Pentstemon caudatus Heller, Minnesota Bot. Stud. 2: 34. 1898. P. angustifolius (f.) caudatus Rydb. Bull. Torrey Bot. Club 33: 151. (7 Apr) 1906. P. angustifolius var. caudatus Rydb. Fl. Colorado (Agric. Exp. Sta. Colorado Agric. College 100:) 309. (Aug) 1906. P. sec- undiflorus var. caudatus A. Nels. in Coult. & Nels. New Manual Bot. Centr. Rocky Mts. 444. 1909. P. angustifolius (subsp.) caudatus Pennell, Contr. U.S. Natl. Herb. 20: 362. 1920. P. angustifolius subsp. caudatus Keck in Kearney & Peebles, J. Wash. Acad. Sci. 29: 490. 1939. TYPE: UNITED STATES. NEW MEXICO. Taos Co.: Barranca, 6900 ft, 26 May 1897, A. & E. Heller 3581 (ISOTYPES: NY-2 sheets).
Sandy or gravelly soils on prairies and hills from 1200 to 2600 m (4000-8500 ft) elevation. Southeastern Colorado and western Kansas to western Oklahoma and northern New Mexico.
Rydberg successively elevated the rank of this taxon. He first (1906, p. 151) characterized it as a mere form, referring to it as "the southern more luxuriant and broad-leaved form of P. angustifolius," to which he added that "the two grade absolutely into each other in Colorado." Later, in Flora of Colorado (1906b, p. 309) he referred to it as "a taller variety with broader leaves from Colorado and New Mexico." In Rocky Mountain Flora (1917 ), he treated it as a species. Pennell's opinion also fluctuated. In 1920 (p. 362) he considered it as "variable and very inconsistantly distinguished; perhaps a robust form, rather than a geo- graphical subspecies . . ." In 1922 (p. 73) he wrote that it "seems to be too inconsistently distinguishable to be considered as more than a form." By 1935, after studying more specimens, he treated it as a species. In the text (p. 266) he wrote with confidence, "appearing not intergrading with the preceding [P. angustifolius]," but later, presumably closer to press time, he wrote an addendum (p. 633) stating that "the distinctness of these two species is only tentatively advocated now; their ranges touch in Colorado and the problem can best be decided by further field-study there."