Prota 2: Vegetables/Légumes
2n = 18
Leaf cabbage, kale, collard, Tronchuda cabbage (En). Chou vert, chou vert non pommé, chou cavalier, chou à grosses côtes (Fr). Couve galega, couve tronchuda (Po). Sukuma wiki (Sw).
Origin and geographic distribution
Brassica oleracea was domesticated about 5000 years ago and is now cultivated throughout the world, although in the tropics it is mostly restricted to higher elevations. Leaf cabbage comprises diverse cultigens developed from wild Brassica oleracea, which has a northern Mediterranean and western European origin. It is probably the first cabbage crop cultivated. Types with tall plants grown for repeated leaf pickings are popular everywhere in East and southern Africa, but less common in Central Africa and rare in West Africa. Leaf cabbage is the most important leafy vegetable in the highlands of Kenya and surrounding countries, and is known as ‘sukuma wiki’. In Zimbabwe the most important types are called ‘rugare’, ‘viscose’ and ‘tronchuda’. There are numerous cultivars and clones of leaf cabbage. Apart from the widely distributed Portuguese kale and marrow-stem kale, Western leaf cabbage types such as curly kale or borecole are rarely found in tropical Africa, and this is also the case for Chinese kale.
Leaf cabbage is grown for the consumption of the fresh leaves, mostly after removal of the thick midribs and petioles. The leaves are used as a vegetable dish or prepared into sauces. Young shoots and tips with young inflorescences are occasionally used for these purposes. Traditional preparation involves boiling of shredded leaves in water to which salt and other ingredients such as onions, tomatoes, garlic, hot peppers, peanuts or sesame are added. In Zimbabwe people occasionally dry leaf cabbage leaves for long-term conservation.
In Portugal the leaves are used for the preparation of a traditional dish ‘caldo verde’, a thick dark green soup. This dish is also known in Angola and Mozambique. Processing by canning and freezing are practised in Western countries. Marrow-stem kale and special leaf cabbage cultivars are used as fodder crops. The medicinal properties of cabbage crops are highly esteemed in Western countries, but as far as known not in Africa. The leaves are often consumed because they are believed to be anticarcinogenic, and they are used to treat gout and rheumatism, whereas the seeds are considered diuretic, laxative, stomachic and anthelmintic.
Production and international trade
Leaf cabbage is among the most important leafy vegetables in East and southern Africa, grown year-round for the domestic market. The total area planted may amount to over 100,000 ha, but no precise data are known. A conservative estimation for the area planted with leaf cabbage in Zimbabwe is 2500 ha for commercial crops and 2500 ha for subsistence crops, as most rural households grow leaf cabbage for family use.
Variation in nutritive composition between the various types of leaf cabbage is considerable and the nutritional composition given here is only indicative. Raw leaves contain per 100 g edible portion: water 84.5 g, energy 209 kJ (50 kcal), protein 3.3 g, fat 0.7 g, carbohydrate 10.0 g, total dietary fibre 2.0 g, Ca 135 mg, P 56 mg, Fe 1.7 mg, Mg 34 mg, Zn 0.44 mg, vitamin A 15,376 IU, thiamin 0.11 mg, riboflavin 0.13 mg, niacin 1.0 mg, folate 29 μg, ascorbic acid 120 mg (USDA, 2003). Compared to most other leafy vegetables, the content of micronutrients is remarkably high and the iron is in an easily digestible form. As all other members of Brassica, leaf cabbage contains high levels of glucosinolates, which during preparation form compounds with antioxidant and anticancer activities. In experiments some of these compounds inhibited cancer growth, some blocked cancer-causing compounds and others prevented the formation of carcinogens. The major glucosinolates in Portuguese kale are sinigrin (2-propenyl-glucosinolate), glucoiberin (3-methylsulphinylpropyl-glucosinolate) and glucobrassicin (indol-3-yl-glucosinolate).
Adulterations and substitutes
Leaf cabbage in dishes may be replaced by leaves of some other Brassica species, e.g. brown mustard (Brassica juncea (L.) Czern.), rape kale (Brassica napus L.) and Ethiopian kale (Brassica carinata A.Braun).
