A coarse annual weed, either branched or not. Flowers very small, yellow, the 4 petals arranged like a cross, about 3/8 inch wide. Blooms April–November. Leaves on long petioles, highly variable, often irregularly lobed to the midrib, generally ovate, some with teeth. Fruits long seedpods (called siliques) that form as flowering continues.
Similar species: There are 4 species of Brassica recorded growing out of cultivation in Missouri. All originated as introduced crop plants. In addition to black mustard, there is brown, leaf, Indian, or Chinese mustard (B. juncea); rutabaga or rapeseed (the source of canola oil) (B. napus); and field mustard or turnip (B. rapa). Because of their many growth forms and hybrids, these can be hard to identify in the wild.