Black Medic, an introduced, invasive species, has been added to the USWildflowers database (02/12/2013.) Scientific name is Medicago lupulina.  It  is also known by the common name Black Hay, Hop Clover, Hop Medic, Yellow Trefoil , and others.  Photo below was taken in my daughter’s yard in Boise, ID, on June 7, 2010.   Go to the Black Medic detail page for more information.

Black Medic – Medicago lupulina