Harvard Pap. Bot. 11: 86. 2006
Perennials; long-lived; reproductive mode unknown; caudex woody. Stems 1 per caudex branch, arising from center of rosette near ground surface, ca. 1 dm, densely pubescent proximally, trichomes stalked, (2 or) 3–6-rayed, 0.2–0.5 mm, glabrate distally. Basal leaves: blade oblanceolate, 4–6 mm wide, margins entire, not ciliate, surfaces moderately pubescent, trichomes long-stalked, 3–6 (or 7)-rayed, 0.2–0.5 mm. Cauline leaves: 2–5, concealing stem proximally; blade auricles absent, surfaces of distalmost leaves pubescent. Racemes 3- or 4-flowered, unbranched. Fruiting pedicels erect-ascending, straight, 4–5 mm, glabrous or sparsely pubescent, trichomes appressed, branched. Flowers not seen. Fruits erect-ascending, sometimes appressed to rachis, not secund, straight, edges parallel, 3–4 cm × ca. 5 mm; valves glabrous; ovules ca. 16 per ovary; style 0.5–0.7 mm. Seeds uniseriate, 5.5–6.5 × 4–4.5 mm; wing continuous, 1–2 mm wide.
Phenology: Flowering Jun–Jul.
Habitat: Rocky soil
Elevation: ca. 1800 m
Boechera ultra-alsa is known only from the holotype, from Snow Mountain in Lake County. Originally identified as Arabis (Boechera) platysperma, this specimen differs from that taxon by having petioles without obvious cilia, basal leaves with long-stalked (0.1–0.2 versus less than 0.1 mm) trichomes, and fruits edges parallel (not undulate or constricted between the seeds).