QUESTION: What plants should not be composted?
ANSWER: While most plant material and garden trash can be added to your compost bin or pile, there are some plants you should take care not to include. Any plants that show signs of disease should be disposed of in the trash instead of in your compost. Using diseased plants in compost can re-introduce the disease to your garden when the compost is added to the garden’s soil.
Similarly, you should avoid using invasive weeds in compost. They’re likely to take root in the compost pile and start growing if the compost isn’t very hot, or they can spill their seeds into the compost, which means they’ll end up growing in the garden again once the compost is used.
Grass clippings are normally a good composting material, but if your grass has been treated with herbicides to kill weeds, the clippings should not be included. Some weed killers don’t break down in the compost pile, so they can persist to end up harming your plants and trees when the compost is spread.
Of particular concern are clopyralid, aminopyralid, and aminocyclopyrachlor. These substances don’t decompose even inside an animal’s digestive system, so they’re certain to survive the composting process.