Arabis pallidifolia Rollins, Cruciferae Continental N. Amer., 181. 1993; A. thompsonii S. L. Welsh; Boechera thompsonii (S. L. Welsh) N. H. Holmgren

Perennials; short-lived; sexual; caudex usually not woody. Stems usually 2-5 per caudex branch, arising from margin of rosette near ground surface, or arising laterally proximal to sterile shoots, (0.5-)1.5-4 dm, densely pubescent proximally, trichomes short-stalked, 2-6-rayed, 0.1-0.3 mm, glabrous or sparsely pubescent distally. Basal leaves: blade oblanceolate to obovate, 5-11(-13) mm wide, margins shallowly dentate or, sometimes, entire, often ciliate along petiole, trichomes (simple and 2-rayed), surfaces moderately pubescent, trichomes short-stalked, 4-8-rayed, 0.1-0.3 mm. Cauline leaves: 3-8(-11), not concealing stem; blade auricles (0.5-)1-2 mm, surfaces of distalmost leaves sparsely pubescent. Racemes 4-15(-20)-flowered, usually unbranched. Fruiting pedicels ascending to divaricate-ascending, straight or, sometimes, recurved, 7-15 mm, glabrous or sparsely pubescent. Flowers ascending at anthesis; sepals pubescent; petals usually lavender, rarely whitish, 5-9 × 1.2-2.5 mm, glabrous; pollen ellipsoid. Fruits usually ascending, rarely horizontal, not appressed to rachis, not secund, straight to curved, edges parallel, 2.5-5(-6.5) cm × 1-2 mm; valves glabrous; ovules 44-62 per ovary; style 0.05-0.4 mm. Seeds uniseriate, 1.1-1.3 × 1-1.1 mm; wing nearly continuous, 0.07-0.15 mm wide.

Flowering Apr-Jun. Rocky slopes and sandy soil in pinyon-juniper, sagebrush communities; 1600-2500 m; Ariz., Colo., N.Mex., Utah, Wyo.

Although Boechera pallidifolia was originally described as endemic to west-central Colorado, recent studies favor a broader circumscription that includes sexual populations previously assigned to Arabis (Boechera) selbyi (see M. D. Windham and I. A. Al-Shehbaz 2006 for detailed comparison). The species has hybridized with nearly every sexual diploid within its range (including B. crandallii, B. fendleri, B. formosa, and B. pendulina), producing a confusing array of apomictic triploids including the type specimen of A. selbyi.