Notul. Nat. Acad. Nat. Sci. Philadelphia 95: 4. 1942.
Herbs. Stems erect, (6–)10–40 cm, glabrous or puberulent, not glaucous. Leaves basal and cauline, ± leathery or not, glabrous; basal and proximal cauline (12–)25–115 × (4–)7–22 mm, blade oblanceolate to lanceolate or elliptic, base tapered, margins entire, apex obtuse to acute; cauline 2–4 pairs, sessile, 18–65 × 3–16 mm, blade oblanceolate to lanceolate, sometimes ovate, base tapered to clasping, margins entire, apex obtuse to acute. Thyrses continuous or interrupted, cylindric, 1–12 cm, axis ± puberulent, rarely glandular-pubescent, verticillasters 1–4, cymes 2–10-flowered, 2 per node; proximal bracts lanceolate, sometimes ovate, 12–44 × 2–13 mm, margins entire; peduncles and pedicels erect, glabrous or ± puberulent, rarely glandular-pubescent. Flowers: calyx lobes ovate to lanceolate, 5–9 × 1.8–2.7 mm, glabrous, rarely glandular-pubescent; corolla light yellow, without nectar guides, funnelform, 12–16 mm, glabrous externally, rarely glandular-pubescent, sparsely to moderately yellow-pubescent internally abaxially, tube 5–7 mm, throat gradually inflated, 4–6 mm diam., 2-ridged abaxially; stamens included or longer pair reaching orifice, pollen sacs opposite, navicular to subexplanate, 0.7–0.9 mm, dehiscing completely, connective splitting, sides glabrous, sutures smooth; staminode 8–9 mm, included, 0.4–0.7 mm diam., tip straight to slightly recurved, distal 2–3 mm moderately pilose, hairs yellow or golden yellow, to 0.8 mm; style 8–11 mm. Capsules 5–8 × 4–5 mm, glabrous. 2n = 48.
Phenology: Flowering Jun–Aug.
Habitat: Open or wooded, rocky slopes.
Elevation: 1800–2500 m.
Penstemon flavescens is known from the Bitterroot Mountains in east-central Idaho (Clearwater, Idaho, and Shoshone counties) and southwestern Montana (Ravalli County). A yellow-flowered, glandular-pubescent hexaploid from near Hoodoo Pass in Shoshone County, Idaho, might be a hybrid between P. attenuatus and P. flavescens (D. V. Clark 1971). Reports of P. flavescens from Oregon might be based on whitish or yellowish flowered forms of P. attenuatus var. attenuatus (D. D. Keck 1945).