I made a quick U-turn to check out these tall yellow flowers which I had not seen on any roadside in the previous day or two. It turns out that this colony of Ragwort was conspicuously associated with a large Texas Leaf-cutter Ant colony. See the last image in the present set and also here:
https://www.inaturalist.org/observations/76428461
BONAP doesn't have a county record for San Augustine Co., but the species is recorded in counties both E and W of here.
http://bonap.net/MapGallery/County/Senecio%20ampullaceus.png
Similarly, USDA PLANTS (new web site) does not show a San Augustine Co. record:
https://plants.sc.egov.usda.gov/home/plantProfile?symbol=SEAM2
Salado Creek Greenway 1604 Trailhead Area
Not yet blooming. Very slender leaves, almost looks like overgrown centipede grass