Herbs annual or biennial, 30-150 cm tall, glabrous or basally sparsely hirsute, often glaucous, with or without fleshy taproots. Stems erect, branched above. Basal and lowermost cauline leaves long petiolate; petiole to 15 cm; leaf blade ovate, oblong, or lanceolate in outline, 5-25(-40) × 2-7(-10) cm, pinnately lobed or lyrate, sometimes undivided; terminal lobes ovate, dentate, repand, or entire; lateral lobes 1-6 on each side of midvein, much smaller than terminal one, entire, repand, or dentate, sometimes absent. Upper cauline leaves sessile, lanceolate, ovate, or oblong, to 8 × 3.5 cm, base amplexicaul, auriculate, margin entire or repand. Fruiting pedicels straight, divaricate, (1-)1.2-2.3(-3) cm. Sepals oblong, (5-)6-10 × 1.5-2.5 mm, ascending or rarely suberect. Petals bright or pale yellow, (0.9-)1-1.6(-1.8) cm × (5-)6-9(-10) mm, broadly obovate, apex rounded; claw 5-9 mm. Filaments (5-)7-10 mm; anthers oblong, 1.5-2.5 mm. Fruit linear, (3.5-)5-9.5(-11) cm × (2.5-)3.5-5 mm, terete or slightly 4-angled, sessile, divaricate or ascending; valvular segment (3-)4-8.5(-9.5) cm, 12-20(-30)-seeded per locule; valves with a prominent midvein, slightly torulose or smooth; terminal segment conical, (0.5-)0.9-1.6 cm, seedless or 1-seeded; style often obsolete. Seeds dark brown or blackish, globose, (1.2-)1.5-2.5(-3) mm in diam., minutely reticulate. Fl. Mar-Jun, fr. Apr-Jul. 2n = 38*.

Native to Europe but not known in the wild, it perhaps evolved in the 16th Century as an allotetraploid derived from Brassica oleracea and B. rapa.

Brassica napus is one of the most important sources of seed vegetable oil. The seed oil is also used in the manufacture of lubricants, grease, lacquers, varnishes, soap, resins, nylon, plastics, insect repellents, stabilizers, and pharmaceuticals. The green parts and fleshy roots are eaten as vegetables. Two varieties are recognized, and both are cultivated in China.

Cultivated. Throughout China [widely cultivated and naturalized elsewhere].