Agave sisalana (sisal hemp)

Agave sisalana is a succulent, tall, perennial monocot native to southern Mexico. It is widely cultivated and naturalised in many other countries. The leaves contain a stiff fibre that is used in making various products and the young central buds can be cooked and eaten as a vegetable. The plant grows for 7–12 years, producing 120–180 leaves, depending on environmental conditions. The plant does not produce much viable seed and is therefore propagated vegetatively, mostly from bulbils that develop in the inflorescence. It can also be propagated by direct planting of suckers. After flowering, the plant dies and needs to be replaced.

A. sisalana is a perennial succulent plant and is included in lists of invasive species in Cuba, Hawaii, Puerto Rico, the Pacific Islands, and Spain. This species has been listed as a Category 2 invader in Florida (i.e., invasive exotic plant that has increased in abundance or frequency but has not yet altered Florida plant communities; Florida Exotic Pest Plant Council, 2011) and also as a Category 2 invader in South Africa’s NEMBA (National Environmental Management Biodiversity Act) list (i.e., invaders with certain qualities such as commercial use, animal fodder, etc). In Australia, A. sisalana is listed as a priority environmental weed in the state of Queensland where it is ranked among the top 200 most invasive plant species. It is also listed as one of the 35 most troublesome weed species occurring on sandy beaches and dunes along Queensland's east coast (Australian Weeds Committee, 2012). It is also known to be invasive in Ethiopia, Kenya, Malawi, and Tanzania.

A. sisalana is adapted to growth in a wide variety of habitats and often escapes from plantations into adjacent natural areas. Additionally, this species has been recorded growing in disturbed areas, roadsides, and arid ecosystems (i.e., deserts and dry forests) which suggest that it has the potential to spread much further than it has to date. A. sisalana reproduces vegetatively by bulbils (developed from sterile meristems) and it is able to develop dense monospecific stands which may prevent the establishment of native vegetation (Badano and Pugnaire, 2004).

While many botanists have adopted the APG III system of classification for the orders and families of flowering plants which places Agave in the family Asparagaceae, the CAB Thesaurus continues to use the Cronquist system which places it under Agavaceae. The taxonomic tree in the Identity section reflects this positioning. The Notes below describe how Agave and A. sisalana are placed within the APG system.

The family Asparagales includes about 153 genera and 2480 species that are distributed worldwide. This family is subdivided into seven subfamilies: Aphyllanthoideae, Agavoideae, Brodiaeoideae, Scilloideae, Lomandroideae, Asparagoideae, and Nolinoideae. For some authors, the classification of this family is highly unsatisfactory. There are no specific traits that characterize the family, some of the subfamilies are difficult to recognize while others are very distinctive. Flowers for the most part are a rather undistinguished "lily"-type. Asparagoideae, and especially Nolinoideae and Agavoideae are very heterogeneous, and several segregate families having been recognized in the past (Stevens, 2012). The subfamily Agavoideae, that comprised the genera Agave and Yucca, has previously been classified as a separate family, Agavaceae: this family is still used in the Cronquist system.

The genus Agave includes 210 species widespread in the tropical and subtropical regions of the world mainly in desert, dry and semiarid environments (Gentry, 1982). A. sisalana is considered by Kew’s World Checklist of Selected Plant Families as “probably a sterile cultivar derived from Agave vivipara”.

 

Tropical succulent perennial of 1.5–2 m height, with thick leaves in a basal rosette of elongated sword-shaped leaves from the base. 

Stems
Two to three years after transplanting, a 20 cm tall stem is formed, which will reach a height of about 1.2 m when flowering. White, fleshy stems develop from underground buds at the base of the plant, first growing sideways and then upwards to form new plants. These new plants are known as suckers (DAFF, 2015).

Trunk
The plant base is a short trunk (30–150 cm), from the top of which the spirally arranged leaves grow (DAFF, 2015). 

The components of the dry weight of sisal fibre are approximately 55–65% α-cellulose, 11–18% hemicelluloses, 7–15% lignin, 1% pectin and 1–8% ash (Elzebroek and Wind, 2008).

