Field Clover (Trifolium sp)
My best guess is that this is Trifolium campestre, but maybe not. According to the USDA, there are 236 genera in Fabaceae (the pea family) and 170 species in Trifolium (clover). Many look quite similar. Regardless, like most (all?) clover, it is an invasive weed. It is native to Eurasia and Africa and was brought to North America to serve as fodder and for soil improvement. Since its introduction it has spread throughout most of the United States and Canada, except for a few states in the arid Southwest. You might as well enjoy its cluster of little yellow flowers because field clover is fully naturalized and is here to stay.
Missouri Plants
USDA Distribution Map & Plant Profile
University of Washington Burke Museum of Natural History
