Annuals, fibrous-rooted or filiform-taprooted. Stems erect to decumbent, sometimes procumbent-trailing, straight or geniculate at nodes, simple or many-branched, 3–22(–40) cm, villous-glandular, hairs greatly variable in length and density, gland-tipped, sometimes 0.2–0.5 mm, sparsely stipitate-glandular. Leaves cauline, basal mostly deciduous by flowering; petiole 1–12 mm; blade pinnately to subpalmately veined, ovate, (3–)8–25(–35) × (1–)5–18(–26) mm, base cuneate to truncate or cordate, margins serrate to sparsely dentate, apex acute, surfaces villous-glandular, hairs greatly variable in length and density, gland-tipped, sometimes 0.2–0.5 mm, sparsely stipitate-glandular. Flowers plesiogamous, 1–20, from proximal to distal nodes. Fruiting pedicels 5–20(–26) mm, villous-glandular, hairs greatly variable in length and density, gland-tipped, sometimes 0.2–0.5 mm, sparsely stipitate-glandular. Fruiting calyces greenish or purplish to red-dotted, cylindric, ± inflated, 4–7 mm, margins distinctly toothed or lobed, villous-glandular, lobes pronounced, erect. Corollas yellow, abaxial limb red-dotted, bilaterally symmetric, weakly bilabiate; tube-throat funnelform-cylindric, (4–)5–10 mm, exserted slightly beyond calyx margin or not; limb expanded 3–4 mm diam., lobes usually oblong, apex notched. Styles glabrous. Anthers included, glabrous. Capsules included, 4–7 mm. 2n = 32.
Phenology: Flowering (May–)Jun–Aug(–Sep).
Habitat: Under overhangs, moist roofs of caves, wet rock crevices, cliff faces, wet cliff bases, below waterfalls, seeps, springs, humus and moist soils over rocks and slabs, moist slopes, ditches and pond edges, wet edges of creeks and rivers, drying mud on margins of wetland depressions, creek beds, wet or swampy meadows, along trails, in lodgepole pine, ponderosa pine, ponderosa pine-Douglas fir, and spruce-fir woodlands.
Elevation: (100–)1800–2600(–3100) m.
Alta., B.C., Ariz., Ark., Calif., Colo., Idaho, Mont., Nev., N.Mex., Oreg., S.Dak., Utah, Wash., Wyo., Mexico (Baja California, Baja California Sur, Chihuahua, Sinaloa, Sonora).
Some plants identified here as Erythranthe floribunda in Arizona and southwestern New Mexico are distinctive in their prominently inflated calyces, sessile to subsessile leaves with attenuate bases and palmately three- to five-veined venation, and much-elongated pedicels (20–43 mm); numerous intermediates in Arizona make it difficult to conclude that the variants represent an entity discontinuous from plants of typical morphology. The variant morphology has not been observed among Mexican populations. Further discussion of this situation was given by G. L. Nesom (2012h).
Erythranthe floribunda has been documented from 12 counties in northern Arkansas (Carroll, Cleburne, Crawford, Franklin, Izard, Johnson, Logan, Newton, Pope, Searcy, Stone, and Washington), where it occurs at 300–500 m. The unpublished name Mimulus floribundus subsp. moorei Iltis appears in various checklists in reference to the Arkansas plants, but there appears to be no basis for treating them as distinct from the rest of the species. Elsewhere in the main range (western states), scattered variants extremely reduced in size, leaves, flowers, and overall stature appear to be at the lower limits of the species rather than taxonomically distinct.