Hibiscus cannabinus (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Cereal / pulse|
|Dye / tannin|
|Forage / feed|
- Protologue: Syst. nat. ed. 10(2): 1149 (1759).
- Family: Malvaceae
- Chromosome number: 2n = 36
- Hibiscus sabdariffa L. subsp. cannabinus (L.) G.Panigrahi & Murti (1989).
- Kenaf, vegetable kenaf, Guinea hemp, Deccan hemp (En).
- Kénaf, chanvre de Guinée, chanvre de Bombay, da (Fr).
- Nacacha, nhacandora, cânhamo brasileiro (Po).
Origin and geographic distribution
Hibiscus cannabinus is a common wild plant in most African countries south of the Sahara. It may have been domesticated as a fibre plant already 6000 years ago in Sudan. Kenaf is now widespread in the tropics and subtropics. As a vegetable it is widely grown in Africa, where it is grown on a much smaller scale as a fibre crop. In the past it has been of some importance as a commercial fibre crop in Côte d’Ivoire, Burkina Faso, Togo, Benin, Niger, Kenya, Tanzania and Malawi. India has long been the largest producer of kenaf fibre.
The shoots or young leaves, and sometimes the flowers and young fruits, are used as a vegetable. In Uganda a local delicacy is made from the seeds. These are roasted, ground and pounded, and the flour and skin are separated in water. The flour is rejected but the floating skin parts are used for the preparation of a paste, mixed with boiled pigeon peas. Children chew the bark for its sweetness.
The stem is a source of fibre used in the manufacture of twine, rope and coarse textiles for sacking and cloth for packaging; special fibre cultivars exist. The production of kenaf fibre in Africa is rather uncommon, but locally important, e.g. in northern Nigeria, Niger and Sudan, where it is used for cordage, twines, fishlines and nets. Dried scraped ribbons are used for twine and cordage for sleeping mats. Ribbons and whole stems are a raw material for the pulp and paper industry. Seeds from the fibre crop are used for oil extraction, the rest being used as feed. The oil is suitable as a lubricant and for illumination, for manufacture of soap, linoleum and in paints and varnishes.
In local medicine in Kenya, pounded roots are administered to spider bites, and leaves are used to treat stomach disorders. In West Africa, powdered leaves are applied to sores and boils, and a leaf infusion is administered for treating cough. In India, juice from the flowers is taken against biliousness, while the seed are considered stomachic and aphrodisiac. Whole young plants are an excellent fodder for cattle. The stem core (xylem) is used in combination with peat moss (Sphagnum) and fertilizers as a growth medium for plants. Kenaf plants accumulate minerals such as selenium and boron and can be used as a bioremedial tool for removing these metals from contaminated soil. In West Africa the plants are used as boundary markers.
Production and international trade
Kenaf leaves are sold at local markets in West and Central Africa. Statistics on production and trade as a vegetable are not readily available.
The average annual world production in 2004–2008 of jute-like fibres, a group including, among others, kenaf, roselle (Hibiscus sabdariffa L.), Congo jute (Urena lobata L.), sunn hemp (Crotalaria juncea L.) and devil’s cotton (Abroma augusta (L.) L.f.), was about 350,000 t, harvested from 250,000 ha. Separate statistics of these species are not available, but kenaf makes up a large part of the total, with India as main producer. In Africa, production is limited and practically all kenaf fibre is produced domestically. Industrial production is reported from Nigeria and Sudan.
The composition of kenaf leaves per 100 g edible portion is: water 79.0 g, energy 280 kJ (67 kcal), protein 5.5 g, fat 1.2 g, carbohydrate 12.2 g, fibre 2.3 g, Ca 484 mg, P 18 mg, Fe 12.1 mg, ascorbic acid 75 mg (Leung, Busson & Jardin, 1968). The composition of kenaf leaves is comparable to other dark green leafy vegetables.
Kenaf produces a bast fibre similar to jute, but with a greater tensile strength, somewhat coarser and more brittle. On a dry weight basis the bast fibre content of the stem ranges from 21% in wild accessions up to 36% in modern cultivars. Individual bast fibres are (1.5–)2–3(–12) mm long and (7–)15–25(–41) μm in diameter with a cell wall thickness of 4–9 μm. In cross-section they are polygonal to round or oval, with a varying lumen width. In longitudinal section the fibre cells are cylindrical. The fibre ends show great variation, but most often they taper into a blunt point. The fibre strands of commerce, consisting of fibre cells cemented together by pectin and hemicelluloses, are 1.5–3 m long. Kenaf fibre contains 36–62% α-cellulose, 14–21% hemicelluloses, 6–19% lignin, 1–5% pectin and 0–3% ash. The tensile strength, elongation at break, and Young’s modulus of kenaf fibre are 295–1190 N/mm², 3.7–6.9% and 22,000–60,000 N/mm², respectively.
