel worms. Avocado pits take months to break down and are better left out. Bread, pasta, rice, and other cooked grains attract pests just like meat does and should go in the trash. The rule is straightforward: if it smells strong or came from an animal, it does not belong in your compost.

Glossy or Waxed Materials

Waxed paper, parchment paper, and paper coffee cups have a moisture-resistant coating that prevents decomposition. Even though they look like paper, they will not break down in your compost pile. Cardboard takeout containers with plastic or wax linings fall into the same category. Tear off any uncoated cardboard flaps and compost those separately, but throw away the lined portions.

Glossy magazines and catalogs are printed on paper coated with clay or plastic to make the colors bright and the surface smooth. This coating is permanent and will remain in your compost as tiny fragments. Stick to plain newsprint, uncoated cardboard, and paper bags without printing or color.

Coal Ash, Charcoal, and Treated Soil

Coal ash and charcoal ash contain substances that damage soil structure and plant health. Wood ash from a fireplace or wood stove can be added in small amounts—a light dusting, not handfuls—because it contains potassium and other nutrients. But if you are burning treated wood, coal, or charcoal, the ash is contaminated and should go in the trash.

Do not add soil that has been treated with pesticides, herbicides, or fungicides to your compost. These chemicals persist through decomposition and will contaminate your finished product. If you are cleaning out a raised bed that was treated with chemicals, dispose of the soil according to your local hazardous waste guidelines rather than composting it.

Frequently Asked Questions

Can I compost eggshells and coffee grounds?

Yes. Eggshells and coffee grounds are among the safest food scraps to compost. Eggshells take longer to break down than other materials, so crush them into small pieces first. Coffee grounds and filters (if unbleached) decompose quickly and add nitrogen to your pile.

What about grass clippings from a lawn that was treated with weed killer?

Do not compost them. Herbicides persist through decomposition and will contaminate your finished compost. Wait at least two to three weeks after treatment before adding grass clippings, and check the product label for how long to wait. When in doubt, throw treated clippings in the trash.

Is it safe to compost leaves from diseased trees?

No. Fungal spores and bacterial pathogens on diseased leaves survive the composting process. If your oak tree had powdery mildew or your maple had anthracnose, bag those leaves and put them in the trash or take them to a municipal composting facility that reaches higher temperatures than home bins.

Can I add vacuum cleaner bags and lint to my compost?

No. Vacuum bags are usually made of synthetic fibers that do not decompose, and they often contain plastic linings. Dryer lint contains microplastics from synthetic clothing. Both should go in the trash.

What if I accidentally added something I should not have?

If you added a small amount of something like glossy paper or a plastic tie, remove it if you can see it. If it is already mixed in, do not worry—it will not harm your compost pile, but it will remain as fragments in your finished compost. Screen your finished compost before using it to catch larger pieces you can remove by hand.