Materials You Cannot Compost

By Cathy Cromell, The National Gardening Association

Composting isn’t a free-for-all. You can’t toss in anything and everything you come across waste-wise and expect it to produce usable, healthy compost. Some materials definitely don’t qualify as compost ingredients because they contain pathogens, attract pests, or cause other problems. Save yourself hassles and headaches by keeping the following items out of your composting operation:

  • Meat, bones, grease, fats, oils, and dairy products: They turn rancid and smelly and attract dogs, cats, raccoons, foxes, and rodents.

  • Feces: Waste from dogs, cats (including soiled cat litter), pet birds, pigs, and humans may contain parasites that are transferable to and infectious for humans.

  • Charcoal barbecue or coal ashes: All gardeners should leave these alone because they contain sulfur oxides and other chemicals you don’t want to transfer to your garden.

  • Wood ashes: Wood ashes are alkaline. If you garden where soils are alkaline (like much of the western and southwestern United States) you don’t want to increase alkalinity by adding ashes to your compost mix. However, if you garden where soils are acidic, wood ashes can be added in small amounts. Sprinkle handfuls throughout as you mix a pile.

  • Treated wood products: Don’t add wood chips or sawdust from chemically treated or pressure-treated wood.

If you become a serious composting enthusiast who likes to monitor and maintain hot piles, the following three items can be composted. Monitoring your pile’s temperature and turning it frequently are essential. If you describe yourself as a laid-back, “compost happens” gardening guy or gal, you’re better off safe than sorry. Dispose of these problem-prone plant materials in the trash:

  • Weeds with seed heads. You can pull weeds before they go to seed and toss them in your compost pile as a good source of nitrogen. But if seeds have set, toss the entire plant in the trash.

  • Disease- or insect-infested plant material.

  • Plants that spread with invasive root systems, such as African couch grass, Bermuda grass, bindweed, Canada thistle and other thistles, dock weed, morning glory, and nettle. Just a smidgen of this root material can survive to sprout another day and spread havoc throughout your garden.

If throwing away organic matter, no matter how weedy and disease-ridden, sends minor guilt pangs up and down your spine but you don’t have time to regularly maintain a hot pile, stockpile all the bad stuff in a separate bin where it can’t inadvertently be mixed in with the good stuff. Or place all the bad stuff in a large (30- to 40-gallon), black, thick plastic garbage bag and seal it. When the quantity is sufficient and you have plenty of green, nitrogen-rich materials (such as grass clippings or manure) to add to it, build one pile to neutralize the problems. Laboring over just one hot heap per garden season isn’t as time-consuming as ensuring that every pile heats up to the red zone.

Another option is to take diseased or invasive plant material to your local recycling center that collects green waste. Ask if they compost at sufficiently high temperatures to destroy your problem plants. If they do, make your contribution; if they don’t, it’s back to Plan A.