Brassica nigra

In black mustard, racemes of yellow flowers arise from the tip of the stems, with the siliques (fruit) of previously fertilized flowers arrayed along the stem below the flowers. The racemes of flowers are 1/2-2″ long. Each flower is about 1/3″ (8 mm) across, consisting of a short (2-4 mm) pedicel, 4 sepals (green in the buds, turning yellow after the flower opens), 4 yellow, oval to round petals with a narrow base (“claw”), 6 greenish-yellow stamens with yellow anthers, and a green ovary with a short style (which persists in the siliques) at its apex. Fertilized flowers produce smooth, elongate fruits (siliques) 2-3 mm across, approximately four-angled in section, and 1/2-1″ long, tapering to a conical or cylindrical beak with the remnant of the style at its tip. The siliques contain 4-15 seeds and are often closely appressed to the stem. The seeds are dark brown or black with networks of furrows on their surface.

Black mustard is an exotic annual that is native to North Africa, western Asia, and eastern Europe, now naturalized across the U.S.; it grows to an imposing eight feet tall. The stem is round in section, hairless except near the base, and covered with a waxy bloom (like a plum). The leaves are alternate, up to 10″ long and 3″ across, decreasing in size higher on the stem. The lower leaves are up to 10″ long and 4″ wide; they are bristly with scattered stiff, white hairs and pinnately lobed with the lobes decreasing markedly in size towards the base of the long, stout petiole. The terminal lobe on all the leaves is much larger than any of the lateral lobes. The upper leaves are often lance-shaped or broadly elliptical and tend to lose their petiole higher on the stem; they may have only a few lobes or none.

Black mustard (Brassica nigra) is an exotic annual that is native to North Africa, western Asia, and eastern Europe, now naturalized across the U.S.; it grows to an imposing eight feet tall. Black mustard has been used as a spice for over 2,000 years, grown in the New World since the late 1700s, and used as a condiment (“mustard”) since the 13th century when people in France mixed the ground seeds with grape juice. Common yellow mustard (the condiment, colored with turmeric) was introduced at the 1904 St. Louis World’s Fair by the R.T French Co. Hot dogs in a bun may have been sold at the Columbian Exposition of 1893 (held in Jackson Park), but nobody could (yet) put yellow mustard on them. Brassica nigra has largely been replaced by a species from India (Brassica juncea) for applications where price is major factor; harvesting B. juncea is more easily mechanized. The young leaves, buds, and flowers of black mustard are all edible, but older leaves are not palatable. Black mustard is also known as Mutarda nigra and Rhamphospermum nigrum, but neither name is considered valid by ITIS.gov. (Both ITIS and USDA Plants recognize Brassica nigra as the valid binomial for black mustard.) Black mustard prefers disturbed areas with good soil; it does not compete well in healthy native ecosystems.