Don't collect grass clippings from herbicide treated lawns. Even after composting the clippings may still be lethal to your plants.
There was a time that adding grass clippings to your garden as mulch or to the compost pile to kick start the decomposition process was sure thing. Well, with the advent of stronger and more persistent herbicides being used on home lawns, you need to be careful. There's word that a new lawn fertilizer/herbicide will feature the chemical aminocyclopyrachlor. This chemical doesn't break down quickly, so any grass clippings saved and used as mulch or added to home compost piles may still be contaminated with this chemical months later and be harmful to your plants.
Researchers tested compost made from herbicide-treated grass clippings and found traces of the herbicide 200 days after composting. When using the compost to container-grown beans, tomatoes, and cucumbers, all of these veggies showed herbicide injury. While the product company says there will be warnings about using grass clippings from treated lawns in gardens and compost, chances are most consumers will miss the warning.
The moral of this story is to know where your grass clippings are coming from. Collecting clippings from your own yard is the best way to control what's in them. Before taking clippings from your neighbors or other sources, check to see what herbicides have been used on their lawns that season. When buying compost locally, ask if grass clippings were used in making the compost and if they screened the lawns for chemical herbicides. When in doubt, don't use the clippings or compost.
For more on this story go to the article in Biocycle Magazine.