How often can a homeowner put out pre emergent for crab grass before they cause damage to an existing St. Augustine lawn? I went to Lowes and purchased a Green Light Crab Grass granular with Dimension in it, applied it heavy and then watered it in. Because of all the crab grass in the St. Augustine last year I want to make sure I get a handle on it. The soil temperature was 57 degrees 4 inches down. I will apply it again in 2-3 weeks if needed but I do now want to damage the yard by applying it too often. Also, Virginia Buttonweed is popping up and I am not sure if it is too early to spray it. Any and all help would be appreciated. Thanks!