Formerly placed in the genus Brassica, and also formerly called Brassica kaber; Sinapis arvensis is a reversion to the name originally given to it by Linnaeus. A common weed along roadsides and in fields. It has the typical four-petaled mustard flowers in the typical mustard-yellow color, but the larger flowers easily distinguish it from the other wild mustards.
Gray describes the genus and the species:
BRASSICA [Tourn.] L. MUSTARD. TURNIP Annuals or biennials, with yellow flowers. Lower leaves mostly lyrate, incised, or pinnatifid. (The Latin name of the Cabbage.)
B. ARVENSIS (L.) Ktze. (CHARLOCK.) Knotty pods fully one third occupied by a stout 2-edged beak; upper leaves rhombic, scarcely petioled, merely toothed; fruiting pedicels short, thick; pods smooth or rarely bristly. 4 cm. long. (B. Sinapistrum Boiss.; Sinapis arvensis L.) Noxious weed in grainfields, etc. (Nat. from Eu.)