Hop trefoil

Hop trefoil (Medicago lupulina), commonly known as black medick, is a prostrate annual or biennial herb belonging to the Fabaceae family. In agronomic contexts, it is classified as an aggressive weed rather than a plant disease, capable of competing with turfgrass and forage crops due to its rapid spreading habit.

The weed typically impacts managed turf, pastures, and newly seeded agricultural fields. Its ability to fix nitrogen allows it to thrive in nutrient-poor soils, making it particularly difficult to eradicate in landscapes where nitrogen fertilizers are not applied consistently.

Key identification features include small trifoliate leaves with serrated margins and tiny, bright yellow flowers arranged in dense, globular heads. As the seeds mature, the flower heads turn black, which distinguishes this plant from other common clovers found in agricultural and residential settings.

Optimal conditions for growth include compacted soils with neutral to alkaline pH levels. It is highly adaptable and can survive close mowing, which often makes it a dominant species in neglected lawns or overgrazed pastures where competition from desirable grasses is significantly reduced.

Management strategies focus on cultural and chemical practices. Maintaining a thick, healthy stand of grass through proper irrigation and aeration is the best deterrent. For severe infestations, selective broadleaf herbicides containing active ingredients such as triclopyr or 2,4-D are effective when applied during the active growth stage before seed set.