Phytoneuron 2012-39: 43. 2012.
Perennials, rhizomatous, rooting at nodes. Stems decumbent-ascending to ascending or erect-ascending, branched, (3–) 10–40 cm, glabrous. Leaves cauline, basal not persistent; petiole 3–10 (–20) mm or 0 mm distally; blade palmately 3–5-veined, suborbicular to depressed-ovate or broadly elliptic-ovate to reniform, 6–25 mm, relatively even-sized or largest often at mid-stem, bracteal reduced, base cuneate to truncate or subcordate, margins shallowly dentate to crenate-dentate, teeth 3–7 (–10) per side, apex rounded, adaxial surface of distals sparsely short villous-glandular or glabrous. Flowers plesiogamous, 2–8 (–12), from distal nodes, sometimes from most nodes, very loosely racemose. Fruiting pedicels 18–30 mm, sparsely short villous-glandular or glabrous. Fruiting calyces obtriangular to broadly obtriangular or deeply cupulate, inflated, sagittally compressed, (7–) 8–12 mm, sparsely short villous-glandular or glabrous, throat not closing, lateral lobes shallowly convex-mucronulate, adaxial ovate with apex rounded. Corollas yellow, sparsely red-dotted or not, bilaterally symmetric, weakly bilabiate; tube-throat cylindric-funnelform, 6–8 mm, exserted 1–3 mm beyond calyx margin; limb expanded 5–8 mm. Styles glabrous. Anthers included, glabrous. Capsules included, (4.5–) 5–8 mm. 2n = 30.
Phenology: Flowering May–Aug(–Oct).
Habitat: Edges of flowing streams, marsh edges, drainage ditches, seepage areas, springs, muddy or moist banks.
Elevation: 200–2500 m.
Alta., Man., Ont., Que., Sask., Ariz., Colo., Ill., Iowa, Kans., Mich., Minn., Mo., Nebr., N.Mex., Okla., Pa., S.Dak., Tex., Wis., Wyo., Mexico (Chihuahua), Mexico (Coahuila), Mexico (Distrito Federal), Mexico (Durango), Mexico (Hidalgo), Mexico (Nuevo León), Mexico (México), Mexico (Querétaro), Mexico (San Luis Potosí), Mexico (Sonora), Mexico (Veracruz), Mexico (Zacatecas)
Erythranthe geyeri has commonly been regarded as conspecific with E. glabrata (Kunth) G. L. Nesom (as Mimulus glabratus var. jamesii), but typical E. glabrata has a different chromosome number and distinct morphology and its range does not reach the United States. In Mexico, the two species are broadly sympatric without intermediates. An allozyme study of the M. glabratus complex (R. K. Vickery 1990) indicated that the Great Plains populations of E. geyeri are distinct from those in New Mexico and Mexico, corresponding to a difference in pedicel vestiture.