The plants not to compost fall into three groups: diseased plants, herbicide-treated material, and invasive weeds. Tossing any of these into your bin can ruin a whole batch of compost. Most home piles don't get hot enough to break down the toxins and pathogens these materials carry.
I made one of the worst composting mistakes of my growing life when I dumped treated grass clippings into my bin. The herbicide lived through months of composting. The next spring I spread that compost on my tomato starts. Every plant curled its leaves into tight cups and stopped growing. I lost a full flat of starts to one bad batch of clippings.
So what not to compost, and why do these materials cause so much harm? It all comes down to heat. Penn State's NOP composting standards say piles must reach 131 to 170°F (55 to 77°C) for three or more days to kill pathogens. Most backyard bins barely hit 130°F in the center. The edges stay far cooler. Disease bugs in the outer layers survive the whole process and come back to life when you spread your finished batch.
Certain plants cause special problems in your bin. When you add diseased plants compost can carry organisms like fusarium wilt into your next crop. Black walnut leaves and bark hold juglone, a toxin that kills tomatoes, peppers, and eggplants on contact. Invasive weeds like bindweed can regrow from tiny root bits that make it through your pile. Even a quarter-inch piece of bindweed root will sprout and take over a bed.
Diseased Plant Material
- Common culprits: Tomatoes with late blight, roses with black spot, and squash with powdery mildew carry spores that survive home piles.
- Risk level: These pathogens can live in your soil for 3 to 5 years after you spread bad compost across your garden beds.
- Safe disposal: Bag sick plants in plastic and send them to city waste rather than risking your whole compost bin.
Herbicide-Treated Plants
- Persistent chemicals: Aminopyralid and clopyralid survive composting and harm sensitive crops like tomatoes and beans at tiny doses.
- Hidden sources: Hay, straw mulch, and grass from treated lawns carry these chemicals into your bin with no visible warning at all.
- Test first: Grow a test pot of beans in your finished compost and watch for leaf curling over two weeks before using it.
Invasive Weed Material
- Worst offenders: Bindweed, quackgrass, and nutsedge all regrow from root pieces that survive home compost piles intact.
- Seed survival: Weed seeds need heat above 140°F to die, and most home piles only hit that mark in the very center.
- Safe option: Drown invasive weeds in a sealed bucket of water for four weeks before you add the slurry to your bin.
When you stand over your bin holding a handful of sketchy plant material, bag it and throw it away. Losing a few scraps of organic matter is nothing next to ruining a pile that took months to build. Guard your compost the way you guard your garden beds. The finished product touches every plant you grow with it.
Read the full article: Potting Soil Guide for Beginners
