Herbicides on your lawn won't necessarily doom your grass clippings to the landfill. Most weed killers will decompose in a well-maintained aerobic compost pile. As long as you ensure that no heavy metals or chemicals classified as "persistent herbicides" are present, your herbicide-treated grass clippings will break down along with the rest of your kitchen and yard waste into healthy, safe and nutrient-rich humus.
Identify precisely which chemicals have been used on the grass you mean to compost. Ensure no "persistent herbicides" or heavy metals are present. If you applied the herbicide yourself, check the label on its packaging. If you've thrown it away since, check online or with the store where you purchased it. If someone else administered the herbicide, such as a homeowners association or a municipal maintenance department, ask to review their records.
Identify the sources of any compost that was used on the grass. If it contained manure from cattle or other livestock, identify which herbicides (if any) were applied to their grazing area.
Add the grass to your level, well-drained outdoor composting site once you've determined that no persistent herbicides or heavy metals are present.
Continue building your compost pile, alternating layers of high-carbon "brown stuff" like dry leaves, straw or sawdust, and high-nitrogen "green stuff" like kitchen waste, hay, manure and more grass clippings. Each layer should be 2 to 4 inches thick.
If you're short on "green stuff," you can add about a 1/2 cup of commercial fertilizer for each 10-inch layer of compost. If you're short on "brown stuff," you can add shredded newspaper, but avoid color inserts as the inks can contain toxic chemicals and heavy metals.
Mark a date 90 days out once your compost heap reaches the ideal size for efficient heating, about 4 to 5 feet in each direction. After three months in a healthy compost pile, all non-persistent herbicides will have been rendered harmless.
Monitor the moisture level in your compost heap. Water it as needed to maintain an environment conducive to aerobic composting--about as damp as a wrung-out sponge.
Aerate your compost heap by punching holes in the sides with a hay fork, narrow shovel or other suitable gardening implement.
Monitor the heap's temperature either with a compost thermometer or by occasionally reaching your hand into the pile.
Begin turning the compost heap when it reaches peak temperatures of 130 to 140 degrees F or when the inside of the pile becomes uncomfortably hot to the touch. Turn the heap every second or third day such that outside and inside materials swap places. A hoe, shovel or pitchfork is perfect for this task.
Continue maintaining your compost pile for the full 90 days. Extend this period as long as necessary to allow your compost to convert completely into a cool, crumbly, soil-like humus.