Black mustard is a coarse annual weed, either branched or not. Flowers very small, yellow, the 4 petals arranged like a cross, about ⅜ 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.
Height: to 5 feet.
Habitat and Conservation
Grows in fields, waste places, roadsides, and other disturbed areas. A native of Eurasia. The genus Brassica includes many important agricultural plants, including broccoli, cabbage, kale, collards, cauliflower, and more. Some kinds are used medicinally or in pharmaceuticals. Many kinds of mustards have escaped from cultivation; all are immigrants with a great variety of leaf shapes.
This species has been cultivated in the Old World for thousands of years. It may be the species Jesus was thinking of when he told his “parable of the mustard seed” in the book of Matthew. Although black mustard has escaped cultivation nearly throughout North America, it is not considered a serious invader because it is an annual, mainly grows only in disturbed soils, and does not threaten native habitats or displace native plants.
Until recently replaced by brown mustard (B. juncea), black mustard was the chief source of seed used in making table mustard, which also contains extracts from another species, white mustard (Sinapis alba). Seed extracts are also used medicinally and in the preparation of some scented soaps.
The familiar cabbage white butterfly was also imported to North America from Europe, apparently in a shipment of cabbage. Its larvae eat mustard plants and are serious crop pests. Our native white butterflies, including the falcate orange tip and checkered white, use mustards as host plants, too.