Erect, glabrous, annual to perennial herb up to 3 m tall; stem often woody at base, sometimes thickened, with ascending branches; root system strongly branched. Leaves alternate, fleshy, more or less coated with a layer of wax, with whitish veins; stipules absent; lower leaves petiolate, pinnately parted with 1–5 pairs of small lateral lobes and large terminal lobe up to 50 cm × 30 cm, cordate at base, rounded at apex, entire to undulate, crispy or toothed; stem leaves oblong or obovate to almost linear, clasping or auriculate at base. Inflorescence a terminal paniculate raceme up to 100 cm long. Flowers bisexual, regular, 4-merous; pedicel up to 2 cm long, ascending; sepals oblong, c. 1 cm long, erect; petals obovate, 1.5–2.5 cm long, clawed, pale to bright yellow or whitish; stamens 6; ovary superior, cylindrical, 2-celled, stigma globose. Fruit a linear silique 5–10 cm × c. 5 mm, with a tapering beak 5–15 mm long, dehiscent, up to 30-seeded. Seeds globose, 1.5–2 mm in diameter, finely reticulate, dark brown. Seedling with epigeal germination, with a taproot and lateral roots; hypocotyl 3–5 cm long, epicotyl absent; cotyledons with petiole 1–2 cm long, blade cordate, 1–1.5 cm long, cuneate at base, notched at apex.
Other botanical information
Leaf cabbage is among the cultivated Brassica oleracea types most closely related to wild cabbage. It comprises many different types, which have been classified in convar. acephala (DC.) Alef. (var. medullosa Thell. for marrow-stem kale, var. palmifolia DC. for palmtree kale, var. sabellica L. for curly kale and var. viridis L. for collard), convar. botrytis (L.) Alef. (var. alboglabra (L.H.Bailey) Musil for Chinese kale), convar. costata (DC.) Gladis (Portuguese kale) and convar. fruticosa (Metzg.) Alef. (var. ramosa DC. for thousand-headed kale). These types can best be considered as cultivar-groups and as such have been called Marrowstem Kale Group, Palmtree Kale Group, Curly Kale Group, Collard Group, Chinese Kale Group, Portuguese Kale Group and Thousand-headed Kale Group, respectively.
A practical classification used in Zimbabwe (with types listed in order of decreasing importance for the domestic market) is:
– Rugare: vegetatively propagated, rarely by seed (flowers only at high altitudes and after some degree of vernalization); plants 2–3 m tall, for repeated leaf pickings, white flowering; many small shoots developing at the base and lower internodes (hence also called thousand-headed cabbage); long life and harvest season; pale blue-green and somewhat curly leaves, but clones available with different leaf colour.
– Viscose: a selection from ‘rugare’ that has gained popularity for commercial production with repeated leaf pickings because of its improved hardiness in the field; vegetatively propagated, rarely by seed (segregates into different types); leaves darker green and more pronouncedly curled than ‘rugare’, some clones in between ‘rugare’ and ‘viscose’.
– Chou moellier: propagated by seed; marrow-stem type, plants comparatively short with short internodes and very thick stems, with large, dark green, somewhat curly leaves, for repeated leaf pickings or once-over harvest; rarely flowering in Zimbabwe; seed imported from Western countries.
– Covo or couve galega: propagated by seed; similar to Portuguese kale, tall plants with long internodes, for repeated leaf pickings, white flowering, shorter than ‘rugare’ and harvest season also shorter; leaves blue -green; mostly grown from seed imported from Europe, but sometimes from local seed production; local cultivars heterogeneous, sometimes close to ‘rugare’ and ‘viscose’, readily bolting, plants from imported seed reluctant to bolt.
– Couve tronchuda: propagated by seed; plant habit half-heading, compact, for single once-over harvest; seed imported from Europe; popular in Mozambique.
– Sukuma wiki: mostly propagated by seed, sometimes vegetatively; plant habit close to ‘rugare’; for repeated leaf pickings, yellow flowering; rare in Zimbabwe, but the most common type in East African countries; heterogeneous, often from local seed.
