Herbs, annual. Stems erect to ascending-erect, 20–100 mm, distal internodes 1–4 mm, short glandular-villous to glandular-puberulent. Leaves usually cauline, relatively even-sized; petiole present proximally, absent distally; blade elliptic-spatulate to obovate or broadly oblanceolate, 10–22 × 2–10 mm, margins entire, plane, apex obtuse to rounded-acute, surfaces minutely glandular-puberulent. Pedicels 1–3 mm in fruit. Flowers 2 per node, or 1 or 2 per node on 1 plant, chasmogamous. Calyces symmetrically attached to pedicels, not inflated in fruit, (4–)5–7 mm, minutely glandular-puberulent, lobes subequal, apex acute to acuminate, ribs dark green or reddish, intercostal areas whitish. Corollas magenta to purplish, usually with a darker narrow line extending from throat onto each lobe midvein, palate ridges yellow with red spots, throat floor glabrous, tube 1.1–1.9 mm diam. at filament insertion, tube-throat 8–10 mm, limb 7–11 mm diam., bilabiate. Anthers (distal pair) exserted, minutely viscid-villosulous. Styles apparently glabrous. Stigmas exserted, lobes usually subequal. Capsules 5–8(–9) mm. 2n = 16.
Phenology: Flowering Jun–Aug.
Habitat: Open pumice flats, scree slopes, sandy soils, juniper-sagebrush, juniper, pine-juniper, yellow pine, lodgepole pine forests.
Elevation: 1400–2400(–2600) m.
Diplacus cascadensis is known from Deschutes, Klamath, and Lake counties. Plants of this species have been identified as D. nanus (similar in its purplish leaves congested on crowded distal nodes, minutely glandular-puberulent vestiture, and purplish and strongly bilabiate corollas), but they differ from D. nanus in their broader distal leaves, shorter calyces, shorter corollas with glabrous throats and magenta tubes, and shorter capsules.