Phytoneuron 2012-39: 43. 2012.
Perennials, rhizomatous, rooting at proximal nodes, sometimes producing creeping, small-leaved runners, forming matted colonies, rhizomes filiform. Stems procumbent or decumbent to decumbent-ascending, delicate, usually in masses, terete or flattish, branched, 3–10 cm, glabrous, minutely hirtellous, or stipitate-glandular. Leaves basal and cauline; petiole: proximals 2–5 mm, distals 0 mm; blade often purple beneath, palmately 3-veined, orbicular to narrowly elliptic or ovate, proximals usually sublyrate, 3–12 mm, becoming larger distally, base cuneate to a short petiole, margins entire, mucronulate, or barely denticulate, apex obtuse, surfaces sparsely to moderately puberulent, hairs minute, stipitate-glandular. Flowers herkogamous, 1–3, from distal nodes, commonly solitary. Fruiting pedicels 10–30(–40) mm, sparsely to moderately villous, hairs short, gland-tipped, sometimes hirtellous. Fruiting calyces broadly campanulate, inflated, sagittally compressed, 7–15 mm, glabrous, minutely hirtellous, or stipitate-glandular, throat closing, proximalmost lobe pair upcurving, distalmost 3–5 mm, prominently protruding. Corollas yellow, dark red-spotted, bilaterally symmetric, bilabiate; tube-throat broadly funnelform to cylindric-funnelform, 15–18 mm, exserted beyond calyx margin; abaxial limb with deflexed-spreading lobes, adaxial with ascending lobes, palate partially closed. Styles minutely hirtellous. Anthers included, glabrous. Capsules included, 4–5 mm.
Phenology: Flowering Jul–Sep.
Habitat: Alpine meadows and slopes, stream banks, wet rocks in streams, wet crevices, talus.
Elevation: 1100–2000(–2300) m.
Erythranthe caespitosa is endemic to northwestern and central Washington (Cascade and Olympic mountains) and adjacent British Columbia (Cascades, Selkirk Mountains and Chilliwack Valley, Coast Mountains). The plants have consistently small leaves with subentire margins, and the stems are consistently procumbent to decumbent-ascending, usually forming matted colonies. Erythranthe caespitosa and E. tilingii appear to be sympatric in counties of northwestern Washington, but this needs to be verified in the field.