Biennial or perennial herbs with a strong tap root sometimes tough and to over 1 m tall. Leaves mostly waxy blue, tough, the margins wavy or variously cut, roundish to lyre-shaped; stalk often broadly winged and stem clasping. Flowers in a long cluster, yellow; spring. Fruits mostly 3-8 cm long with valves having a pronounced midvein.
Variants of this wild species have been long in cultivation and given rise to a wide and complex range of selections for vegetables, salads and commercial crops. These are sometimes treated as botanical varieties or subspecies but are now generally divided into groups with similar characters and cultivation requirements; some of those of horticultural interest include those selected for their leaves (cabbage and kale), stems (kohlrabi), flower shoots (broccoli, cauliflower), and buds (brussels sprouts).
Acephala Group Kales, Collard and Flowering Cabbage. The leaves are thick and waxy but generally only in loose heads. This group includes the ornamental kales with leaves various combinations of white, pink and purple.
Alboglabra Group Chinese Broccoli, Chinese Kale, Gai-lohn, Fat-shan. Annuals. Leaves thick, lower ones round, waxy blue, wavy-edged.
Botrytis Group Broccoli and Caulifower. A group characterised by short thick stems with thickened flower clusters.
Capitata Group Cabbage, Savoy Cabbage, Red Cabbage. Leaves blistered and contorted. Stem short with a dense rounded head of compact leaves.
Gemmifera Group Brussels Sprouts. Bearing compact lateral buds.
Gongylodes Group Kohlrabi. Biennials with stems swollen just above the ground to form a turnip-like tuber.
Italica Group Asparagus Broccoli, Cape Broccoli, Italian Broccoli, Sprouting Broccoli, Purple Cauliflower. Stems shorter and thicker than those of the Botrytis group with the flowerheads not aggregated into a solid head.
Source: (1997). Brassicaceae. In: . Horticultural Flora of South-eastern Australia. Volume 2. Flowering plants. Dicotyledons. Part 1. The identification of garden and cultivated plants. University of New South Wales Press.