Thlaspi idahoense Payson, Univ. Wyoming Publ. Sci., Bot. 1: 159. 1926; T. aileeniae Rollins; T. fendleri A. Gray var. idahoense (Payson) C. L. Hitchcock; T. idahoense var. aileeniae (Rollins) Rollins; T. montanum Linnaeus var. idahoense (Payson) P. K. Holmgren

Stems (0.3-)0.4-1.1(-1.2) dm. Basal leaves: petiole not differentiated from blade, about as long as blade; blade linear to narrowly oblanceolate, 1.1-4(-5) mm wide, base attenuate. Cauline leaves (2 or)3-7(-9). Racemes lax or slightly compact. Fruiting pedicels horizontal or subhorizontal, forming an angle more than 70º with rachis. Flowers: petals white, (3.4-) 3.6-6 × 1-2 mm. Fruits only slightly winged, obovate or, rarely, elliptic, (4-)5-8(-9.5) × (2.5-) 2.7-4(-4.5) mm, 1-2 times as long as wide, apex obtuse, truncate, or emarginate; style (0.5-)0.8-2(-2.5) mm. Seeds 2-8 per fruit. 2n = 14, 28.

Flowering Jun-Aug. Alpine or subalpine slopes, meadows, rocky crevices, rocky and gravelly soil in valleys, steep slopes, knolls; 2100-3700 m; Idaho.

Subspecies idahoensis is restricted to Blaine, Boise, Custer, Elmore, and Valley counties of central Idaho. Holmgren treated it as a variety of Thlaspi montanum, whereas R. C. Rollins (1993) recognized it as a species, T. idahoense, with two varieties. In my opinion, his varieties are based on continuous characters such as the loosely versus closely branched caudex, oblanceolate versus linear-oblanceolate basal leaves, and cuneate versus minutely auriculate distalmost cauline leaves.