Q: What the heck is this weed growing in my St. Augustinegrass?
A: I am glad you brought this weed into the office for me to identify because it is so similar looking to grass. The weed you brought in is Doveweed, Murdannia nudiflora (L.) Brenan or Aneliema nudiflorum (L.) Kunth. It is classified as an annual grass-like weed which makes it difficult to kill because it is a distant relative of turfgrasses such as St. Augustine. It loves moist areas and tolerates shade very well.
Doveweed produces attractive, small purple or blue flowers but they are not very showy. The good news is that is reproduces by seed only, which means you should use an herbicide (pre-emergent) that will discourage the seeds from germinating next year. Pre-emergents, such as atrazine, should be used only twice a year (once in the spring and once in the fall). This product is very potent and the label must be followed to avoid environmental damage. Keep it away from water areas such as retention ponds and wells.
Nothing will kill the adult, seed producing plant now except a non-selective herbicide like Round-up. Of course, if you use a non-selective herbicide you run the risk of killing any green plant it touches. You might consider pulling this weed since it is an annual. The upside to pulling is you get rid of the adult and the future offspring all in one fell swoop.