Subshrubs. Stems erect, 300–1000(–2000) mm, glandular-puberulent and short-villous. Leaves usually cauline, relatively even-sized; petiole absent; blade elliptic to lanceolate, elliptic-lanceolate, or elliptic-oblanceolate, 25–65(–80) × 4–15(–25) mm, margins entire or serrate, revolute, apex acute to obtuse, abaxial surfaces densely hairy, hairs branched, adaxial glabrescent. Pedicels 5–16 mm in fruit. Flowers 2 per node, chasmogamous. Calyces not inflated in fruit, 22–32 mm, glandular-puberulent and short glandular-villous to hirsute-villous, tube slightly dilated distally, lobes unequal, apex acute, ribs green, intercostal areas light green. Corollas light orange to pale yellow-orange, palate ridges orangish, tube-throat 34–45 mm, limb (25–)28–40 mm diam., bilabiate, lobes oblong, apex of adaxial 2 each shallowly incised. Anthers included, glabrous. Styles minutely glandular. Stigmas included, lobes equal. Capsules 18–28 mm. 2n = 20.
Phenology: Flowering (Mar–)Apr–Jul.
Habitat: Rocky hillsides and slopes, talus, chaparral, live oak woodlands.
Elevation: (50–)100–1300(–1800) m.
Calif., Mexico (Baja California).
Diplacus longiflorus occurs in southwestern California and northeastern Baja California.
Plants and populations intermediate between Diplacus longiflorus and D. puniceus are found where their ranges meet in Los Angeles, Orange, Riverside, and San Diego counties. The intermediate morphology and geography indicate that these are hybrids (as has been hypothesized by, for example, M. A. Streisfeld and J. R. Kohn 2005; D. M. Thompson 2005; M. C. Tulig and G. L. Nesom 2012), which have been identified as D. ×australis (McMinn ex Munz) Tulig. Streisfeld and Kohn found that in San Diego County, D. longiflorus and D. puniceus are discrete in morphology and separated in geography, with a narrow zone of hybrids and putative introgressants between.
Plants identified as Diplacus ×lompocensis McMinn (as species) occur where the geographic ranges of D. aurantiacus and D. longiflorus meet in Santa Barbara County and southern San Luis Obispo County; these plants have floral features intermediate between these two species. Stable populations of the putative hybrid are found throughout this region, although at either end of its distribution, the populations may more closely resemble the nearer parent. Considering that both D. aurantiacus and D. longiflorus are morphologically consistent across broad regions, D. ×lompocensis is perhaps best interpreted as a zone of introgression.
Diplacus ×australis and D. ×lompocensis are similar to D. longiflorus as well as to each other in most features; they are easily separated only by geographic range. Diplacus longiflorus is distinct from both in its larger corolla features and, frequently, calyx indument.