Black medic (Medicago lupulina) is a spreading, low growing summer annual.
It reproduces exclusively from seed. Young plants have leaves with three leaflets and hairy petioles. As the plant matures, the stems become hairy, somewhat square, and abundant as they emerge from the base of the plant. Older plants also have foliage consisting of three leaflets, which gives black medic a “clover-like” appearance when viewed from above. The leaflets have very fine teeth along the edge and the center one has a longer stalk. This helps identify black medic from the clovers.
The yellow flowers are produced through most of the growing season: May through September. Individual flowers are produced in clusters with each consisting of five yellow petals. The formation of the flowers again gives the plant a “clover-like” appearance. The fruit is a distinct black color and contains one seed. There are a number of plants that are trifoliate (three leaflets) that can be confused with black medic. The excellent book Weeds of the Northeast, by Richard Uva, Joseph Neal and Joseph DiTomaso, has a table that goes through the characteristics of each to help with identification.
Management
Black medic can be a weed in turfgrass and in ornamental plant beds. It thrives in nutrient poor soil and during drought conditions. (It is a legume and can fix its own nitrogen.) A first option for management is to address these environmental conditions. Hand removal is also an option in ornamental beds, particularly when the soil is moist.
Pre-emergent herbicide options are limited to dithiopyr (Dimension) and oxyfluorfen and oryzalin (Rout). Post-emergent herbicide applications focus primarily on two- or three-way mixtures of 2,4-D, dicamba, MCPP, or MCPA.
Source: psu.edu