Herbs. Stems ascending to erect, (18–)25–45(–61) cm, retrorsely hairy, sometimes also ± glandular-pubescent distally, not glaucous. Leaves basal and cauline, basal often withering by anthesis, not leathery, glabrous or sparsely retrorsely hairy, especially along midvein and margins, abaxially, glabrous or sparsely puberulent adaxially; basal and proximal cauline 20–95 × 5–28 mm, blade spatulate or obovate to ovate, base tapered, margins subentire to ± serrate or dentate, apex rounded to obtuse or acute; cauline (6–)8–12 pairs, short-petiolate or sessile, 22–80 × 4–20 mm, blade lanceolate, base tapered to clasping, margins ± serrate or dentate, apex acute to acuminate. Thyrses interrupted, conic, 6–24 cm, axis retrorsely hairy and ± glandular-pubescent, verticillasters 4–8, cymes 3–13-flowered, 2 per node; proximal bracts lanceolate, 6–40 × 2–12 mm, margins entire or ± dentate; peduncles and pedicels ascending, retrorsely hairy and ± glandular-pubescent. Flowers: calyx lobes ovate, 2.8–4.5 × 1.1–2 mm, sparsely glandular-pubescent; corolla white, with reddish purple nectar guides, tubular to tubular-funnelform, 15–19 mm, glandular-pubescent externally, moderately whitish lanate internally abaxially, tube 4–6 mm, throat slightly inflated, 4–7 mm diam., 2-ridged abaxially; stamens included, pollen sacs opposite, navicular, 0.8–1 mm, dehiscing completely, connective splitting, sides glabrous, sutures papillate; staminode 10–12 mm, exserted, 0.2–0.4 mm diam., tip straight to slightly recurved, distal 5–7 mm ± villous, hairs yellow or golden yellow, to 1.2 mm; style 9–11 mm. Capsules 4–7 × 4–5 mm, glabrous. 2n = 16.
Phenology: Flowering Apr–Jun.
Habitat: Rocky oak-hickory woodlands, glades, rocky stream bottoms.
Elevation: 60–300 m.
Ark., Mo., Okla.
Penstemon arkansanus is centered in the Interior Highlands. The species is most similar to P. pallidus, from which it differs by its narrower, less hairy, and more numerous cauline leaves. Some specimens from the narrow zone of sympatry in north-central Arkansas and south-central Missouri exhibit varying degrees of morphologic intermediacy. Penstemon arkansanus has been reported from Illinois (R. H. Mohlenbrock 1986) and Texas (S. D. Jones et al. 1997); those reports seem unlikely.