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tall weeds with black seeds

Tall weeds with black seeds

Erect annual, taprooted forb, 2 to 8 feet tall; stems usually glabrous and glaucous, sometimes with scattered stiff hairs toward the base; upper stems terminate in narrow racemes of yellow flowers.

Leaves: The alternate leaves are 2 to 10 inches long, 1 to 6 inches wide, becoming smaller as they ascend the stems; lower leaves are pinnately lobed and obovate in outline, tapering to a long and rather stout petiole (not clasping), terminal lobe much larger than the lateral lobes, upper surface, often bristly with scattered hairs that are stiff, short, and white, lower surface usually glabrous, except for a few hairs along the central vein; upper leaves often lanceolate, broadly elliptic, or some other odd shape, 1 to 2 lobed or none.

Flowers: Flowers May to July; narrow racemes of yellow flowers, 6 to 24 inches long when fully mature; flower up to 5/16 inch across, consisting of 4 sepals and 4 yellow petals. The sepals are initially green, but become yellow while the flower blooms. The petals are well rounded toward their tips.

Fruit: Fruit is a silique, 5/8 inch long, tapering to a conical beak, appressed against the stalk of the raceme as it matures; petiole of silique (or flower) is about 5/16 inch long; seeds are dark brown or black.

Habitat

Black mustard occurs in dry disturbed sites such as waste places, pastures, and along roadsides and railroad rights-of-way within elevations that generally range below 7,000 feet.

Propagation/Phenology

Reproduces by seed.

Comments

Native to Eurasia; black mustard seeds and foliage have a pungent taste. Black mustard grows profusely and produces allelopathic chemicals that prevent germination of native plants; in addition, the seeds contain an alkaloid and the sinapina the glucoside sinigrin. This species generally occurs as a weed in wildland areas of the Southwestern Region rather than as an invasive plant.