There are no herbicides that will kill nimblewill grass while sparing zoysia grass. Nimblewill grass is very difficult to remove, so the best way to control it is to use a general herbicide and replant the zoysia in that area, per the University of Illinois Extension.
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Marie Notnats enjoys learning about new topics and teaching people new skills. She has been writing professionally since 2008. She is a professional horticulturist with a Bachelor of Science degree in horticulture from Brigham Young University. She writes for About.com, Examiner.com, eHow, and assorted gardening blogs.