Perennials; long-lived; (somewhat cespitose); sexual or apomictic; caudex woody. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, or arising laterally proximal to sterile shoots, 0.5-2 (-2.5) dm, glabrous or sparsely pubescent proximally, trichomes short-stalked, 2-6-rayed, 0.1-0.2 mm, glabrous distally. Basal leaves: blade oblanceolate to obovate, 1.5-5 mm wide, margins usually entire, rarely slightly dentate, ciliate along petiole, surfaces densely to sparsely pubescent, trichomes short-stalked, 3-9-rayed, 0.1-0.2 mm. Cauline leaves: 2-8(-12), not concealing stem; blade auricles absent or 0.1-0.5 mm, surfaces of distalmost leaves glabrous or sparsely pubescent. Racemes 3-12(-17)-flowered, usually unbranched. Fruiting pedicels divaricate-ascending to slightly descending, usually slightly recurved, rarely straight, 2-6 mm, glabrous or sparsely pubescent, trichomes appressed, branched. Flowers divaricate-ascending at anthesis; sepals glabrous or sparsely pubescent; petals purple to lavender, 3.5-6 × 1-1.5 mm; pollen ellipsoid or spheroid. Fruits divaricate-ascending to slightly descending, not appressed to rachis, secund, straight or curved, edges parallel, (1.6-) 2-4.4 cm × 1.6-2.3 mm; valves glabrous; ovules 28 40(-44) per ovary; style 0.1-0.2 mm. Seeds uniseriate, 1.3-2 × 1-1.5 mm; wing continuous, 0.1-0.5 mm wide. 2n = 14.
Flowering Jun-Aug. Cliffs, talus slopes, and gravelly soil in alpine and subalpine habitats; 2100-4400 m; Alta, B.C., Yukon; Alaska, Calif., Colo., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.
Boechera lemmonii is easily recognized by its combination of secund fruits, mat-forming habit, purplish sepals, and obovate-oblanceolate cauline leaves. Both sexual and apomictic collections are known; further study is needed to determine whether they truly are conspecific. The taxa traditionally treated as Arabis (Boechera) lemmonii vars. depauperata, drepanoloba, and paddoensis are apomictic hybrids here recognized as distinct species (see M. D. Windham and I. A. Al-Shehbaz 2007 for detailed comparison).