Leaves
Stiff, heavy, persistent leaves that are 0.6–1.2 m long, 10.2–20.3 cm wide, and 2.5–10.2 cm thick when mature. Leaves are spirally arranged around the trunk, greyish-green in colour and covered by a layer of wax. Leaves contain coarse, cream-coloured or pale-yellow fibres (3%) (DAFF, 2015). Young leaves may have small spines along their margins; they disappear when the plant matures. Leaves have a terminal, dark brown, rigid, very sharp spine, 2–3 cm long. The cross-section at the base of the leaf resembles a flattened triangle (Elzebroek and Wind, 2008).

Inflorescences
A large panicle with flowers arranged on the terminal portion in dense clusters, sessile, 4–5 cm long. Perianths with 6 segments, 6 stamens, filaments longer than the perianth segment, 3–4 cm long anthers. Style exserted, stigma 3-lobed (EOL, 2018). It only flowers once at around 2 years. Before flowering, a flower stalk of 4.5–6.0 m develops from the growth point. The flower stalk subdivides to form branches that bear the flowers. The flowers do not produce seed, but form bulbils, which are used for reproduction. Bulbils are borne in the axils of the bracteoles of the inflorescence after flowering. Flowers are yellowish green, with reddish filaments. 

Roots
A. sisalana has a shallow, fibrous root system up to 60 cm deep. The 2–4 mm thick root arises from leaf scars at the base of the bole beneath the soil surface, and extends up to 5 m horizontally way from the mother plant, forming suckers. These can be used for propagation (DAFF, 2015). Sisal produces subterraneous rhizomes from buds in the axils of the lower leaves. Along the rhizomes there are buds that may grow into new plants, forming colonies. Most of the roots are concentrated in the upper 40 cm of the soil, where they spread horizontally up to 5 m. A number of roots grow deeper than 40 cm, which results in good anchorage (Elzebroek and Wind, 2008).

Fruit 
This species is monocarpic (i.e., dies after fruiting). Fruits are capsules up to 6 cm long, 2–2.5 cm diameter, stipitate and beaked. Capsules rarely formed, and seeds (if any) are probably not viable. Vegetative bulbils are commonly produced below the flowers in the axils of bracts (Weber, 2003; Acevedo-Rodriguez and Strong, 2005).

A. sisalana is native to Mexico. It has been introduced into tropical and subtropical areas in North America, South America, Caribbean Islands, Africa, Australia and Asia. It has been used as an ornamental, vegetable  and cultivated plant. Currently, A. sisalana is widely cultivated in tropical areas in Brazil, Tanzania, Uganda, and Kenya to produce fibres.

In the 19th century, the cultivation of A. sisalana spread to Brazil, Florida and the Caribbean Islands, as well as countries in East Africa (i.e., Kenya, Tanzania, and Uganda) and Asia (ISSG, 2012; USDA-ARS, 2012). In Florida, the first plants of A. sisalana were introduced at Indian Key in the year 1826 by Dr. Henry Perrine, to establish a plantation of this species (Brown, 2002). In Brazil, the first commercial plantings of sisal were made in the late 1930s. In the 1960s the production of sisal in Brazil was accelerated and today Brazil is the major world producer of sisal fibres (FAO, 2012). A. sisalana and other Agave species were introduced in Spain in the 1940s as ornamental and cultivated plants (Martín-Galindo, 1988), and a recent study indicates that these species are spreading into new habitats, mainly on coastal sandy soils in the south-eastern end of the country (Badano and Pugnaire, 2004).

For the West Indies, A. sisalana appears reported as “established” in a 1913 collection made by W. Trelease in the island of St. Croix (US Virgin Islands). Collected in the Bahamas by Eggers in 1888, but only reported by Britton in 1920. This species was also reported as a “cultivated plant” by I. Boldingh in 1914 for the islands of Aruba, Curaçao and Bonaire (Boldingh, 1914). In Puerto Rico, this species is known from Mona Island and Guánica Dry Forest Reserve, where it is spreading and forms dense monospecific stands (Acevedo-Rodríguez, 2005). Its occurrence has been reported in Puerto Rico since 1923 (Britton and Wilson, 1923) and in the Virgin Islands (St. Croix) since 1913 (Trealease, 1913).

