Perennials; usually short-lived; sexual; caudex not woody. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, (1.5-)2.5-8 dm, densely pubescent proximally, trichomes simple and short-stalked, 2-rayed, to 1 mm, sparsely pubescent or glabrous distally. Basal leaves: blade oblanceolate, 1-7 mm wide, margins entire or dentate, ciliate along petiole, trichomes (simple), to 0.7 mm, surfaces moderately to densely pubescent, trichomes short-stalked, 5-8-rayed, 0.15-0.4 mm. Cauline leaves: (8-)15-40, concealing stem proximally; blade auricles 1-3 mm, surfaces of distalmost leaves usually sparsely pubescent. Racemes (15-)30-90-flowered, usually unbranched. Fruiting pedicels reflexed, abruptly recurved at base, otherwise straight, 4-13 mm, glabrous or sparsely pubescent, trichomes usually simple. Flowers descending to pendent at anthesis; sepals pubescent; petals usually white, rarely lavender, 4-6 × 0.8-1.5 mm, glabrous; pollen ellipsoid. Fruits strongly reflexed, usually appressed to rachis, rarely somewhat secund, straight, edges parallel, 3.5-6 cm × 0.9-1.5 mm; valves glabrous; ovules 70-126 per ovary; style 0.2-0.5 mm. Seeds uniseriate, 1-1.4 × 0.8-1 mm; wing continuous (rarely absent), to 0.1 mm wide. 2n = 14. Flowering May-Jun. Rocky and gravelly hillsides, prairies, open woods, floodplains; 600-1400 m; Alta., Man., Que., Sask.; Idaho, Minn., Mont., Nebr., N.Dak., S.Dak., Wyo. Though often treated as a variety of Arabis (Boechera) holboellii (e.g., R. C. Rollins 1993; G. A. Mulligan 1996), B. collinsii is easily distinguished from that species by having simple and 2-rayed (versus 4-8-rayed) trichomes proximally on stems, and narrower (0.9-1.5 versus 2-2.5 mm) usually non-secund fruits. The two taxa also show allopatric distributions, with B. collinsii found on the North American continent (usually east of the Rocky Mountains) and B. holboellii apparently confined to Greenland.