I have been trying to take over a completely overgrown lawn at a house I purchased since the beginning of April. We removed several trees, bushes and shrubs to the best of our ability and sodded the entire yard with St. Augustine grass. For the most part, the St. Augustine has been thick enough to choke out the other plants from coming back. It's also worth mentioning, that any of the overgrowth that had a root or stump larger than about 1/2" I drilled and poured RoundUp into the hole to hopefully kill off.
However, the plant picture below is popping up in the most random places in my yard. I don't actually remember removing anything that looked like this, so I'm guessing it was dormant back in April. I live in the Dallas, TX area so it's a hot, dry climate right now. These plants have started coming up in the cracks in my concrete, around my pool equipment and along my deck drains. They grow at insanely fast rates, the one pictured was cut down last time I mowed which was less than 2 weeks back.
The ones along my deck drain get just as big and ugly, I even removed my deck drains about a week and half ago and dug out as much dirt and roots as I could in an attempt to kill them and they're already back. Roundup knocks them out for a few days, but they always come right back.
Can anyone identify this plant and help come up with a plan to kill them off for good?