"Round Up" is a brand. Under that brand they make a few different products. One of the products, the one they got famous for, will kill any live plant it touches. But they do make other products designed for weeds intermingled with grass. You can get this stuff on the grass and it wont harm it. Read the label. It will tell you which one you have. You might have the one that doesn't kill grass, which unfortunately is not as effective on other types of weeds.

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Jonathan

2011 Mazda2
2000 E320 4Matic Wagon
1994 C280 (retired)