46. Boechera howellii (S. Watson) Windham & Al-Shehbaz, Harvard Pap. Bot. 11: 69. 2006.
Arabis howellii S. Watson, Proc. Amer. Acad. Arts 25: 124. 1890; A. conferta Greene; A. inamoena Greene var. acutata Jepson; A. platyloba Greene; A. platysperma A. Gray var. howellii (S. Watson) Jepson; A. platysperma var. imparata Jepson
Perennials; long-lived; sexual; caudex usually woody. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, 0.6-2(-3) dm, glabrous throughout. Basal leaves: blade narrowly oblanceolate, 1-7 mm wide, margins entire, ciliate proximally, trichomes (simple mixed with a few short-stalked, 2-rayed ones) 0.1-0.4 mm, surfaces glabrous. Cauline leaves: 2-4, not concealing stem; blade auricles (0-)0.2-1 mm, surfaces of distalmost leaves usually glabrous, rarely ciliate proximally. Racemes 2-5-flowered, unbranched. Fruiting pedicels ascending, straight, 4-10 mm, glabrous. Flowers erect to ascending at anthesis; sepals glabrous; petals white to dark lavender, 4-8 × 1-2.5 mm, glabrous; pollen ellipsoid. Fruits suberect to ascending, usually not appressed to rachis, not secund, straight to curved, edges often somewhat undulate (not parallel), 2.5-6.5 cm × 3-7 mm; valves glabrous; ovules 10-20 per ovary; style 0.05-0.3 mm. Seeds uniseriate, 3-6 × 2-4 mm; wing continuous, 1.3-2.5 mm wide.
Flowering Jun-Aug. Rock outcrops, talus slopes and gravelly soil in alpine and subalpine habitats; 1800-3800 m; Calif., Nev., Oreg.
Boechera howellii usually is subsumed under B. platysperma, but the two taxa are distinct (see M. D. Windham and I. A. Al-Shehbaz 2006 for detailed comparison).