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B. olearacea, along with the cultivars of B. juncea and B. rapa, have such a long history of cultivation and diversification that it can be difficult to ascertain and classify the relationships among species and varieties (or subspecies). Varieties of B. olearacea are generally placed into the following eight groups:
1) var. acephala, which includes some types of kale, collards, palm cabbage, and Portuguese cabbage, all used for their leaves, which are generally used as a cooked vegetable or in soups, but are sometimes marinated in dressing and used as a salad.
2) var. alboglabra, Chinese broccoli or Chinese kale, which produces fleshy stems and crowded flower buds; the stems, flower buds, and young leaves are generally used as a cooked vegetable and are typical in Asian stir fries, soups, and noodle dishes.
3) var. botrytris, cauliflower, including purple cultivars (Cape broccoli), as well as the pyramid-shaped Italian cultivars (Romanesco), which have large fleshy stems and flower buds that do not open; the stems and buds, which form heads, are served raw in salads or used as a cooked vegetable.
4) var. capitata, which includes cabbage (sometimes referred to as white cabbage), red cabbage, and Savoy cabbage, in which the large, rounded leaves grow together densely together with a fleshy stem to form a “head,” which is used raw in salads, pickled or fermented (in the German sauerkraut as well as in numerous other regional dishes), or cooked as a vegetable in many northern European cuisines.
5) var. gemmifera, Brussels sprouts, which forms numerous large axillary buds (where leaf joins stem), with leaves so densely packed as to form small heads, and numerous of these buds ascending a central stalk that can grow to 1 m (3 ft) tall; the buds are used as a cooked vegetable, featured in various typical French dishes. The large stem leaves may also be used as a cooked green.
6) var. gongylodes, kohlrabi, which has a greatly enlarged, bulbous, above-ground stem, with the leaves often growing in a crown around the outer edge; the stem may be used raw in salads but is more typically cooked and prepared similarly to turnips or used in soups. The leaves may also be used as a cooked green.
7) var. italica, broccoli, which has thick fleshy stalks and a large head of densely packed flowerbuds; the stems are harvested while flower buds are still immature and tightly closed, and used raw or as a cooked vegetable. Broccoli is a highly nutritious vegetable, with a high vitamin C content and numerous other vitamins and minerals, while low in calories.
8) var. sabellica, curly and Portuguese kales, which are short-lived perennials from which the leaves can be harvested for several years; the leaves are generally used as a cooked green, notably in Portuguese dishes including the traditional kale soup known as “caldo verde,” but may also be marinated in dressing and used raw in salads.
Statistics on the production of this diverse group of vegetables are difficult to aggregate. The FAO tracks a category called “cabbages and other Brassica species,” which covers all the varieties of B. olearacea but includes related species as well. However, this category suggests at least the general level of commercial production of these vegetables, which the FAO estimated was 60 million metric tons worldwide in 2010, harvested from 2.1 million hectares. Leading producers were China, India, the Russian Federation, Japan and Korea; the U.S. ranked 9th in total production.
(Bailey et al. 1976, FAOSTAT 2012, Hedrick 1919, van Wyk 2005.)