Subshrubs. Stems ascending to erect, (10–)20–80 cm, retrorsely hairy, not glaucous. Leaves cauline, opposite, sometimes alternate, glabrous, not glaucous; cauline (4–)7–10 pairs, sessile or proximals short-petiolate, 20–80 × 5–17 mm, blade elliptic to lanceolate or oblanceolate, base tapered, margins irregularly dentate or serrate, apex acute. Thyrses interrupted, cylindric, 3–25(–50) cm, axis glandular-pubescent, verticillasters 4–10, cymes 1–11(–18)-flowered, 1 or 2 per node; proximal bracts lanceolate, rarely ovate, 9–45 × 2–16 mm; peduncles and pedicels spreading to ascending, glandular-pubescent. Flowers: calyx lobes lanceolate, (2–)3–5.2 × 0.9–1.3 mm, glandular-pubescent; corolla bluish lavender to violet or purple, with reddish purple nectar guides, funnelform, 13–19 mm, glandular-pubescent externally, glabrous internally or sparsely white-lanate abaxially, tube 5–6 mm, throat gradually inflated, 5–6 mm diam., rounded abaxially; stamens included, filaments glabrous or sparsely white-pubescent distally, pollen sacs parallel, 0.9–1.3 mm, distal 1/2 indehiscent, sides glabrous, sutures denticulate, teeth to 0.1 mm; staminode 9–11 mm, reaching orifice or barely exserted, 0.4–0.5 mm diam., distal 1–4 mm sparsely to moderately lanate, hairs white or yellowish, to 1.5 mm; style 8–12 mm. Capsules 4–6 × 2.8–4.5 mm.
Phenology: Flowering Jun–Aug(–Sep).
Habitat: Talus slopes, cliffs, rocky banks.
Elevation: 1000–2300 m.
Idaho, Mont., Wash.
Penstemon diphyllus occurs in northeastern Idaho, western Montana, and southeastern Washington (Whitman County).