Growth and development
Leaf cabbage is grown from seed or stem cuttings. Young plants grown from seed develop a strong taproot with lateral roots, whereas rooting shoots develop several strong lateral roots. Depending on the desired leaf tenderness and size, the first leaves can be picked in 4–6 weeks from planting. Removal of older leaves promotes development of new ones and therefore a high leaf yield. Leaf production stops when plants start to flower, or when the crop becomes too old and the stem rots. Flowering is controlled by temperature. Local seed-propagated cultivars bolt easily when grown above 500 m altitude. Vegetatively propagated types do not flower easily and grow indeterminately if flowering is not induced by growing them at high altitudes or by artificial vernalization. European cultivars from imported seed usually do not flower at low altitudes; they require vernalization with some weeks below 10°C. Regular leaf pickings and high nitrogen fertilizing retard bolting. Flowers are insect-pollinated, mainly by bees. The fruit reaches its maximum length 3–4 weeks after anthesis. For a seed crop, continuous leaf harvesting is discouraged to ensure that healthy, viable seed is produced. Leaf cabbage flowers during 1–2 months, then the plants senesce and die. Seed crops should be well isolated (1000 m) from other flowering Brassica oleracea plants because of the strong outcrossing character.
Leaf cabbage grows best under full sunlight. The optimum temperature for growth is 15–25°C, but there are great differences in cold or heat tolerance between cultivars. Leaf cabbage tolerates low temperatures, European types even tolerate frost. A regular supply of water is essential for good growth, either by rain or irrigation (about 5 mm per day). Soils should be well drained and fertile, having good moisture retaining capacity, high organic matter content and pH > 6.0. On acid soils leaf cabbage is often heavily affected by club root.
Crop management is similar to that for headed cabbage. Rotation with crops other than Brassicaceae is recommended. The uptake of minerals is extremely high. Soil preparation includes deep ploughing with incorporation of up to 30 t/ha organic manure. Before planting fertilizer is applied, e.g. NPK 15–15–15 at a rate of 500 kg/ha, followed by a top dressing of N fertilizer for good vegetative growth. Planting on ridges during the wet season to improve drainage is recommended. The crop should be kept free of weeds, especially in the first month after transplanting.
Propagation and planting
The 1000-seed weight of leaf cabbage is 2–4 g. Dry seed (6% water) remains viable for at least 4 years when stored below 20°C. Freshly harvested seed may show poor germination, which is overcome by soaking in water overnight. After storage for 3–4 months dormancy disappears. Seeds germinate within 3–6 days at 15–20°C. Chilling the seed for 3 days prior to planting may speed up germination. A fine tilth is essential for good germination and emergence. Direct seeding and transplanting are feasible options but the former requires more seed. Seeds are usually sown on a seedbed. Young seedlings may require light shading. About 300–500 g seed and 200 m2 of seedbed are required to plant 1 ha. Seedlings are ready for transplanting 4–6 weeks after sowing.
Vegetative propagation is much applied, especially for African types that produce few or no seed. Shoots are usually planted directly, but it is also possible to plant them first in a nursery bed for rooting. The disadvantage of vegetative propagation is an uneven stand caused by planting material being of variable size, and high plant losses due to rotting and wilting diseases. Plant spacings of 20–30 cm between plants in the row and 50–80 cm between rows is used, depending on the size of the cultivar.
Diseases and pests
Diseases and pests of leaf cabbage are essentially the same as for the better studied headed cabbage, but in general leaf cabbage is more resistant. Grey leaf mould (Alternaria brassicae) and downy mildew (Peronospora parasitica) can be controlled by fungicides; bacterial soft rot (Erwinia carotovora) occurs under hot and humid conditions; black rot (Xanthomonas campestris pv. campestris) is controlled by disease-free seeds and avoidance of overhead irrigation. Club root (Plasmodiophora brassicae) has been spreading fast during the last decades and has become the most detrimental disease in many highland areas. Tolerance to it differs between clones and cultivars. Club root is controlled by crop rotation, liming and cultivation on soils with pH > 6.5. Other diseases are ringspot (Mycosphaerella brassicicola), cabbage yellows (Fusarium oxysporum) to be controlled by crop rotation, and turnip mosaic virus, which can be prevented by control of the aphid vectors. Blackleg (Leptosphaeria maculans) can be controlled by chemicals or planting resistant cultivars. In Portuguese kale resistance was found against Peronospora parasitica, Xanthomonas campestris pv. campestris, Plasmodiophora brassicae and Leptosphaeria maculans.
