Bull. Torrey Bot. Club 34: 39. 1907.
Herbs, perennial, 2–5.5 dm; from a remote woody caudex; with a taproot. Stems solitary, sometimes few, proximally creeping, becoming rhizomatous, erect to ascending distally, unbranched, sometimes branched, often glabrate proximally, hairy distally, hairs spreading, medium length and long, soft, mixed with much shorter stipitate-glandular ones near inflorescence. Leaves green to purplish, linear-lanceolate to broadly lanceolate or narrowly oblong, 1.2–7.1 cm, not fleshy, margins plane, sometimes ± wavy, slightly involute, 0-lobed, apex acute to acuminate. Inflorescences 4.5–18 × 1.5–6 cm; bracts white, cream, pale yellow, pink, salmon, orange, or dull red throughout, or proximally greenish, distally as above, broadly lanceolate to oblong, 0–3(–5)-lobed; lobes ascending to erect, lanceolate to triangular, often short, arising above mid length, apex obtuse or rounded, sometimes acute or acuminate. Calyces colored as bracts, pigmentation often confined to lobes, 15–22 mm; abaxial and adaxial clefts 7–14 mm, 40–50% of calyx length, deeper than laterals, lateral 2–6.5 mm, 10–20% of calyx length; lobes narrowly triangular to narrowly lanceolate, apex acute (to sometimes acuminate in Logan Valley). Corollas ± straight, 19–30 mm; tube 11–19 mm; beak exserted from calyx, adaxially green, 7.5–11 mm; abaxial lip deep green, reduced, 1–2 mm, 20% as long as beak; teeth incurved to erect, green, 0.5–1 mm. 2n = 48.
Phenology: Flowering May–Aug.
Habitat: Mesic or wet, usually flat meadows, sometimes near hot springs, along stream banks, montane.
Elevation: 1600–2500 m.
Idaho, Mont., Oreg., Wyo.
Castilleja gracillima populations are centered around the Greater Yellowstone region, but its range extends sporadically west to central Oregon. It is sometimes confused with C. miniata, but differs from that species in its floral dimensions, mostly single-stemmed growth form, primarily white, yellow, or pinkish orange bract coloration, puberulent stems, and weakly rhizomatous habit. Where the two grow in the same general region, there is no clear evidence of hybridization; however, C. cusickii and C. gracillima form an extensive, sporadically intergrading population in the Logan Valley, Grant County, Oregon.
Plants attributed to this species from the Rocky Mountain trench near the head of the Columbia River in southeastern British Columbia or adjacent Alberta are a combination of several other species, especially Castilleja lutescens and C. miniata.