The risk of introduction of A. sisalana is high. The international trade of this species is not prohibited. The species is dispersed vegetatively by bulbils and rhizomes and the risk of introduction of vegetative material as a contaminant of soil remains high in those countries where the plant is well established. Additionally, this species may escape from active or abandoned plantations into natural areas where it often persists indefinitely (Acevedo-Rodríguez, 2005).

Propagation of A. sisalana can be by dividing rhizomes, tubers, corms or bulbs. This species also developed bulbils from sterile meristems (Arizaga and Ezcurra, 1995; Arizaga and Ezcurra, 2002). The species may produce up to 4000 bulbils/plant.

A. sisalana has been dispersed by man to many countries for use as a fibre crop.

A. sisalana can be found in deserts, dry forests, grasslands, roadsides, disturbed and secondary forests, caatinga, coastal beaches, coastal dunes, and plantations. Vegetative reproduction by bulbils allows the species to colonize large areas and replace the native vegetation with its rosettes (Weber, 2003).

Genetics
The number of chromosomes for this species is 150 with a ploidy level = 5x (Cavallini et al., 1996).

Reproductive Biology
The principal mechanism of reproduction in A. sisalana is clonal (i.e., bulbils). Seeds are rarely produced and germination rates and seedling establishment probabilities are very low (Arizaga and Ezcurra, 1995, Arizaga and Ezcurra, 2002; Acevedo-Rodríguez and Strong, 2005).

Physiology and Phenology
A. sisalana is a CAM species with characteristic nocturnal stomatal opening and tissue succulence (Arizaga and Ezcurra, 2002). CAM metabolism is an adaptation to increase the efficiency in the use of water, and so is typically found in plants growing in arid conditions such as Agave species.

Longevity
A. sisalana  has a life-span ranging from 7 to 10 years (ISSG, 2012).

Environmental Requirements
A. sisalana is a tropical succulent plant that needs full sunlight and moderate water availability to grow. It grows best in regions with an average annual rainfall of 800–1000 mm (or less). The species is drought-resistant; it is morphologically adapted to manage water scarcity by its extensive root system and the arrangement and shape of the leaves, which, like a funnel, concentrate rainwater on a small area. Moreover, it is a xerophytic plant, which means that its photosynthetic pathway is the crassulacean acid metabolism (Elzebroek and Wind, 2008). The maximum temperature should not exceed 32°C, with minimum temperatures of 5°C. A. sisalana is damaged by frost and it does not tolerate hail or waterlogging. Under dry and arid conditions or at low average temperatures it forms fewer leaves per year and has a longer life cycle. This species prefers sandy-loam soils but can be grown on a range of soils with pH ranging from 4 to 6. It does not tolerate water-logging (FAO, 2012; PROTA, 2012).

A. sisalana is relatively free from pests and diseases. The only serious insect pest of sisal is the agave weevil or Mexican sisal weevil (Scyphophorus acupunctatus). Bole rot, caused by Aspergillus niger, enters through leaf bases after cutting and causes a wet rot (DAFF, 2015). 

Scyphophorus interstitialis [S, acupunctatus] is the major pest of Agave species worldwide. This insect (Coleoptera) is native to Mexico where it has devastated plantations and natural populations of different Agave species. The international trade of ornamental Agave plants worldwide has facilitated S. interstitialis to establish in many parts of the world, particularly in Central and South America, the Caribbean, Africa, and Asia. On its host species, S. interstitialis causes rot and sometimes mortality due to its larvae boring holes which then facilitates microorganism and fungi entering and colonizing the host agave. The list of symptoms/signs described for Agave plants infected includes: necrotic areas and external feeding on leaves and internal red necrosis, internal discoloration, internal feeding on stems and rhizomes (ISSG, 2012).