The fibre cells in the woody core are (0.4–)0.5–0.9(–2.4) mm long and 18–33 (–37) μm in diameter, with an average cell wall thickness of 4–8 μm. The length of both bast and core fibre cells increases from the base to the top of the stem. Whole stems contain 77–79% holocellulose, 37–50% α-cellulose, 16–20% lignin and 2–4% ash. The core fraction contains 65–72% holocellulose, 34–39% α-cellulose, 24–29% hemicelluloses, 14–20% lignin and 2–6% ash.
Paper made from kenaf bast fibre by chemical pulping is stronger than paper from softwood pulp; paper made from whole stems by chemical pulping has strength properties between paper made from chemical pulp from softwood and that made from hardwood; it is comparatively tight and nonporous compared to paper made from wood.
Seeds contain up to 22(–26)% oil, with a fatty acid composition of palmitic acid 14–20%, stearic acid 3–7%, oleic acid 28–51% and linoleic acid 23–46%. The oil has phytotoxic and fungitoxic properties. The presscake contains: moisture 9%, crude protein 32%, oil 8%, crude fibre 8% and practically no antinutritional components.
Adulterations and substitutes
Kenaf leaves in dishes can be replaced by roselle (Hibiscus sabdariffa) or other leafy vegetables.
For many purposes, such as packaging and cordage, kenaf, jute (Corchorus spp.), roselle (Hibiscus sabdariffa) and congo jute (Urena lobata) may be substituted for each other, although kenaf and roselle are coarser and therefore cheaper than jute.
Annual herb, up to 2 m tall in the wild, up to 5 m in cultivars; taproot well developed, with lateral roots spreading horizontally to 1 m and adventitious roots on lowest stem section; stem erect, slender, cylindrical, prickly on wild accessions. Leaves alternate, simple; stipules filiform, 5–8 mm long, pubescent; petiole 3–30 cm long; blade 1–19 cm × 0.1–20 cm, very shallowly to very deeply palmately 3–7-lobed in lower part of plant, often unlobed in upper part or even bractlike near the apex, base cuneate to cordate, apex acuminate, margins serrate or dentate, upper surface glabrous but with a prominent, 3 mm long nectary at the base of the midrib, lower surface hairy along the veins. Flowers axillary, solitary or sometimes clustered near the apex of the plant, bisexual, 5-merous, 7.5–10 cm in diameter; pedicel 2–6 mm long, articulated at the base; epicalyx of 7–8 linear segments 7–18 mm long, persistent; calyx campanulate with acuminate to subcaudate lobes 1–2.5 cm long (up to 3.5 cm in cultivars), persistent, green, bristly and with a characteristic white, woolly, arachnoid tomentum especially near the base and margins, with a prominent nectary gland on each midrib; petals free, usually spreading, twisted clockwise or anticlockwise, obovate, 4–6 cm × 3–5 cm, outer side stellate-pubescent, usually cream to yellow with red inner base, sometimes blue or purple; stamens numerous, filaments united into a column surrounding the style, 17–23 mm long, dark red, with yellow or red anthers; ovary superior, ovoid, villose, 5-celled, style branching into 3–5, hairy arms 2–4 mm long, each branch ending in a capitate stigma. Fruit an ovoid, shortly beaked capsule 12–20 mm × 11–15 mm, densely appressed pubescent, 20–25(–35)-seeded. Seeds reniform to triangular with acute angles, 3–4 mm × 2–3 mm, grey to brown-black with pale yellowish spots, hilum brown. Seedling with epigeal germination.
Other botanical information
Hibiscus comprises 200–300 species, mainly in the tropics and subtropics, many of which are grown as ornamentals. The estimated number of species varies because opinions differ about inclusion or exclusion of several related groups of species in the genus. Kenaf belongs to Hibiscus section Furcaria, a group of about 100 species which have in common a pergamentaceous calyx (rarely fleshy) with 10 strongly prominent veins, 5 running to the apices of the segments and bearing a nectary, and 5 to the sinuses. Interspecific hybridization has been attempted with variable success between Hibiscus cannabinus and other species within the same section, such as Hibiscus sabdariffa L., Hibiscus radiatus Cav., Hibiscus diversifolius Jacq. and Hibiscus acetosella Welw. ex Hiern.
Hibiscus cannabinus can easily be distinguished from the related species Hibiscus sabdariffa by the white, arachnoid tomentum on the calyx. Hibiscus cannabinus is very variable and various subclassifications have been proposed, but none is generally accepted. The vegetable types such as ‘Malakwang’ grown in Uganda have a bushy form.
