Monogr. Acad. Nat. Sci. Philadelphia 1: 237. 1935.
Herbs. Stems ascending to erect, (15–)35–55 cm, spreading-puberulent or retrorsely hairy proximally, glandular-pubescent medially and distally, not glaucous. Leaves basal and cauline, basal often withering by anthesis, not leathery, puberulent or retrorsely hairy; basal and proximal cauline 25–80(–110) × 5–24(–32) mm, blade spatulate to obovate, oblanceolate, or lanceolate, base tapered, margins subentire or ± denticulate or serrate, apex rounded to obtuse or acute; cauline 2–9 pairs, short-petiolate or sessile, (25–)60–120 × 4–20 mm, blade lanceolate to linear, base tapered to clasping, margins entire or ± denticulate to serrate, apex acute. Thyrses interrupted, cylindric, (5–)8–18(–25) cm, axis glandular-pubescent, verticillasters (2 or)3–6, cymes 2–4(–6)-flowered, 2 per node; proximal bracts lanceolate to linear, 60 × 6 mm, margins entire; peduncles and pedicels ascending to erect, glandular-pubescent. Flowers: calyx lobes ovate to lanceolate, 5–7 × 3–4 mm, glandular-pubescent; corolla white to ochroleucous, without nectar guides, personate, tubular, 24–32 mm, glandular-pubescent externally, densely yellowish lanate internally abaxially, tube slightly differentiated from throat, 6–10 mm, throat slightly inflated, 6–8 mm diam., 2-ridged abaxially; stamens included, pollen sacs divergent, navicular to subexplanate, 1–1.5 mm, dehiscing completely, connective splitting, sides glabrous, sutures papillate; staminode 18–20 mm, included or slightly exserted, 0.4–0.6 mm diam., tip straight, distal 14–16 mm ± villous, hairs yellowish orange, to 1.5 mm; style 20–22 mm. Capsules 8–13 × 3–5 mm, glabrous. 2n = 16.
Phenology: Flowering Apr–Jun.
Habitat: Tallgrass prairies, open woods.
Elevation: 200–300 m.
Penstemon oklahomensis is known from 26 counties primarily in east-central Oklahoma and in Lamar County, Texas (J. N. Mink et al. 2010).