A: I believe the specimen you brought into the office is called Annual Jewgrass or Japanese Stiltgrass, Microstegium vimineum. This warm season annual grass can be creeping or erect. The good news: it reproduces only by seed. The bad news: it is difficult to manage and control in St. Augustinegrass since they are both grasses. It prefers moist, shady areas which would keep the St. Augustinegrass thin and allow Japanese Stiltgrass to become better established. Your best plan of action now is to dig up what you can before it produces a seed head. You could dab small amounts of glyphosate (Round-up) on the leaves (do not spray) and it will die within the week. If this weed is too widespread then you must be diligent to spray a pre-emergent next summer to prevent the weed seeds from germinating.
Q: What is this weed growing in my yard?