Growth and development
In general kenaf is an obligate short-day plant. Flowering is influenced by the time of planting; long days and high temperatures prolong the vegetative growth phase, an advantage for vegetable and fibre crops. Most cultivars remain vegetative until the photoperiod falls below 12.5 hours. In Ghana, day-neutral early maturing varieties exist that take 45–56 days from sowing to flowering.
Kenaf is mainly an out-breeding plant, but up to 30% self-pollination occurs. Flowers open before daybreak and begin to close about midday. The flower structure promotes cross-pollination. The pistil is functional when the flower opens, while the stamens are not dehiscing until shortly after sunrise. While the pistil is still functional, the stigmatic lobes hang down, almost touching the unopened anthers. Later in the day, the stigmatic lobes become turgid and soon stand above the anthers, which lose their pollen. Cross-pollination is mainly effected by insects such as bees. Seeds ripen in about 5 weeks after anthesis. In wild and vegetable types the fruit wall bursts and the seeds are spread on the ground, whilst in fibre types the fruits are indehiscent.
Kenaf grows naturally in grassland and as a weed in fields and wasteland. It is grown from sea level up to an altitude of 2700 m but does not do well above 2500 m and does not tolerate frost. It does well at day temperatures between 16°C and 27°C with 500–650 mm rainfall distributed over a period of 4–5 months. Lower temperatures retard plant growth. It does best on well-drained, neutral sandy loams, rich in humus. It does not tolerate waterlogging.
Propagation and planting
Kenaf is usually propagated from seed but may also be propagated through cuttings. Seed yield is 1–28 g/plant; the 1000-seed weight 25–27 g, in wild forms only 9–12 g. Under ordinary storage conditions at ambient temperatures and humidity, seeds remain viable for about 8 months. The optimum temperature for seed germination is about 35°C. In home gardens or on small plots of vegetable kenaf, seeds are sown two or three at a time, at spacings of 15 cm × 15 cm. This system is also used when interplanting with other crops. Commercial farmers may broadcast the seed for a crop that will be uprooted once the stems are 20–30 cm long. For ratoon cropping farmers usually sow in rows 30 cm apart. After a first thinning the within-row spacing is 5–7 cm. Germination of untreated seed takes about 7 days, longer than for most vegetables; for this reason some farmers soak their seed in water for 24 hours prior to sowing, obtaining emergence after 3 days. The latter method can only be used when rain is expected soon after sowing or when irrigation facilities are available.
For fibre production kenaf is broadcast at a seed rate of 15–30 kg/ha or drilled at a spacing of 15–30 cm between rows and a 3.5–10 cm within rows, but somewhat wider to produce fibre for making paper. Broadcast crops require thinning to about 400,000 plants/ha.
Kenaf responds well to fertilizers and organic manure, but only farmers producing for the market are prepared to invest in this crop. Manure at a rate of 10–20 t/ha is preferred, but if it is not available, an initial application of 250 kg/ha compound fertilizer (e.g. NPK 15–15–15) is recommended. An additional application of nitrogen as side dressing of 50 kg/ha is needed after the first cutting, and this can be repeated after every harvest. Ratoon crops can be picked up to five times when adequate fertilizer is provided. Kenaf may be grown alone or intercropped with other plants. In some regions it is semicultivated as a protected weed for use as a vegetable.
Kenaf grown for fibre grows rapidly and requires little weeding after the first month after sowing.
Diseases and pests
Diseases and pests of vegetable kenaf are the same as reported for the fibre crop, and most of them are also similar to these of cotton and okra. The main diseases are: foot, stem and collar rot and wilting caused by Phytophthora; Sclerotium rolfsii causing collar rot; powdery mildew caused by Leveillula taurica; Coniella musaiaensis causing leaf spot; Selenosporella species causing root rot and wilting; Rhizoctonia solani causing stem rot and lodging; Pythium deliense causing root rot; Phomopsis species causing stem spot; Verticillium dahliae causing wilting; Fusarium oxysporum causing wilting and necrosis; tobacco necrosis virus (TNV); and hibiscus latent ringspot virus (HLRSV), which is seed transmitted.
The cotton flea beetle Podagrica puncticollis is a notable pest, most serious in the seedling stage. Oxycarenus spp. and Dysdercus superstitiosus are seed infesting bugs. Kenaf is especially susceptible to root-knot nematodes (Meloidogyne spp.) that may reduce growth and yield especially on light-textured soils. Nematodes also predispose affected plants to pathogenic soil fungi. In order to control nematodes, crop rotation is recommended especially with amaranth or a cereal crop, and liberal application of organic fertilizer. Chemical spraying for control of pests and diseases is rarely applied.
