Plants lose between 50 and 75 percent of their volume in composting, so a lot of plant material can be processed effectively. Composting can be effective on most yard wastes such as leaves, vegetable and flower plant parts, straw, and a limited amount of woody prunings, grass clippings and weeds. Avoid highly resinous wood and leaf prunings from plants such as junipers, pine, spruce and arborvitae. 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.