Rose vervain (Glandularia canadensis, formerly Verbena canadensis) of the Vervain (Verbenaceae) family is an herbaceous, low-growing plant with spikes of showy flowers. The genus name refers to glands found on many of the species. The specific epithet refers to the species’ occurrence in what was historically considered to be Canada but is now part of the northeastern U.S. (it does not occur in Canada, as known today). The species occurs from New Mexico and Colorado, northeast to Minnesota, then east to New York and thence south and east to the Gulf and Atlantic Coasts. In Arkansas, it occurs statewide except for portions of the Mississippi Alluvial Plain. Other common names include rose verbena, trailing verbena and clump verbena. Preferred habitats are sunny to mostly sunny, mesic to dry rocky sites in woodlands, glades, prairies, sand hills and disturbed areas where soils may be thin and competition by other plants is limited.
An established plant develops a central ascending stem to 1½ feet long along with an apron of ground-hugging stems growing off the caudex. Ascending stems develop long lateral stems, and all stems eventually recline along the ground, with roots developing where stems contact the soil. Where stems become rooted, additional ascending stems may develop. Over winter months, portions of stems that are elevated above the soil (or not covered) freeze, while ground-hugging stems survive and actually continue to grow, producing short lateral ground-hugging branches that will produce apical flower spikes in spring. After fruits have been produced, fruit-bearing stems develop lateral branches that intermix with the central ascending stems noted above. Thick mats may form. In areas of especially harsh winters, rose vervain may survive only as annuals
Photo 1: This late-January photo shows a single, newly rooted stem that produced a flower spike the previous year (dead stem at upper left). Thereafter, lateral axillary stems developed that will produce apical flower spikes in the current year.
Stems, varying from dark green in spring to purplish in winter, are densely pubescent. Persistent stem pubescence is at first soft (villous), but becomes scabrous as stems become somewhat woody with age. Smallest stems may appear to be round (terete), but larger stems have four rounded sides.
Photo 2: In late January, previous year’s stems (purple) that were elevated were subject to freezing while ground-hugging stems thrived, producing new side stems with fresh leaves.
Leaves of rose vervain, broadly lanceolate in outline and loosely up-folded along midveins, are dark green above and a lighter green below. Leaves, up to 3 inches long and broad, are widest at the base with a gradual tapering to a gently-pointed apex. Leaf margins have prominent, equal-sided, blunt-tipped teeth (dentate margins) with the leaf apex having a similar tooth. One to several opposite pairs of shallow to deep clefts may occur between marginal teeth with deeper clefts near the base. Venation is off-set pinnate with secondary branched veins terminating at tips of marginal teeth. Veins are suppressed above (with a sunken upper midvein) and expressed below. The blade surface between secondary veins is bullate (raised). The upper leaf surface has scattered short pubescence across its surface while longer lower leaf pubescence occurs along veins. Leaves have ½ to ¾ inch grooved petioles. Leaf blades continue down petioles (decurrent) as very narrow wings that gradually disappear toward petiole bases. Petioles, with a flat grooved upper surface, have long ciliate pubescence angled toward the leaf blade and shorter flattened underside pubescence.
Flower buds, at the ascending tips of the ground-hugging branches, become apparent in mid-winter. The principal bloom period is in early spring with first flowers typically opening about mid-March. Flowering continues for up to two months and, thereafter, depending on weather, blooms may occur sporadically into fall. At first, flowers seem to be in 2-inch-wide domed clusters, but with flowering advancing as the rachis elongates, the true spike arrangement becomes apparent. Spikes, stout and pubescent, may be 5 inches long (including a 2 inch peduncle) with ten to thirty flowers. By the time the uppermost flowers are at anthesis, the lowermost flowers already bear immature fruit.
Photo 3: Lavender flower buds appear from calyxes rimmed with linear tooth-like lobes. Note leaf shape, venation and pubescence as well as glandular pubescent calyx lobes. Photo in mid-March.
Flowers, to ½ inch across, have slender ½-inch-long corolla tubes that flare out 90 degrees at the throat before dividing into five ¼-inch-long lobes (salverform shape). Lobes have wide bases and wider, wavy and notched apexes. The lavender to purple flowers have a throat opening that is edged in a darker color as well as covered by an encircling thick fringe of ascending hairs (floccose). Corollas are set in slender, densely glandular-pubescent, medium green, ascending, ribbed calyxes that are rimmed with five long, purplish, linear to lanceolate tooth-like lobes. Half of the corolla-tube length extends beyond the calyx.
Photo 4: Early flowers of two spikes are shown along with a developing spike (upper left center). Flowers have long corolla tubes (see lower flower of upper spike). Leaves of this plant are more elongate than plant in previous photo.
Photo 5: Throats of flowers are covered by a thick fringe of hair so that reproductive parts cannot be seen.
A pistil and four stamens of the perfect flowers remain hidden within the floccose corolla tube. Stamens occur in two pairs, with one pair bearing large anthers above a two-lobed stigma while the second pair of large anthers is below the stigma. Stamens are a light green with slightly yellow anthers; style and stigma are also light green. Stamens, with short filaments, are adnate (fused) to the upper portion of corolla tube. The corolla tube has internal, decurrent pubescence below the stigma. The ovary comprises four “knobs” (each with a single ovule) with the style attached at the ovary’s depressed center.
With fertilization, ovules become seeds within the four nutlets of the fruit. Individual nutlets, about 1/8 inch long, have an elongate cylindrical shape. Nutlets have a round apex and truncated base. Most of surface is covered with small longitudinally arranged pocks, except for the relatively narrow section where the nutlets join. Dry seeds are dark brown overall with lighter coloration where nutlets join.
Rose vervain is an ideal plant for borders, rock gardens and containers. It can develop into an attractive ground cover with a heavy bloom in early spring and occasional later blooms. As spring approaches, frozen stems can be clipped off for a better floral display. Flowers are similar to phlox species, but phlox leaves are linear and smooth-margined.
Photo 6: Rose vervain in a sunny rock garden setting. Photo in late March.
Other species of the genus found in Arkansas are Dakota vervain (Glandularia bipinnatifida var. bipinnatifida, a native species of conservation concern that is recorded from Blackland Prairie sites in four southwestern counties), moss vervain (Glandularia pulchella [= G. aristigera], a non-native species recorded from a few scattered counties), and pink vervain (Glandularia pumila, a native species recorded from one southwestern county). Rose vervain can be readily distinguished from Dakota vervain and moss vervain by its more coarsely divided leaves and from pink vervain, an annual, by its larger, more purplish flowers.
Article and photographs by ANPS member Sid Vogelpohl