The resins protect these materials from decomposition and extend the time needed for composting in comparison with other plant materials. The easiest way to handle grass clippings is to leave them on the lawn. Research shows that they return valuable nutrients back to the soil. Some grass clippings can be used for compost if other green plant material isn't available. Many, but not all, plant disease organisms are killed if the compost reaches 122 degrees F. Temperatures will vary throughout the compost.
Outer layers stay cooler than the center and cause uneven kill of disease organisms.
If a plant is severely diseased, it is better to dispose of it in the trash. In general, avoid plants treated with weed killers. Small amounts of herbicide-treated plants may be mixed in the pile as long as you allow for thorough decomposition. Weed killers and other pesticides break down at various rates. If you use treated grass clippings, the breakdown of these chemicals should be at least as fast as breakdown in the soil.