Medicago

1. Fruit, even when immature, covered with hooked spines; stipules deeply toothed or lacerate.

M. polymorpha

1. Fruit without spines; stipules at most shallowly toothed.

2. Corolla 1.8–2.2 (–2.5) mm long, yellow; plant low and ± prostrate or slightly ascending; fruit ± reniform, 1-seeded, nearly black when mature.

M. lupulina

2. Corolla (7–) 8–10 (–11) mm long, blue-purple or (less often) yellow, cream, or variegated (white in albinos); plant erect; fruit curved to spirally coiled, several-seeded, green when mature.

M. sativa

All species found in Medicago

Medicago lupulinaBLACK MEDICK 
Medicago polymorphaBUR-CLOVER 
Medicago sativaALFALFA 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. September 20, 2017. http://michiganflora.net/genus.aspx?id=Medicago.