Description. Medicago lupulina is an annual or short-lived perennial plant, growing each year from adventitious buds on the roots. Mature plants measure from fifteen to eighty centimeters (32 inches) in height, with fine stems often lying flat at the beginning of growth and later erecting. The leaves are compound, each with three oval leaflets, carried on a short petiole; the center leaflet usually has a longer petiole. The leaflets are hairy, toothed toward the tip, and differ from those of the similar Trifolium dubium in that they end in a short point.
Black medick has small yellow flowers, often grouped in tight bunches, and typically measuring between two and four millimeters in diameter; but on larger plants the flowers may reach eight millimeters or more (1/3 inch).
The fruit is a small, ovoid pod, between one and two millimeters in length, that does not open upon maturation, but hardens and turns black when ripe. Each pod contains a single amber-colored seed.
Like other legumes, the roots of black medick contain nodules hosting nitrogen-fixing bacteria. Plants that survive for more than one year may develop a deep tap root.