Proc. Amer. Acad. Arts 44: 575. 1909
Herbs or subshrubs, perennial, 1.4–7.5 dm; from a woody caudex; with a strongly woody taproot or branched, woody roots. Stems few to many, erect to basally ascending, sometimes with subtle, curvy zigzags, unbranched, sometimes branched, proximally or with short, leafy axillary shoots, hairs loosely spreading, long, soft, ± wavy, eglandular and shorter stipitate-glandular. Leaves green, linear-oblong to oblong-lanceolate, narrowly lanceolate, or narrowly oblanceolate, 2–5.5 cm, not fleshy, margins plane, involute, 0-lobed, apex acute, distalmost obtuse to rounded. Inflorescences 2–7.5 × 2–3.5 cm; bracts proximally greenish, distally abruptly rose red, bright red, or red-orange, narrowly to broadly oblong-obovate to obovate or ovate-elliptic, 0-lobed; apex rounded to obtuse. Calyces colored as bracts, sometimes with a yellow central band, 20–32(–36) mm; abaxial and adaxial clefts (5–)7–14 mm, ca. 40% of calyx length, deeper than laterals, lateral 0–0.5 mm, 0–7% of calyx length; lobes poorly developed, if present, rounded, apex rounded to truncate, sometimes emarginate. Corollas straight or slightly curved, (24–)28–36(–42) mm; tube 15–22 mm; beak exserted, adaxially green to yellow-green, (8–)10–14 mm; abaxial lip dark green, reduced, 0.5–1.5 mm, 7–12% as long as beak; teeth reduced, green to yellowish green, 0.5–1.5 mm.
Phenology: Flowering Mar–Jul(–Nov).
Habitat: Desert scrub, rocky slopes, ledges, ridges, flats, swales, roadsides, commonly with Agave lechuguilla.
Elevation: 300–1500 m.
Tex., Mexico (Chihuahua, Coahuila, Durango, Nuevo León, Zacatecas).
Castilleja rigida is endemic to desert scrub on limestone deposits in the Chihuahuan Desert region and the adjacent western edge of the Edwards Plateau. It is almost always closely associated with, and most likely parasitic on, Agave lechuguilla. The majority of its range is in the Mexican states of Chihuahua, east of the Sierra Madre Occidental, as well as in Coahuila and Nuevo León, where it is a characteristic component of the regional flora, but also extends into a few counties in southwestern Texas. It is sometimes confused with the similar species, C. nervata, which is widespread in the sierras of western Mexico and is known from a single extant population in the western Chiricahua Mountains of southeastern Arizona. Castilleja nervata and C. rigida are largely parapatric in Mexico.