re: What is this weed and how do I kill it?

Whatever you decide to do to combat it, start ASAP. That stuff spreads fast. In my yard, it usually pops up in mid July, so I started fighting it about 2 weeks ago.

Two years ago, I didn't pay attention to it until the end of August, and it had consumed huge areas of my back yard.

This year, I walk around every 3 or 4 days and spray heavily where I find it. When it starts to dry up a few days later, I'll pull it out, careful not to pull up the St. Augustine with it. I'm hoping there's enough time for the grass to grow back in the empty spots.