Herbs. Stems ascending to erect, 20–80 cm, retrorsely hairy and ± glandular-lanate, retrorse hairs sometimes few or distal only, not glaucous. Leaves basal and cauline, not leathery, moderately to densely glandular-lanate, especially along major veins abaxially; basal and proximal cauline 40–138 × 7–37 mm, blade spatulate to oblanceolate or elliptic, base tapered, margins entire or ± serrate to dentate, apex rounded to obtuse or acute; cauline 4–7 pairs, sessile or proximals sometimes petiolate, (11–)20–110 × 3–30(–40) mm, blade ovate to lanceolate, base truncate to clasping, sometimes tapered, margins entire or crenate-serrate to dentate, apex acute to acuminate. Thyrses interrupted, conic, (3–)8–37 cm, axis moderately to densely glandular-pubescent to glandular-lanate, verticillasters 3–6(or 7), cymes 3–10(–13)-flowered, 2 per node; proximal bracts lanceolate, 6–40 × 1–10 mm, margins entire or ± serrate; peduncles and pedicels ascending, moderately to densely glandular-pubescent to glandular lanate. Flowers: calyx lobes ovate to broadly lanceolate, 2–5 × 1.3–2 mm, glandular-pubescent; corolla white, without nectar guides, personate, tubular, 21–28 mm, sparsely glandular-pubescent externally, moderately to densely white-villous internally abaxially, tube 6–8 mm, throat slightly inflated, 5–8 mm diam., 2-ridged abaxially; stamens included, pollen sacs opposite, navicular to subexplanate, 1–1.2 mm, dehiscing completely, connective splitting, sides glabrous, sutures papillate; staminode 13–16 mm, exserted, 0.4–0.6 mm diam., tip straight, distal 10–12 mm moderately to densely pubescent, hairs yellow, to 1.6 mm; style 16–19 mm. Capsules 5–7 × 3–5 mm, glabrous.
Phenology: Flowering Apr–Jun.
Habitat: Dry woods, cedar glades, prairies, barrens, cliffs.
Elevation: 30–80 m.
Ala., Ky., Miss., Tenn.
Penstemon tenuiflorus is concentrated in the Interior Low Plateaus region of western Kentucky, central Tennessee, and northwestern Alabama, and in the Black Belt subsection of the Coastal Plain of central Alabama and northeastern Mississippi.
R. K. Clements et al. (1998) observed that the leaves of Penstemon tenuiflorus bear both simple and stellate hairs, reporting only simple hairs on the leaves of closely related P. hirsutus. However, stellate hairs occur in the leaf axils of plants in some populations of P. hirsutus, and they are common on the stems. Stellate hairs also occur on other diploid, tubular-flowered eastern North American species, including P. arkansanus, P. australis, P. brevisepalus, P. canescens, P. laxiflorus, P. oklahomensis, and P. pallidus. Stellate hairs have not been observed on any of the polyploid, ventricose-flowered eastern North American species.