Plants hispid proximally, glabrous distally. Stems (1-) 2-10(-15) dm. Basal leaves: petiole 1-5 cm; blade with 1-5 lobes each side, 2-25 cm, margins irregularly dentate, lateral lobe oblong or ovate, terminal lobe suborbicular or ovate, larger than lateral. Cauline leaves: blade simple or sinuately lobed, margins subentire or dentate. Fruiting pedicels erect, (appressed to rachis), 1.5-5 mm. Flowers: sepals 2.5-5 mm; petals pale yellow, 6-11 × 2.5-4 mm; filaments 4-7 mm; anthers 1.2-1.5 mm. Fruits: valvular segment ellipsoid, 0.7-3 × 0.5-1.5 mm; terminal segment globose or ovoid, 1.5-3.5 × 1-2.8 mm, usually rugose or ribbed, rarely smooth; style 1-3(-5) mm. Seeds 1.5-2.5 mm. 2n = 16. Flowering Apr-Jul (Nov-Jun in Texas). Roadsides, disturbed sites, waste places, fields, grassy banks, ballast; 0-2000 m; introduced; Ont., Que.; Calif., Ind., La., Mass., Nev., N.J., N.Y., Oreg., Pa., Tenn., Tex., Wash., Wis.; Europe; introduced also in Mexico, South America, Asia, Africa, Atlantic Islands, Australia. Three subspecies of Rapistrum rugosum have been recognized, including: subsp. rugosum, subsp. linnaeanum Rouy & Foucaud, and subsp. orientale (Linnaeus) Arcangeli. They are distinguished mainly on the basis of silique shape, rugosity, prominence of ribs, and the length and thickness of fruiting pedicels. These features vary considerably and intergradation occurs in such a wide array of combinations that it seems rather meaningless to recognize them without further studies. All three subspecies and some of their intermediates have been found in the United States (I. A. Al-Shehbaz 1985; R. C. Rollins 1993). The species was first collected in the flora area in 1873 as a ballast plant from Boston, Massachusetts (Al-Shehbaz).
Annual herb 20 cm - 1 m tall Stem: roughly hairy below. Leaves: mostly basal, pinnately divided to coarsely toothed, reverse lance-shaped to reverse egg-shaped. Stem leaves few, smaller than basal leaves, and toothed. Flowers: in branched clusters (raceme). Sepals four, ascending, tips hood-shaped. Petals four, yellow, 6 - 10 mm long, bases narrowed, tips rounded. Fruit: a short pod, cylindrical, leathery, divided into two members. The lower member is dehiscent, 2 - 3 mm long, cylindrical to reverse egg-shaped, and contains one to three suspended seeds. The upper member is indehiscent, 3 - 4 mm long, nearly round, wrinkled, hairy, and contains one upright seed. The beak is 1 - 4 mm long.
Similar species: The variety venosum is very similar, but does not have hairs on the larger section of its fruit.
Flowering: May to late July
Habitat and ecology: Introduced from Europe. A weed of cultivated ground and waste places.
Annual, 2-10 dm, hispid at least below; lvs mostly basal, oblanceolate to obovate, pinnatifid to coarsely dentate; cauline lvs few, smaller, merely dentate; pet 6-10 mm; mature pedicels stout, erect, 2-4 mm; lower member of the fr 2-3 mm, upper member 3-4 mm; beak 1-4 mm; 2n=16. Native of Eurasia, rarely adventive in waste places with us. All summer.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.