Black medick
Medicago lupulina, commonly known as black medick, nonesuch, or hop clover, is a familiar lawn plant belonging to the legume or clover family. Plants of the genus Medicago, or bur clovers, are closely related to the true clovers and sweet clover.
Photo: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=145007