Black medic (Medicago lupulina) is a common low-growing, broadleaf plant that invades lawns and gardens, often causing frustration for those seeking uniform turf. Frequently mistaken for clover, this pervasive species presents a unique challenge for control due to its persistence and ability to spread. This guide covers identifying the plant, understanding its growth, and implementing effective strategies for its removal and prevention.
Identifying Black Medic
Identifying Medicago lupulina is crucial for effective management, as it is often confused with other trifoliate plants like clover or woodsorrel. The foliage consists of three dark green, oval-shaped leaflets with fine teeth near the tip. A distinctive feature separating it from true clover is that the central leaflet is attached by a slightly longer stalk (petiolule) than the two side leaflets.
The plant’s growth habit is prostrate, with stems spreading low and close to the ground, radiating from a central taproot and forming dense mats up to two feet in diameter. These somewhat hairy, trailing stems do not root along their length. Black medic anchors itself firmly with a deep taproot, allowing it to survive in dry or compacted soils.
During the growing season (late spring through fall), black medic produces small, bright yellow flowers in dense, globe-shaped clusters, each containing up to 50 tiny individual flowers. After the flowers fade, they develop into small, coiled seed pods. These pods turn black upon maturity, which is the source of the plant’s common name. The hard, black, kidney-shaped pods each contain a single seed, indicating the reproductive stage.
Ecological Classification and Growth Habits
Black medic is classified as an annual or occasionally a biennial broadleaf plant, reproducing exclusively by seed. It is widely considered a weed, particularly in turfgrass settings, due to its competitive nature and ability to thrive where desirable plants struggle. It is common in areas with thin turf, disturbed soil, or along sidewalks where the soil is compacted.
A key factor in its success is its membership in the legume family. This allows it to fix atmospheric nitrogen through a symbiotic relationship with Rhizobium bacteria in its root nodules. This ability gives black medic a significant advantage in nitrogen-deficient soils, allowing it to flourish where turfgrass growth is stunted. The plant’s deep taproot further enables it to tolerate drought and low mowing heights, making it a persistent invader.
The plant’s aggressive, low-growing mat formation effectively outcompetes turfgrass and other desirable species. By spreading close to the soil surface, it shades out grass seedlings and prevents them from establishing, creating large, distinct patches. Furthermore, the plant produces thousands of seeds per season. These seeds can remain viable in the soil for several years, contributing to a persistent seed bank and making long-term control challenging.
Strategies for Eradication and Prevention
Effective control of black medic requires a multi-faceted approach that integrates cultural, mechanical, and chemical strategies. The most effective long-term method involves improving turf density to prevent the weed from establishing a foothold. This cultural control begins with maintaining a healthy, dense lawn through proper fertilization, as adequate nitrogen levels discourage the growth of this nitrogen-fixing weed.
Mowing turf at a higher height, typically in the three-inch range, helps shade the soil surface, preventing black medic seeds from germinating and reducing the sunlight available to existing low-growing plants. Aerating compacted soil is also highly beneficial because black medic often thrives in areas where soil density is high. Soil testing to correct any nutrient deficiencies or pH imbalances, aiming for a pH between 6.0 and 6.8, creates an environment more favorable for grass than for the weed.
For small infestations, mechanical control through hand-pulling is a viable option, but it must be done carefully to remove the entire taproot. The best time for manual removal is when the soil is moist, which makes extracting the deep taproot easier and reduces the chance of the plant regenerating. It is important to pull the weed before it produces its characteristic black seed pods to prevent new seeds from entering the soil.
If the infestation is extensive, chemical control with selective, post-emergent broadleaf herbicides is often necessary. Products containing active ingredients such as 2,4-D, dicamba, or MCPP (mecoprop) are effective against black medic. These herbicides work best when applied to actively growing plants in the spring or early fall, which are the optimal periods for weed uptake. Applying the herbicide as a spot treatment is recommended, and always follow the product label directions for application rates and timing.