Medicago lupulina, commonly known as Black Medic, is a low-growing, mat-forming weed that quickly colonizes lawns, often signaling underlying issues with turf health. This broadleaf plant is classified as an annual or biennial, completing its life cycle within one or two years. Its prolific seed production makes it a persistent problem. Controlling Black Medic requires a comprehensive strategy that includes proper identification, cultural practices, and preventative measures.
Confirming the Target: Black Medic Identification
Accurate identification is important because Black Medic is often confused with clover or wood sorrel due to its trifoliate leaves. The leaves grow in groups of three, with each leaflet being oval or wedge-shaped and having fine serrations toward the tip. A distinct feature separating Black Medic from true clovers is the central leaflet, which sits on a short, separate stalk (petiolule).
The plant produces dense, ball-shaped clusters of bright yellow flowers from late spring through early fall. After the flowers fade, they are replaced by tiny, kidney-shaped seed pods that turn black upon maturity, giving the weed its common name. Black Medic exhibits a prostrate growth habit, spreading along the soil surface and forming dense patches that suppress healthy turf.
Non-Chemical Removal and Cultural Control
For small, isolated patches, manual removal is an effective control method, provided the entire plant, including the taproot, is extracted. Hand-pulling is easiest when the soil is moist, as the root system releases more readily from wet ground. It is important to remove the plants before the flower clusters mature into black seed pods to prevent seed deposition.
Cultural control practices focus on strengthening the lawn so it naturally crowds out the weed, which thrives in thin, stressed turf. Mowing at a higher setting, typically between 3 and 4 inches, encourages the grass to develop a dense canopy. This increased leaf density shades the soil surface, inhibiting the germination and establishment of Black Medic seeds. Deep, infrequent watering also encourages deep grass roots, making the turf more competitive against the shallow-rooted weed.
Selecting the Right Chemical Treatment
When the infestation is widespread and manual or cultural methods are insufficient, selective post-emergent herbicides are necessary. These products are formulated to control broadleaf weeds like Black Medic without harming the surrounding turfgrass. Effective herbicides often contain a combination of active ingredients such as 2,4-D, dicamba, and mecoprop (MCPP), or triclopyr.
These multi-component herbicides target the plant’s growth hormones, causing the weed to distort and cease growth. Application should be timed when Black Medic is actively growing, typically in the spring or early fall, and ideally when temperatures are below 85°F. Always use a product specifically labeled for broadleaf weeds in turf and follow all mixing and application rates precisely.
Preventing Future Outbreaks
Long-term control relies on correcting the underlying soil conditions that allow Black Medic to flourish, such as low nitrogen or compaction. Black Medic is a legume, meaning it fixes its own nitrogen, giving it a competitive advantage in nutrient-poor lawns. Regular, adequate fertilization, particularly with nitrogen, helps turfgrass outcompete the weed by promoting a vigorous, dense stand.
Addressing soil compaction is also necessary, as Black Medic indicates hard, poorly aerated ground. Aerating the lawn in the spring or fall alleviates compaction, improving water infiltration and root growth for the turfgrass. Overseeding with quality grass seed in the fall helps fill in thin spots, creating a uniform, dense turf that prevents Black Medic seeds from germinating.