Erect biennial, sometimes winter annual or short-lived perennial, to 1.2 m tall. Immature plants exist as basal rosettes until flowering stems develop at maturity. Plants are highly competitive and often grow in dense colonies. Dyer’s woad is primarily a noxious weed of rangeland, agronomic crops, and undisturbed natural areas in the intermountain west region of the northwestern U.S. It was cultivated for several centuries in Europe as a medicinal herb and source of blue dye. Foliage contains compounds that appear to have insecticidal and fungicidal properties. The native rust [Puccinia thlaspeos] can significantly reduce seed production and may have potential as a biocontrol agent. Dyer’s woad was introduced from Europe as a cultivated plant of the early settlers of the Eastern U.S.

SEEDLINGS: Cotyledons ovate, glabrous, ~ 15-20 mm long. Tips +/- squared, often slightly indented. Bases +/- wedge-shaped, tapering into a stalk ~ 5-12 mm long. First leaves alternate, elliptic to obovate with smooth margins, ~ 15-20 mm long, +/- rounded at the tip, sparsely covered with long hairs. Bases taper to a hairy stalk ~ 4-10 mm long. Subsequent few leaves resemble first leaves.

MATURE PLANT: Flowering stems typically several per rosette, gray to purplish, glabrous, typically branched near the top. Leaves +/- bluish-green, often covered with a powdery white bloom (glaucous). Midveins conspicuously pale. Rosette leaves oblanceolate to elliptic, ~ 3-18 cm long, 1-4 cm wide, tips +/- rounded, bases gradually tapered to stalk ~ 1/2-3/4 the length of the blade, sparsely covered with simple long hairs, especially on veins. Margins weakly toothed to +/- wavy. Stem leaves alternate, sessile, broad to narrowly arrowhead-shaped (sagittate) with smooth margins, sometimes broadest near the tip, clasping and lobed at the base, +/- glabrous

ROOTS and UNDERGROUND STRUCTURES: Taproots of rosette and mature plants penetrate the soil to an average depth of about 1 m. Most lateral root growth occurs in the top 30 cm of soil during the second year.

FLOWERS: April-June. Panicles of racemes +/- flat-topped (corymb-like) or umbrella-shaped. Petals 4, bright yellow, spoon-shaped, ~ 3-4 mm long. Sepals 4, separate to base, shorter than petals. Stamens 6, 4 long, 2 short. Insect- or self-pollinated.

FRUITS and SEEDS: Fruits (silicles) pendant, black to blue- or purplish-black, flattened, oblong to oblanceolate, 8-18 mm long, 5-7 mm wide, longitudinally ridged at the center of each side, gradually tapered to a slender stalk. Stigma sessile. Fruits do not open, mostly contain 1 seed. Seeds oblong, +/- round in cross-section, grooved into 2 unequal halves, dull yellowish- to orangish-brown, ~ 3-4 mm long.

HABITAT: Disturbed and undisturbed sites, roadsides, railroad rights-of-ways, fields, pastures, grain and alfalfa fields, forest and rangeland. Often grows on dry, rocky or sandy soils.