re: What is this weed that's running through my St. Aug?

That my friend is the early spawn of Virginia buttonweed. It will bloom in summer and spread like wildfire, creating a mat that kills out all grass underneath it

It can’t really be killed in a single season. Took half my lawn one year when I didn’t know what it was.

I often try to rip it up now so that my grass will take over and slow it’s growth in the summer when it’s most deadly. But that ultimately helps it grow back the next year so it’s not always the best way to manage it if you have it in multiple locations

Celsius herbicide is the only answer I’ve found. Apply at the higher rate. Do it now before it gets too hot. Just spot treat if that’s your only weed problem