All of our species are known to hybridize with one or more other species, but in Michigan only V. hastata × V. urticifolia is at all frequent. The corollas are normally some shade of blue to purple or white, with rose-colored forms occurring rarely.
1. Spikes (even excluding corollas) at least (8–) 12 mm thick; plant ± spreading-decumbent, with incised-pinnatifid leaves.
1. Spikes (at least excluding corolla) less than 8 mm thick; plant stiffly erect, with leaves unlobed or lobed only at the base (pinnate only in V. officinalis).
2. Leaves linear-lanceolate, 5.5–12 times as long as broad, the blade less than 1.5 cm wide and tapering into ± indistinct petiole; plant glabrous or with scattered, mostly appressed stiff hairs.
2. Leaves ovate-lanceolate or broader, 1.3–6 (–6.3) times as long as broad, (1.3–) 2–6.5 cm wide, sessile to subsessile or mostly with distinct petiole; plant with hairs on stems and leaves numerous and/or ± spreading.
3. Leaves (except uppermost) deeply pinnately lobed or incised; flowers at maturity remote in spike; stem glabrous or sparsely scabrous.
3. Leaves at most (V. hastata) with 1–2 lobes at base of the blade; flowers remote or crowded; stems glabrous or pubescent.
4. Plant ± densely hoary-pubescent; leaves sessile (or the lower subsessile on winged petioles), 1.3–3 times as long as broad; limb of corolla ca. 7–11 mm broad; calyx over 3.5 mm long.
4. Plant with more sparse pubescence, not hoary; leaves distinctly petioled, 2–6.3 times as long (including petiole) as broad; limb of corolla ca. 2.5–4 mm broad; calyx less than 3.5 mm long.
5. Spikes with flowers and fruit (except sometimes lowest ones) densely overlapping; corolla blue to purple (rarely rose), the limb ca. 2.8–4 mm broad; style 1–1.5 mm long, noticeably exceeding calyx after shedding of corolla; calyx 2.3–3 mm long; leaves often with 1–2 lobes at base of blade.
5. Spikes with flowers (except in bud) and fruit distinctly separated on the axis; corolla white, the limb ca. 2.5 mm or less broad; style ca. 0.4–0.6 mm long (or less), scarcely if at all exceeding calyx; calyx (1.5–) 1.8–2.3 (–2.5) mm long; leaves all unlobed.