A detailed study of the Hedeoma drummondii complex in Central Texas (i.e., H. drummondii, H. reverchonii var. reverchonii, and H. reverchonii var. serpyllifolia, using more recent names) was presented by Robert S. Irving in his 1968 U. Texas at Austin dissertation. Irving's dissertation actually treated this complex as a single species, and expressed difficulty in determining individual herbarium specimens (p. 141):
In treating the H. drummondii complex as a single species divided into 2 subspecies, each grouping 2 varieties, it should not be surmised that the taxa are clearly defined morphologically; rather, each variety is no more than a "catchall" for a host of interrelated elements or races. Despite intravarietal variation, I have, using herbarium material, been unable to distinguish, with any degree of certainty, among any of the sub-varietal units.