Herbs, perennial, 2.5–8(–10) dm; from a woody caudex; with a taproot or branched root system. Stems few to many, ascending to erect, unbranched or often strongly and diffusely branched distally, hairs sparse to dense, spreading to matted, long proximally on stem, becoming puberulent distally, ± stiff, eglandular, often mixed with retrorse shorter ones. Leaves green, linear-lanceolate or narrowly to broadly lanceolate, 2–6.5(–8) cm, not fleshy, margins plane, flat to involute, 0(–3)-lobed, apex acute; lobes ascending, lanceolate, apex acute to obtuse. Inflorescences (2.5–)5–15 × 2–4.5 cm; bracts proximally greenish, distally scarlet to red or orange-red, rarely yellow or crimson, veins usually yellow or yellow-green, contrasting conspicuously with base color, lanceolate or elliptic to oblanceolate or obovate, 0–3(–5)-lobed; lobes ascending, lanceolate to triangular, medium length, arising above mid length, apex rounded to obtuse. Calyces mostly yellowish throughout, with a thin reddish apex, 15–27 mm; abaxial clefts (5–)9–11 mm, adaxial 4.5–9.5 mm, clefts 25–50% of calyx length, deeper than laterals, lateral 2–4 mm, 10–20% of calyx length; lobes linear-lanceolate to triangular, apex acute to acuminate, rarely ± obtuse. Corollas slightly curved, 15–35 mm, subequal to calyx or beak partially to strongly exserted; tube 15–17 mm; beak adaxially yellowish green, 10–16 mm; abaxial lip green, reduced, ± pouched, 0.5–1.5 mm, 4–10% as long as beak; teeth incurved, deep green, 0.7–1 mm. 2n = 24.
Phenology: Flowering Jun–Oct.
Habitat: Rocky slopes, meadows, riparian zones, moist ground in open forests, montane to subalpine.
Elevation: 1900–3100 m.
Ariz., N.Mex., Mexico (Chihuahua, Durango, Nayarit, Sonora).
Castilleja nelsonii is fairly common in the upper elevations of the so-called sky island ranges from central and eastern Arizona to adjacent New Mexico, southward into the Sierra Madre Occidental, at least as far south as southern Chihuahua, where the type collection was obtained on Cerro Mohinora. Although it was long known in the United States as C. austromontana, the name C. nelsonii has priority. Some specimens from southern Coconino County, Arizona, approach C. miniata, but most material is easily separable. Castilleja nelsonii occasionally hybridizes with C. mogollonica in Apache County, Arizona, near the border of the range of the former.