Important pests include diamondback moth (Plutella xylostella), for which chemical control is increasingly ineffective because of the quick build-up of resistance to all except neem-based insecticides. In headed cabbage good results were obtained with biological control by sex pheromones and parasitoids, and this is probably effective also in leaf cabbage. Other insect pests are leaf webber (Crocidolomia binotalis) and web worm (Hellula undalis) particularly in southern Africa. Occasional pests are cut worm (Spodoptera littoralis), flea beetle (Phyllotreta spp.), cabbage butterfly (Pieris spp.) and cabbage aphid (Brevicoryne brassicae).
Leaves are harvested when needed at any stage, depending on consumer preference and intended use. Leaf harvesting continues until flowering when leaf productivity goes down.
Data on yield are scarce. Average yields are around 20 t/ha from a once-over harvest. An estimated total yield of up to 50 t/ha from about 10 repeated leaf pickings in 6 months can be obtained. A seed crop may yield 1–2 t seed per ha.
Handling after harvest
Leaf cabbage is more perishable than headed cabbage because the leaves are open and have a large surface to volume ratio. However, the leaves are less perishable than those of Ethiopian kale (Brassica carinata), brown mustard (Brassica juncea) and rape kale (Brassica napus) because of their more waxy surface.
Leaves to be sold on the market should be harvested in the evening or early morning and immediately taken to the market. In Zimbabwe, traders sprinkle cold water onto the leaves or suspend the stalks in water to maintain freshness. The sweetness of leaves that have been harvested in warm weather can be improved by keeping them in a refrigerator for a couple of days before cooking. Locally, the fresh leaves are sun-dried for long-term storage.
Collections of leaf cabbage are held in European countries, notably in Portugal (Banco Português de Germoplasma Vegetal (BPGV), S. Pedro de Merelim, Braga) and the Netherlands (Centre for Genetic Resources, Wageningen), and in the United States (Northeast Regional Plant Introduction Station, PGRU, USDA-ARS, Cornell University, Geneva NY) and Russia (N.I. Vavilov All-Russian Scientific Research Institute of Plant Industry, St Petersburg). Leaf cabbage is a main source of genes for conferring resistance to environmental stress to other Brassica oleracea types. Traditional leaf cabbage clones and cultivars have developed desirable characters including tolerance to diseases, pests and environmental stress. Collection and conservation of this germplasm is urgently needed, because the expected increasing popularity of improved cultivars will cause genetic erosion.
Several international seed companies have created improved cultivars of the European leaf cabbage types including Portuguese kale, which are sold in seed shops everywhere in East and southern Africa. Commercial seed from a local cultivar of ‘sukuma wiki’ is available in seed shops in Kenya. However, breeding efforts on local leaf cabbage types in Africa are almost absent. Imported cultivars can be outstanding in yield and uniformity, but they lack resistance to pests and diseases as found in the local cultivars, are less suited to the consumer taste and are not suitable for cultivation at lower altitudes. In Zimbabwe, East West Seed Company recently started breeding work on local cultivars. It was observed that seed-propagated crops from plants that are normally vegetatively propagated show earlier flowering in the following generation, implying a less desirable selection for early bolting. They also showed a wide variation, showing the highly heterogenic nature of leaf cabbage.
Leaf cabbage is an extremely important leaf vegetable in East and southern Africa, in many regions at least as important as headed cabbage. The high yield capacity, popularity among consumers and excellent nutritional properties all indicate the need for more breeding and research work on cultural practices including integrated control of pests and diseases. There is especially a great need for healthy seed of improved and adapted cultivars.
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Correct citation of this article:
Mvere, B. & van der Werff, M., 2004. Brassica oleracea L. (leaf cabbage) In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, leafy shoot; 2, flowering and fruiting branch; 3, flower.
Drawn from life plant by R.H.M.J. Lemmens
Portuguese kale, plant habit
sukuma wiki (left) and Brassica juncea (right)
flowering plant of ‘Portuguese kale’
cv. group Chinese Kale, market
cv. group Marrowstem Kale in the field
cultivar of the ‘Viscose’ group on the market in Zimbabwe
field of seed-propagated Portuguese kale
cultivar of the ‘Rugare’ group with side shoots and basal shoots for vegetative reproduction
sukuma wiki planted as border