Bean family (Fabaceae)
Description: This plant is a winter or summer annual with prostrate or ascending stems up to 2½' long. These stems are light green or reddish green and densely covered with white hairs (although older stems become less hairy); they branch occasionally. The alternate compound leaves are trifoliate. Young trifoliate leaves toward the tips of the stems have short hairy petioles, while older trifoliate leaves have longer petioles. At the base of each petiole, there is a pair of stipules that are lanceolate to ovate and variable in size. The leaflets of the compound leaves are up to 2/3" (17 mm.) long and about one-half as much across; they are medium to dark green, obovate or oval-ovate, hairy or nearly hairless, and slightly dentate along their margins. Each middle leaflet has a short stalk, while the lateral leaflets are sessile. The upper surface of each leaflet has fine lateral veins that are light green and straight.
Occasionally, individual flowerheads are produced from the axils of the trifoliate leaves on peduncles up to 3" long. Each flowerhead is about ¼" across and globoid in shape; it consists of a dense cluster of 15-50 yellow flowers. Each flower is about 1/8" (3 mm.) long; when fully open, it has a pea-like floral structure with an upper standard and lower keel; the former is relatively larger than the latter in size. The base of each flower consists of a small green calyx with 5 narrow teeth. The blooming period occurs from late spring to early fall and can last several months. Each flowerhead is replaced by a dense cluster of seedpods. Each seedpod is dark-colored, hairy, strongly curled, and about 1/8" (3 mm.) long; it contains a single dark seed that is somewhat flattened and reniform (kidney-shaped). The root system consists of a coarse branching taproot that can form nodules.
Cultivation: This plant thrives in areas with full to partial sun, moist to mesic conditions, and soil containing loam, clay-loam, or gravel. It is weedy and aggressive, but often overlooked because of its low growth habit and relatively small size. The roots add nitrogen to the soil by forming an association with rhizobial bacteria.
Range & Habitat: Black Medic occurs in every county of Illinois and it is quite common. This plant was introduced from Eurasia into North America for agricultural purposes. Habitats include prairies (black soil, clay), weedy meadows, old fields, cropland, pastures, vacant lots, landfills, cemeteries, lawns, areas along railroads and roadsides, and miscellaneous waste areas. This plant is usually found in highly disturbed areas, although it can invade high quality prairies. In the latter case, it becomes one of the understory plants that can tolerate the shade of taller prairie vegetation.
Faunal Associations: The nectar and pollen of the flowers attract various insects, particularly bees (Andrenid bees, honeybees, bumblebees, & Halictid bees). Other insects sucking nectar from the flowers include thick-headed flies (Conopidae) and small butterflies; see Müller (1873/1883) for a discussion. The foliage is edible to mammalian herbivores.
Photographic Location: A lawn in Champaign-Urbana, Illinois. The flowerheads in the photograph are still in the bud stage.
Comments: Black Medic looks like a sprawling Trifolium sp. (Clover) with small yellow flowerheads, however it is more closely related to Medicago sativa (Alfalfa). This is because their seedpods have a similar structure, although the seedpods of Alfalfa are more coiled than those of Black Medic. Other species in this genus include Medicago arabica (Spotted Medic) and Medicago orbicularis (Round Medic). These latter species are rare in Illinois and their seedpods are strongly coiled and spiny. Generally, all of these species have been introduced from Eurasia into North America as a source of forage for livestock or to replenish agricultural soil with nitrogen.