Plants can be divided according to how they use carbon in photosynthesis. Grasses are C3 plants, since they form atoms with 3 carbons. Broad-leaf weeds form atoms with 4 carbons and are called C4. Weed killers are developed to affect one type of plant, so they will kill weeds and not grass.
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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.