Herbs annual, (20-)30-100(-180) cm tall, pubescent or rarely glabrous, glaucous or not, sometimes with fleshy taproots. Stems erect, branched above. Basal and lowermost cauline leaves long petiolate; petiole (1-)2-8(-15) cm; leaf blade ovate, oblong, or lanceolate in outline, (4-)6-30(-80) × 1.5-15(-28) cm, lyrate-pinnatifid or pinnatisect; terminal lobe ovate, repand, dentate, or incised; lateral lobes 1-3 on each side of midvein, much smaller than terminal lobe, crisped incised, dentate, repand, or entire. Upper cauline leaves petiolate or subsessile, oblanceolate, oblong, lanceolate, or linear, to 10 × 5 cm, base cuneate to attenuate, margin entire or repand, rarely dentate. Fruiting pedicels straight, divaricate, (0.5-)0.8-1.5(-2) cm. Sepals oblong, (3.5-)4-6(-7) × 1-1.7 mm, spreading. Petals yellow, (6.5-)8-11(-13) × 5-7.5 mm, ovate or obovate, apex rounded or emarginate; claw 3-6 mm. Filaments 4-7 mm; anthers oblong, 1.5-2 mm. Fruit linear, (2-)3-5(-6) cm × 3-4(-5) mm, terete or slightly 4-angled, sessile, divaricate or ascending; valvular segment (1.5-)2-4.5 cm, 6-15(-20)-seeded per locule; valves with a prominent midvein, slightly torulose; terminal segment conical, (4-)5-10(-15) mm, seedless; style often obsolete. Seeds dark to light brown or gray, globose, 1-1.7 mm in diam., minutely reticulate. Fl. Mar-Jun, fr. Apr-Jul. 2n = 36*.

Variation in the basal leaf morphology of Brassica juncea is tremendous, and minor variants have been recognized at specific, subspecific, and varietal ranks. All these "taxa" have 2n = 36, and they can be readily crossed and produce fully fertile offspring. Seven varieties and three species were recognized in FRPS. Of these, only three major types are recognized here as varieties. Brassica juncea var. megarrhiza and B. napiformis represent one taxon (var. napiformis), while B. juncea var. tumida is quite distinct in leaf morphology. Both varieties are recognized here. The other varieties, which are based solely on the type of leaf margin, are reduced to the synonymy of var. juncea. These include var. gracilis (margin doubly serrate or incised), var. multisecta (margin with linear or filiform lobes), var. foliosa (margin sinuate-dentate), var. crispifolia (margin sharply dentate or incised), and var. multiceps (margin unlobed, slightly incised, or irregularly doubly serrate). Brassica integrifolia was described from European plants of B. juncea that became naturalized in St. Croix, an island in the Caribbean Sea. It is said to differ from B. juncea in having undivided leaves and smaller fruit, but these alleged differences are unrealistic, and B. integrifolia does not merit any taxonomic status.

Fields, waste places, roadsides. Cultivated throughout China, sometimes naturalized especially in SW China [widely cultivated and naturalized elsewhere].