Some grass clippings can be used for compost if other green plant material isn't available. In general, catching and bagging grass clippings is not worth the effort when they are easily recycled right on the lawn. If clippings are too long to leave on a lawn, composting is a better alternative than disposal in the trash. Some weeds can be composted, particularly if they are pulled before they produce seed. 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 are careful to 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.