Phytoneuron 2012-40: 80. 2012.
Annuals, fibrous-rooted, usually dark purplish. Stems erect, simple, weakly 4-angled, 4–16(–24) cm, slender, glabrous or sparsely villous-glandular near nodes. Leaves basal and cauline, basal usually persistent, largest at mid stem or above, cauline relatively few on long internodes; petiole: proximals to distals 1–4 mm; blade palmately 3-veined, ovate to ovate-lanceolate, 5–16(–20) × 3–11 mm, base truncate to subcordate, margins shallowly, evenly crenulate to serrate-dentate or denticulate, apex acute to obtuse, surfaces: proximals glabrous or sparsely villous, distals and bracteals sparsely hirtellous or glabrous abaxially or along distal margin, sparsely villous adaxially, hairs vitreous, flattened, eglandular. Flowers plesiogamous, (1–)4–7, usually from all nodes, usually beginning about mid stem, cleistogamous. Fruiting pedicels deflexed 45–90º at calyx, 6–19 mm, longer than subtending leaves, glabrous or sparsely villous-glandular. Fruiting calyces sometimes purple-dotted, broadly elliptic-ovoid, inflated, sagittally compressed, 10–13 mm, minutely hirtellous, sometimes also sparsely glandular, throat closing, adaxial lobe not distinctly longer than abaxial, not falcate. Corollas yellow, sparsely red-dotted, bilaterally or nearly radially symmetric, bilabiate or nearly regular; tube-throat narrowly cylindric, 4–6 mm, exserted 0.5–1 mm beyond calyx margin; limb barely expanded. Styles glabrous. Anthers included, glabrous. Capsules included, stipitate, 6–8 mm.
Phenology: Flowering Apr–Jul(–Aug).
Habitat: Grassy slopes, damp soils, moist rocks.
Elevation: (900–)1700–2400(–2800) m.
Erythranthe charlestonensis is characterized by its annual duration, autogamous reproduction, small stature, commonly purplish color, regularly ovate, short-petiolate leaves with shallowly crenate margins and (distal leaves) sparsely villous or glabrous adaxial surfaces, hirtellous calyces without a prominently longer adaxial lobe, and lack of glandular hairs. It is endemic to the Charleston (Spring) Mountains of Clark County, Nevada, and one locality (Union Pass) in Mohave County, Arizona.