Mint family (Lamiaceae)
Description: This introduced perennial plant is usually 1' or less, branching frequently and forming a low-growing mat of stems and leaves across the ground. The 4-angled stems are prostrate to slightly ascending, and often form rootlets near the axils of the leaves when they touch the ground. The opposite leaves are about 1" long and across. They are green to purplish green, orbicular, and crenate along the margins. There is a flat indentation where the long petiole joins the base of a leaf. The pubescent upper surface has conspicuous palmate venation. Clusters of 1-3 tubular flowers develop from the leaf axils. These flowers are bluish violet to reddish purple and about ½" in length. The corolla of each flower is narrow at the base, but flares outward like a trumpet into spreading lobes. There is a notched upper lobe, a notched lower lobe, and 2 smaller side lobes. The lower lobe is larger than the others and functions as a landing pad for visiting insects. It has darker violet lines that function as nectar guides. Within the throat of the corolla, there are fuzzy hairs. Each flower has a single pistil with a divided style, 2 long stamens, and 2 short stamens. The pubescent calyx is about 1/3 the length of the tubular corolla, with 15 veins running along its length and 5 teeth along its outer edge.
The blooming period usually occurs from mid-spring to early summer for about 2 months, although some plants may bloom later in the year if they remain in cool shade or a major disturbance prevents earlier bloom. Upon maturity, each flower is replaced by 4 dark brown nutlets. Each nutlet is ovoid, with 2 flat sides and an outer side that is rounded. The root system is fibrous and shallow. This plant often forms dense colonies by forming rootlets along the stems.
Cultivation: The preference is partial sun, moist conditions, and fertile loamy soil in an open situation where there is little ground cover. This species can spread aggressively and is difficult to destroy without resorting to herbicides. During hot summer weather, it has a tendency to become dormant.
Range & Habitat: Ground Ivy is occasional to locally common in most areas of Illinois (see Distribution Map). Habitats include openings of floodplain forests, semi-shaded areas along rivers, powerline clearances in woodland areas, cemeteries, lawns and gardens, and miscellaneous waste areas. This plant can withstand regular mowing, but flourishes better without it. It prefers disturbed areas, but occasionally invades higher quality natural areas. Sometimes homeowners tolerate its presence in lawns because they like the flowers.
Faunal Associations: The nectar of the flowers attracts long-tongued bees primarily, including honeybees, bumblebees, mason bees (Osmia spp.), a cuckoo bee (Nomada superba), a digger bee (Synhalonia speciosa), an Anthophorine bee (Anthophora abrupta), and small carpenter bees (Ceratina spp.). Occasionally, the flowers attract green metallic bees (Augochlorella striata, Augochloropsis metallica), bee flies (Bombylius spp.), a Syrphid fly (Rhingia nasica), Sulfur and White butterflies, and skippers (Robertson, 1929). The caterpillars of a polyphagous moth, Xanthotype urticaria (False Crocus Geometer), feed on the foliage Ground Ivy, while the larvae of Liposthenes glechomae (Ground Ivy Gall Wasp) form galls on the stems, petioles, or leaves (Covell, 1984/2005; Felt, 1917). The foliage of this plant is considered toxic to domesticated livetock, especially horses. However, the European Wild Boar, a species that has naturalized in the United States, reportedly uses the leaves as a source of food (Waggy, 2009). The leaves are also eaten by the Ruffed Grouse (Bennetts, 1900).
Photographic Location: At the edge of a flower garden in Urbana, Illinois.
Comments: Ground Ivy is an important source of nectar during the spring for bees. Otherwise, its ecological value is rather low. The size and color of the flowers are somewhat variable. This species resembles another introduced member of the Mint family, Lamium amplexicaule (Henbit), which is also an aggressive spreader. However, the opposite leaves of Henbit are more widely spaced along its spreading stems and they strongly clasp the stems, while the leaves of Ground Ivy have long petioles. The flowers of Henbit occur in whorls from the axils of the upper leaves, and they are more erect and pink than the flowers of Ground Ivy. Other members of the Mint family usually have an erect habit, while the stems of Ground Ivy often sprawl about and form loose mats. Another common name for this species is Gill-over-the-Ground, which is more often used in England.