Verbena L.
Family: Verbenaceae
Vervain
Verbena image
Emmet J. Judziewicz  

Key to Wisconsin Verbena

Author: John G. Zaborsky

    • 1a. Plants prostrate-ascending, often mat-forming; at least the lower leaves incised pinnatifid; bractlets much longer than the calyx V. bracteata

    • 1b.Plants erect; leaves unlobed or only lobed at the base; bractlets equaling or shorter than (rarely surpassing) the calyx 2

    • 2a.Leaves linear to narrowly oblong or spatulate, the blades 3–11 mm wide; plants with appressed hairs V. simplex

    • 2b.Leaves ovate, lanceolate, or elliptic, with many blades over 10 mm wide; plants with spreading hairs 3

    • 3a.Spikes with flowers (except in bud) and fruits distinctly separated, not overlapping; inflorescence branches widely-spreading; corollas white, the tubes 1.8–2.5 mm long V. urticifolia

    • 3b.Spikes with flowers and fruits (except the lowest ones) distinctly overlapping; inflorescence branches erect, not spreading; corollas blue to purplish or lavender (rarely rose, pink, or white), the tubes 2.5–6.5 mm long 4

    • 4a. Leaves narrowly ovate to elliptic, on narrow petioles, sometimes hastate-lobed; plants sparsely to moderately pubescent, not hoary; corolla limb 3–4.5 mm wide V. hastata

    • 4b.Leaves narrowly elliptic, orbiculate or ovate, subsessile or sessile, unlobed; plants densely hoary pubescent; corolla limb 7.5–9 mm wide V. stricta

All of our species, with the exception of V. simplex, have benefited from human impacts on the land. These species can often be found in highly disturbed and low-quality habitats as well as more natural ones. Part of their success is likely due to the presence of the bitter glucoside verbenalin in their stems and tissues. This chemical deters herbivores and gives the plants an advantage in grazed pastures or forests. Our species are all capable of hybridizing with one another and most of the possible hybrid taxa have been collected in the state. A thorough overview of the hybrids is given by Tans & Iltis (1979). The most commonly encountered hybrids are V. ×rydbergii Moldenke (V. stricta X V. hastata) and V. ×engelmannii Moldenke (V. hastata X V. urticifolia). Hybrid taxa are uncommon and exhibit intermediate leaf morphology between the parents.

Verbena bonariensis L., native to South America, has been observed (but not collected) growing in waste areas and alleyways.  It is commonly grown as an ornamental, but it seems unlikely that it is, or will ever become, established.