Roadside Flowers of Texas names it simply Eryngo, and eNature goes a step further with "Leavenworth's Eryngo". It's sooo tempting to call it a thistle but it isn't. It is in the carrot family but you can't eat it. Aren't they beautful? I took these pictures on a lane near my home recently.
They start out a blueish green and then when fully developed, they begin turning purple. Eventually, the entire plant will be purple. These are easily recognized from the road, even when driving past at highway speeds.