Vervain family (Verbenaceae)
Description: This native plant is an annual or short-lived perennial that branches at the base to form multiple stems. It is only about ½–1' tall because the stems sprawl across the ground, often forming a circular or semi-circular mat of leaves and stems in open areas. The stems are about ½–1½' in length and more or less 4-angled. They are thinly to moderately covered with long white spreading hairs. The opposite leaves are up to 3" long and 1" across. The lower leaves are sharply divided into 3 primary lobes consisting of a large terminal lobe and 2 side lobes that are much smaller. These primary lobes are themselves divided into secondary lobes, or they may have a few large blunt teeth along their margins. The terminal lobe has a wedge-shaped bottom and tapers to a blunt tip. The side lobes are narrow. The upper leaves lack the side lobes and are a little smaller in size, otherwise they are shaped similarly to the lower leaves. The lower surface and margins of these leaves have scattered white hairs, while their upper surface is less hairy or nearly glabrous.
The stems terminate in dense spikes of small flowers. These spikes are up to 6" in length and at least 2/3" across because of the large bracts. The flowers bloom near the apex of each spike where the new growth occurs. Each flower is about 1/8" across, consisting of a light blue or light purple corolla. This corolla is narrow and tubular at the base, but flares outward into 5 rounded lobes. The throat of each flower is white or greenish white. The small narrow calyx is usually a darker shade of the flower and is rather inconspicuous. Underneath each flower, there is a single large bract that is lanceolate to linear, with an abundance of white hair on its underside and margins. The blooming period usually occurs during the summer and early fall, and lasts about 2-3 months for a colony of plants. Each flower is replaced by a small hairy capsule containing 4 nutlets. Each nutlet has an oblong rectangular shape with a reticulated outer surface. The root system consists of a taproot. This plant spreads by reseeding itself.
Cultivation: This plant prefers full sunlight, rather dry conditions, and a barren soil that is sandy, gravelly, or rocky. During a bad drought, some of the lower leaves or entire stems may wither away. This plant withstands foot traffic to some extent.
Range & Habitat: Prostrate Vervain occurs in most areas of Illinois, except many counties in south-central Illinois, where it is rare or absent (see Distribution Map). Elsewhere, this species is occasional to locally common. Habitats include upland areas of sand prairies, gravel prairies, barnyards and pastures, cracks in sidewalks and driveways, and dry sunny waste areas where sand, gravel, or rocky material is abundant. This species is more common in disturbed areas of cities where there is evidence of a decaying infrastructure. It is a member of an ecologically significant group of plants that pioneer in badly degraded areas – the first step in restoring the productivity of the land.
Faunal Associations: The nectar of the flowers attracts primarily small bees, including Lasioglossum tegularis (Halictid Bee sp.) and Calliopsis andreniformis (Dagger Bee sp.). Small butterflies and skippers may visit the flowers for nectar as well. The foliage is eaten by the caterpillars of Autographa precationis (Common Looper Moth) and Crambodes talidiformis (Verbena Moth). The seeds are a minor source of food to various birds, including the Mourning Dove, Cardinal, Slate-Colored Junco, and various native sparrows. The hairy leaves are bitter-tasting and rarely eaten by mammalian herbivores, although the Cottontail Rabbit may nibble on the foliage when little else is available.
Photographic Location: The cracks of a little-used paved driveway in Urbana, Illinois.
Comments: Prostrate Vervain is a weedy little plant that is often overlooked by passersby. The foliage is rather coarse in appearance and the flowers are quite small in size. It is a fine example of a plant that can survive in the harsh conditions of cities, including high temperatures near pavement, trampling underfoot and motor vehicle traffic, air pollution, summer drought, and poor soil lacking organic material. This species can be distinguished from other vervains by its prostrate habit and thick spikes of flowers (at least 2/3" across). Other vervains have an erect habit, with the exception of Verbena angustifolia (Narrow-Leaved Vervain), which has a tendency to sprawl. However, the slender spikes of this species are less than ½" across and its flowers are usually white.