Medicago lupulina is an annual or biennial herb, taprooted with thin secondary roots forming a dense mat; roots form nodules containing nitrogen-fixing Rhizobium bacteria; stems erect, 4-angled, prostrate or reclining, 10-80 cm long; stipules lanceolate, 0.5-1.5 cm long, 0.5 cm wide (Radford et al. 1968; Yatskievych 2006).
Leaves alternate, 3-parted, leaflets 1-2 cm long, 0.3-1 cm wide, elliptic to nearly circular, tip blunt with a small abrupt point, upper surface sparsely hairy, hairy below, upper (distal) margin toothed, terminal leaflet stalk to 0.4 cm long.
Flowers yellow, to 0.4 cm, pea-flower-shaped, standard longer than keel, calyx lobes to 0.1 cm long, about equal, stamens 10, 9 together, one solitary, ovary superior; inflorescence of rounded to cylindrical heads to 1.5 cm long with 10-50 flowers (Radford et al. 1968); blooms and fruits May-Sept. Self-fertile (autogamous), although they are often visited by bees, (Mulligan and Kevan 1973; Turkington and Cavers 1979), petals deciduous after fruit set; stalk held above leaves.
Fruit dry pods, almost black, 0.2-0.3 cm long 0.1 cm wide, kidney-shaped, to coiled, tip tightly curled inward, surface with conspicuous netted veins, base cupped in persistent calyx; 1-seeded (Radford et al. 1968). One plant can produce up to 6600 seeds. Seeds dispersed by birds and grazing animals and can float in water for 12 days. Seeds usually germinate within a year in moist soil. Seeds and/or pods eaten by some birds and small mammals (Martin et al. 1951).
Wetland status: UPL.
Frequency in NYC: Occasional.
Habitat: Open areas, roadsides, lawns, fill, conspicuous for its small, bright yellow flowers in low clumps. Prefers mineral soil, pH 5.9-7.8; intolerant of drought, shade, fire, anaerobic soils, somewhat tolerant of salt (USDA, NRCS 2006).
Notes: Cultivated as animal fodder. Seeds germinate March-June, those emerging after mid-July often are winter killed. New plants may flower after six weeks and produce seeds after 9 weeks. Under normal environmental conditions, black medick plants usually die by the end of their second growing season. Black medick is host to the fungus Rhizoctonia leguminicola and is also attacked by the root rot fungus Thielaviopsis basicola as well as a leaf spot fungus, Pseudopeziza trifolii and a rust, Uromyces striatus. The larvae of the gall midge Dasyneura lupulinae is also reported to attack black medick (Turkington and Cavers 1979). Medicago lupulina.Max Licher.SEINet.swbiodiversity.org (Accessed 4/2014).