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Medicago polymorpha L., burclover, is a shallow-rooted annual legume. The characteristic growth habit of burclover is one of numerous prostrate stems branching from the crown and spreading outward 6 to 30 inches. Where thick stands develop stems may become erect, obtaining heights of 18 to 24 inches in more favorable years. The leaves are subglabrous and clover-like in appearance, with leaflets normally wedge-shaped and toothed toward the top. The inflorescence is usually quite limited, presenting only a few small, yellow, pea-like flowers. The several-seeded fruit is a flattened, coiled pod, commonly up to 1/4 inch in width and fringed with a double row of conspicuous, hooked spines. Well developed plants may produce more than 1,000 pods. The seed is rather large for a legume of this type, usually developing to over 3/32 inch in length.
Although often considered an indigenous California plant, burclover was introduced from southern Europe. Burclover has become extensively naturalized in the United States from cultivation as a hay or cover crop. It is one of the more widely recognized Medicago species, especially west of the Sierra Nevada and Cascade Mountains, where it is most abundant.