The taxonomy, as well as application of both scientific and common names, has long been confused in this genus, especially among the species with strains cultivated for vegetables. Some species have been moved into the genus Sinapis.

Besides Brassica napus and B. rapa, another important cultivated species is B. oleracea L., rather similar to B. napus in characters of flowers and fruit, but with sepals closely appressed to the corolla rather than slightly spreading. Brassica oleracea apparently is native to Europe but long grown in various forms including cabbage, broccoli, cauliflower, kale, kohlrabi, Brussels sprouts, etc.

1. Leaves (especially upper ones) strongly clasping with auriculate bases.

2. Sepals 6.5-9 mm long; petals 9-14 (-16) mm long; beak of fruit ca. 5-8 (-12) mm long, ca. 10-25% as long as valves; open flowers not clearly overtopping the unopened buds; foliage glaucous.

B. napus

2. Sepals 3.4-6.5 mm long; petals 6.5-9 (-10) mm long; beak of fruit ca. 8-15 (-23) mm long, ca. 30-50% as long as valves; open flowers overtopping the buds of the inflorescence; foliage green or glaucous.

B. rapa

1. Leaves sessile or petioled, not clasping.

3. Fruit (2.5-) 3.5-4.5 cm long at maturity (even when immature soon exceeding 1.5 cm), including beak 7-9 (-10) mm long, on pedicels 7-10 mm long and diverging from axis; young tips of inflorescences ± corymbose, with flowers overtopping the unopened buds.

B. juncea

3. Fruit 0.9-1.7 (-2) cm long, including beak less than 4 mm, on pedicels 3-5 (-6) mm long and closely appressed to the axis of the inflorescence; young tips of inflorescences with unopened buds overtopping open flowers.

B. nigra