Herbs or subshrubs. Stems ascending to erect, (15–)35–60 cm, retrorsely hairy, hairs pointed, at least proximally, not glaucous. Leaves cauline, opposite, glabrous, glaucous; cauline 4–8 pairs, short-petiolate or sessile, (15–)20–60 × (2–)6–13(–20) mm, blade elliptic or proximals oblanceolate to lanceolate, base tapered, margins entire, apex obtuse to acute. Thyrses continuous, cylindric, 5–22 cm, axis glabrous or retrorsely hairy, verticillasters 4–8(–11), cymes 1- or 2(–4)-flowered, 2 per node; proximal bracts lanceolate to elliptic, 8–66 × 1–13 mm; peduncles and pedicels ascending to erect, peduncles glabrous or sparsely retrorsely hairy, pedicels papillate or glandular distally. Flowers: calyx lobes lanceolate, 4–6(–7) × 1.4–2.6 mm, glabrous; corolla lavender to light violet or violet, with or without faint lavender nectar guides, ventricose-ampliate, (20–)22–25(–30) mm, glabrous externally, glabrous internally, tube 6–8 mm, throat gradually inflated, 7–9 mm diam., 2-ridged abaxially; stamens: longer pair exserted, filaments glabrous, pollen sacs parallel, 1.5–1.9(–2.1) mm, distal 1/2–2/3 indehiscent, sides glabrous or hispidulous to lanate, hairs white, to 0.5 mm, sutures denticulate, teeth to 0.2 mm; staminode 12–14 mm, reaching orifice or exserted, 0.6–0.8 mm diam., glabrous; style 18–20 mm. Capsules 7–11 × 3.5–4.5 mm. 2n = 32.
Phenology: Flowering Jun–Aug.
Habitat: Talus slopes, rocky canyons, foothills.
Elevation: 1400–2400 m.
Penstemon platyphyllus is known from the Wasatch Mountains in Davis, Salt Lake, Utah, and Weber counties, with one somewhat isolated population in Indian Canyon in extreme southwestern Duchesne County. Reports of P. platyphyllus from White Pine County, Nevada, are based on a specimen cited by D. D. Keck (1932) that is referable to P. leonardii var. patricus, as are most collections cited by Keck from western Juab and southwestern Tooele counties, Utah.