GRASS CLIPPINGS--WEEDKILLERSGrass clippings can be composted if weed killers or herbicides have been applied. Herbicides on the market will decompose within a matter of months and not affect the final product.
If fresh grass clippings are used as a mulch, avoid using clippings from the first three (3) cuttings following the application of a pre-emergent or broadleaf weed killer. Otherwise, damage to desirable vegetable, fruit or flower plants may occur.
Source Uni Illinois:
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