Perennials, rhizomatous, sometimes rooting at proximal nodes. Stems erect to ascending-erect, branched distally, sometimes fistulose, to 10 mm wide, pressed, (6–)15–65(–80) cm, villous-glandular or moderately to densely hirtellous, hairs eglandular or glandular and eglandular. Leaves basal and cauline or basal not persistent; petiole 0 mm or proximals 1–95 mm; blade subpinnately, sometimes palmately, 5–7-veined, ovate-elliptic to ovate or suborbicular, 4–125 mm, 1–2 times longer than wide, gradually or abruptly reduced in size distally, base rounded to cuneate to truncate, margins crenate to coarsely dentate, proximally shallowly toothed to irregularly small-lobed or lyrate-dissected, apex rounded to obtuse, surfaces glabrous. Flowers herkogamous, (1–)3–20(–28), from distal nodes, sometimes in relatively compact racemes with reduced bracts. Fruiting pedicels 15–40(–60) mm, villous-glandular or moderately to densely hirtellous, hairs eglandular or glandular and eglandular. Fruiting calyces nodding, usually without red markings, ovate-campanulate, inflated, sagittally compressed, 11–17(–20) mm, villous-glandular or moderately to densely hirtellous, hairs eglandular or glandular and eglandular, throat closing. Corollas yellow, red-dotted, bilaterally symmetric, bilabiate; tube-throat funnelform, (10–)12–20 mm, exserted 3–5 mm beyond calyx margin; limb expanded 12–24 mm. Styles minutely hirsutulous to villosulous. Anthers included, glabrous. Capsules included, 7–11(–12) mm. 2n = 28, 56.
Phenology: Flowering Apr–Sep.
Habitat: Springs and seeps, marshes, beaver dams, along rivers, streams, and irrigation canals, loamy soils in conifer forests, wet and damp meadows, wet roadsides.
Elevation: 20–3200(–3700) m.
Alta., B.C., N.B., N.W.T., Sask., Yukon, Alaska, Ariz., Calif., Colo., Conn., Idaho, Mich., Mont., Nebr., Nev., N.Mex., N.Y., Oreg., Pa., S.Dak., Tex., Utah, Wash., Wyo., Mexico (Baja California, Baja California Sur, Chihuahua, Coahuila, Nayarit, Sonora), introduced in Europe.
Erythranthe guttata is markedly variable in stature, leaf shape, vestiture, flower size, and the separation distance between anthers and stigma; it ranges from subalpine and near-alpine habitats into desert situations where water is available.
In all of Colorado, the Four Corners area, and north-central New Mexico, the vestiture of stems and calyces is consistently densely hirsute-hirtellous, without glandular hairs. Plants with similar vestiture also occur in British Columbia, Oregon, and Washington, and in scattered localities elsewhere. In northwestern Arizona, California Nevada, and southern Oregon, vestiture is consistently villous-glandular, without eglandular hairs. Elsewhere in the geographic range the vestiture is a mix of hirsute-hirtellous (eglandular) and villous-glandular hairs. Other morphological variants and patterns, as well as variation in ploidy level, within Erythranthe guttata were discussed by G. L. Nesom (2012i).
Plants of Erythranthe guttata with extremely large corollas have been frequently collected on the Aleutian Islands, Kodiak Island, and in other Alaskan localities (for example, Admiralty Island Amakuk, Juneau, and Yakutat Bay). Corolla tube-throats are 19–26 mm, and the limbs are expanded to 18–25 mm. The type collection of E. guttata is one of these plants, and the name E. guttata may prove to apply most appropriately only to Alaskan populations. Diploids and tetraploids appear to be sympatric in Alaska.
Mimulus guttatus subsp. haidensis was described as an endemic subalpine race that occurs in and along the flanks of the Queen Charlotte Mountains on Graham Island and Moresby Island. The subspecies was distinguished on the basis of its hirtellous vestiture, but plants of similar hirtellous vestiture occur over the whole range of the species. A tetraploid chromosome number (2n = 56) was reported for subsp. haidensis from a total of five localities on Graham Island and Moresby Island (J. A. Calder and R. L. Taylor 1968, vol. 2), and diploids (2n = 28) were documented from one locality on each of the two islands. At least one of the diploids has densely hirtellous stems, pedicels, and calyces, matching the morphology of subsp. haidensis.
Erythranthe guttata is naturalized in Europe and has been introduced to the northeastern United States (Connecticut, Michigan, New York, and Pennsylvania) and eastern Canada (New Brunswick).