Boechera is a North American lineage that has traditionally been placed into Arabis. But it is clear that the two genera are not closely related and are currently placed in different tribes (Al-Shehbaz et al. 2006). Boechera is a difficult genus in which hybridization, polyploidy, and apomixis play a major role in its complexity (see Windham & Al-Shehbaz, 2006, 2007a, 2007b).
Plants without mature or nearly mature fruit are often puzzling to identify. If one is confronted with an immature specimen, it may be necessary to try both sides of the key where fruit is required and to continue until further descriptions fail to apply on one side to the plant at hand, thus justifying the inference that the other choice is the correct one. Sterile hybrids do occur occasionally, and parentage is often difficult to discern.
1. Cauline leaves not clasping at the base.
1. Cauline leaves clasping with auriculate or sagittate bases.
2. Pedicels becoming distinctly reflexed even before shriveling of the petals, the fruit definitely pendent; sepals ca. half as long as mature petals or a little shorter.
3. Stem with stellate pubescence only at the very base, otherwise glabrous; sepals glabrous; mature fruit spreading to loosely pendent, the pedicels more arched than reflexed.
B. grahamii (in part)
3. Stem with stellate pubescence at least on lower half; sepals at least sparsely stellate-pubescent; mature fruit strongly pendent, the pedicels sharply reflexed.
2. Pedicels spreading or ascending to strongly appressed, even after anthesis, the fruit spreading to erect; sepals various.
4. Fruiting pedicels strongly ascending to appressed, the fruits straight, erect and closely appressed to the stem.
4. Fruiting pedicels ± spreading or divaricate, the fruits straight or somewhat curved and clearly spreading from the axis.
5. Upper cauline leaves (i.e., below the lowermost pedicels or branches) ± dentate and pubescent on both surfaces.
5. Upper cauline leaves entire or nearly so, glabrous.
6. Basal leaves ± lyrate-pinnatifid, with at least a few simple hairs at the tips of the teeth or lobes; cauline leaves below the inflorescence numerous, ca. 30–40; sepals ca. half as long as the petals.
6. Basal leaves entire or merely serrate (or absent at anthesis), completely glabrous or stellate-pubescent on both surfaces; cauline leaves various; sepals various.
7. Cauline leaves ca. (11–) 25–35 or more below the inflorescence, the longest (2–) 2.5–4 (–6) cm; stem at the base and both surfaces of basal leaves ± stellate-pubescent; petals pink or pale purple; sepals at most barely more than half as long as the petals.
B. grahamii (in part)
7. Cauline leaves ca. 10–15 (–20) below the inflorescence, the longest (8–) 9–15 cm; stem and leaves completely glabrous at base of plant (and elsewhere); petals white; sepals much more than half as long as petals.