Herbs, annual. Stems erect to erect-ascending, 40–150 mm, distal internodes 1–2 mm, minutely glandular-puberulent. Leaves usually cauline, relatively even-sized or gradually larger distally; petiole absent, proximal base short petiole-like; blade broadly ovate or obovate to elliptic-ovate or elliptic-oblanceolate, 10–15(–25) × 4–13 mm, margins entire, plane, apex acuminate or cuspidate, surfaces glabrous or sparsely glandular-puberulent. Pedicels 1–1.5 mm in fruit. Flowers usually from proximalmost to distal nodes, 2 per node, or 1 or 2 per node on 1 plant, chasmogamous. Calyces symmetrically attached to pedicels, not inflated in fruit, 7–8 mm, glandular-puberulent, lobes subequal, apex linear-acuminate, ribs green distally, intercostal areas whitish. Corollas light pink to magenta or rose purple, usually with a darker narrow line extending from throat onto each lobe midvein, throat yellow, palate ridges yellow, tube-throat 8–12 mm, limb 10–16 mm diam., bilabiate. Anthers included, glabrous or sparsely hispidulous. Styles puberulent, at least on distal 1/2. Stigmas exserted, lobes subequal, abaxial slightly longer. Capsules 7–9 mm. 2n = 16.
Phenology: Flowering Jun–Aug.
Habitat: Sandy and ashy soils, pumice sand and gravel, red clay slopes, hillsides, roadsides, bare areas, sagebrush, sagebrush-juniper, juniper, yellow pine and lodgepole pine forests.
Elevation: 700–1500 m.
Diplacus deschutesensis is endemic to Crook, Deschutes, Jefferson, Klamath, Lake, and Wheeler counties of central Oregon. D. M. Thompson (2005) regarded these plants as a zone of stabilized hybrids, intermediate between Mimulus cusickii and typical M. nanus, the range just outside and west of the wider range of typical M. cusickii. In an earlier study that included both of the latter species, W. L. Ezell (1971, and by annotation in 1987) identified the same set of plants simply as M. cusickii, not associating them at all with M. nanus. A. L. Grant (1924, and by annotation of MO collections) identified them variously as either M. cusickii or M. ovatus. Thompson did not say what features of intermediacy he observed in the putative hybrids, but he did note that they produced leaves with acuminate-cuspidate apices and that they would key to M. cusickii.
Leaves of Diplacus deschutesensis are broad with abruptly and sharply acuminate apices like those of D. cusickii, and the corolla coloration also is similar. The flowers (calyx length, corolla tube-throat length, limb width) and capsules of D. deschutesensis are considerably smaller, and the distal leaves are smaller with glabrous surfaces.