Vegetable kenaf takes 3–4 weeks from emergence to the first harvesting. As a first thinning, plants of about 20 cm are pulled up and marketed with their roots attached. When ratoon cropping is practised, the second harvest is at the 6-week stage, 2–3 weeks after the thinning round. Cutting is done at a height of 6–8 cm, leaving 3 leaves and buds for regrowth. Up to 4–5 harvests may be carried out at 2–3-week ratooning intervals. After that, the remaining loose leaves are collected for home consumption. In commercially grown broadcast crops whole plants are pulled up when 20–30 cm tall and are sold at the market with their roots attached.
The recommended time of harvesting kenaf as a fibre crop for an optimum balance in fibre yield and quality is when about 50% of the plants are flowering. Plants are cut near the ground and tied into loose bundles that are placed upright in the field for 2–3 days to induce defoliation and drying. Stems are then graded and tied into bundles of about 10 kg and of even stem thickness.
The yield for once-over harvest as a vegetable by uprooting is 20–30 t/ha. For ratoon cropping, a total of 60 t/ha is feasible from 5 harvest rounds, depending on soil fertility and moisture. The highest yields are obtained from the second and third cuts.
World yield of kenaf as a fibre crop averages about 1.2 t dry fibre per ha. The potential yield, obtained in experimental fields with improved cultivars, is 3–5 t dry fibre per ha.
Handling after harvest
Fresh shoots are easily transported and can be kept in good condition for 1–2 days especially in shade or cool places. Sprinkling with water helps to keep the leaves fresh. Leaves can be preserved by sun-drying. The dried product is broken into small pieces or ground to powder and used in soups.
For fibre production kenaf stems are usually retted in clear and slow-moving water for a period of 10–15 days at temperatures around 30°C to liberate the fibres from the bark. Sometimes stems are decorticated (‘ribboned’) and only the ribbons are steeped in water, cutting retting time by half. When retting is complete, the fibres are stripped manually from the stems, washed thoroughly in clean water and dried well in dust- and sand-free conditions. The dried fibres are transported in crude bales of 60–150 kg to the spinning mills.
Whole stems may also be transported to pulp- and paper-making factories. Kenaf can be pulped by chemical, semi-chemical and mechanical processes. The alkaline sulphite-anthraquinone process is suitable for kenaf bark and whole stems, giving better yield, strength, viscosity and brightness of the pulp than soda and soda-anthraquinone pulping.
Local landraces of vegetable kenaf are presently not at great risk of genetic erosion, but the genetic variability in fibre cultivars is narrow. In Africa no major collections of kenaf have been reported. The Crop Research Institute in Kumasi, Ghana, has a germplasm collection of local cultivars of the vegetable type. In other countries (India, Bangladesh) large collections of kenaf as fibre crop are maintained. The Bangladesh Jute Research Institute (BJRI) at Dhaka, Bangladesh, has been designated as the world germplasm depository for kenaf and maintains a collection of some 920 accessions, including old and new cultivars, landraces, wild and semiwild accessions of kenaf and related species.
Almost no research has been carried out to enhance the genetic potential. The existing landraces are mixtures of genotypes; the available diversity should be studied and selections made from the desirable types. Purple-flowered strains and the purple false roselle (Hibiscus acetosella) are resistant to the main disease of kenaf, root-knot nematodes; hence they are potential sources of desirable genes in breeding programmes.
Breeding of fibre cultivars with high potential yields under suboptimal conditions is urgent as kenaf grown for fibre is being pushed increasingly towards marginal environments. Other breeding objectives are plants without prickly stems and bristly capsules to facilitate harvesting, and resistance to diseases, nematodes and pests.
Kenaf is a high-yielding and increasingly popular vegetable for the city markets. Contrary to the popular roselle (Hibiscus sabdariffa), it can be grown near the equator. At present its main constraint is its sensitivity to nematodes. If this problem can be solved, vegetable kenaf may well face a bright future.
Kenaf fibre is a biodegradable and environment-friendly raw material suitable for many applications, such as woven and non-woven fabrics, geotextiles and semi-rigid and laminated sheets for packaging and panelling. Kenaf stems are an excellent substitute for softwood as raw material for the pulp and paper industry. Prospects for increased kenaf fibre and pulp production are good in view of growing concerns about environmental pollution and dwindling forest resources.
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Sources of illustration
- Nabakooza, J., 2003. Illustration Hibiscus cannabinus L. Unpublished.
- R. Bukenya-Ziraba, Department of Botany, Makerere University, P.O. Box 7062, Kampala, Uganda
Correct citation of this article
Bukenya-Ziraba, R., 2011. Hibiscus cannabinus L. [Internet] Record from PROTA4U. Brink, M. & Achigan-Dako, E.G. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.
Accessed 13 November 2020.
- See the Prota4U database.