Black medic, sometimes called yellow trefoil or black clover, is frequently confused with clover. The low-growing, hairy, trailing stems of this summer or winter annual form dense mats that crowd out desirable plants. Cloverlike leaves grow in threes on the stems. Small, bright-yellow flowers bloom and produce seeds in black, kidney-shape pods from May to September. In warm-weather areas, blooming may last until December. Black medic grows in lawns and gardens with low nitrogen fertility and in cracks in sidewalks and driveways. If not controlled, a single black medic plant enlarges into a thick mat that crowds out lawn grasses.
Prevent black medic from becoming established by mulching landscaped beds. Where appropriate, improve nitrogen fertility with regular fertilization to reduce black medic invasion. This weed can also be hand-pulled.