Black medic

Black medic is a low, sprawling, prostrate broadleaf weed in the legume family whose clover-like leaves are divided into three small, finely toothed leaflets with prominent veins; the middle leaflet sits on a short distinct stalk, a useful trait separating it from true clovers and from yellow woodsorrel. It often goes unnoticed until it produces tight, round clusters of tiny bright-yellow flowers about an eighth of an inch across. These flowers mature into small, distinctively coiled, kidney-shaped seed pods that turn black at maturity, giving the plant its name. The plant grows from a taproot and forms low mats that hug the ground beneath mowing height.

Black medic forms low, dark-green to yellow-flowered mats that sprawl over thin turf, creating patches of contrasting texture and a scattering of small yellow flowers that mar a uniform lawn. By occupying bare and weak areas and spreading its prostrate stems beneath mowing height, it competes with already-stressed grass for space and moisture, and its presence signals a thinning, under-fertilized stand that is losing density to the weed.

Black medic behaves as a summer annual, though it can act as a winter annual in milder climates, and it reproduces by seed. As a legume it fixes its own nitrogen, which lets it thrive on nutrient-poor soils where turf is weak. Plants germinate and grow through the warm season, flower and set the characteristic black seed pods, and rely on a persistent seed bank for reinfestation; its taproot also helps it endure dry conditions during the growing season.

Black medic is a classic indicator of low-fertility, low-nitrogen turf and favors dry, compacted, nutrient-poor sites in full to partial sun. It moves into lawns that are thin, drought-stressed, or under-fertilized, where its nitrogen-fixing ability lets it outcompete weak grass that cannot access enough nitrogen. Compacted, droughty soils and thin turf are the conditions that most reliably invite it in.