The introduction of A. sisalana as a farming species causes environmental degradation mainly because sisal plantations replaced native forests. Additionally, A. sisalana often escapes from plantations into adjacent natural areas and studies have shown that this species has the potential to outcompete native vegetation (Badano and Pugnaire, 2004; Australian Weeds Committee, 2012). A. sisalana can develop dense monospecific stands (due its large size and rapid propagation) which may prevent the arrival and establishment of native plant species (Badano and Pugnaire, 2004; ISSG, 2012). Porembski (2000) reports that in Madagascar A. sisalana has invaded inselbergs where it has become a serious threat to the indigenous vegetation as its large size and rapid propagation enable it to out-compete many native outcrop species.

This species may impact negatively the fertility of soils in cultivated areas. Studies have demonstrated that the fertility of soils in areas subject to continuous cultivation of A. sisalana has declined, while the pH of the topsoils has decreased (Hartemink et al., 1996; FAO, 2012). Finally, the effluent from the sisal fibre production process cause serious pollution when it is allowed to flow into watercourse (FAO, 2012).

Flower stalk sap from A. sisalana can be used to make beer and a brandy-like spirit. The central buds are cooked and eaten as a vegetable.

It is cultivated as a source of fibre traditionally used in the production of twine, ropes, carpets, mattresses, and handicrafts. Sisal leaf waste has been used profitably for cattle and rabbit feed. The succulence of fresh sisal waste makes it a useful feed during dry periods. Leaf waste also has been used as a material to produce bio-fuel (methane). This species is also used as “live fences” or as an ornamental plant in gardens. Other products developed from sisal fibres include spa and cosmetic products, cat scratching posts, lumbar support belts, rugs, slippers, and cloths (FAO, 2012; PROTA, 2012). In Africa, extracts of A. sisalana leaves and leaf waste are used in traditional medicine as a fungicide. A study evaluating the antimicrobial activity of extracts of the leaves and leaf waste discarded in the process of obtaining the hard fibres of A. sisalana showed significant inhibition of Candida albicans when treated with sisal extracts (Santos et al., 2009).

Economic Value
A. sisalana is the sixth most important fibre crop globally; representing 2% of the world's production of plant fibres (plant fibres provide 65% of the total world production of fibres). The world largest producers of sisal fibre are Brazil, Kenya, Tanzania, and Madagascar. After fibre extraction, approximately 95–96% of the leaves weight still remains (leaf waste). The leaf waste is used as fertilizer, animal feed, or the dried pulp as a fuel for methane production (FAO, 2012).

Countries where sisal is cultivated
Sisal is cultivated for fibre in Angola, Brazil, China, Cuba, Haiti, Indonesia, Kenya, Madagascar, Mozambique, Mexico, South Africa. Tanzania and Thailand (FAO, 2018). Production patterns differ between countries (FAO, 2018). In Tanzania and Kenya, sisal is predominantly a plantation crop, whereas in Brazil, it is mostly small-scale (FAO, 2018). 

Site preparation
For transplants raised from bulbils, existing vegetation should be removed, and the land should be ploughed to a depth of at least 30 cm.  A ripper should be used first, followed by ordinary ploughing. For direct planting of suckers, the soil should be cleared mechanically or by hand and ploughed shallowly (DAFF, 2015).

Soil tolerances
Soil should be free-draining, acid or neutral, and of a light to medium texture. The crop can tolerate shallow and saline soil.

Transplanting to the field
Transplants usually have the fibrous roots and lower leaves cut off before planting out by hand. Plants are placed into holes made by a hoe in the ploughed ground. The distance between rows should be 1.0–1.5m and 4 m at a depth of 3 cm. Transplanting can be carried out at any time of year as it is a succulent with good drought tolerance. However, it is usually done before the beginning of the rainy season when the land is dry. This is to reduce diseases and weevil attacks. Insecticide is often applied to the soil during planting (DAFF, 2015). Plant densities range widely, from 4000 to 7000 plants/ha, depending on cropping system, climate, soil conditions and the end use of the crop. For obtaining the perfect crop for manufacturing paper, the highest plant densities are used. When plants are cultivated in rows, a much-used density of 5000 plants/ha can be obtained by a spacing of 2.5 m between the rows and 0.8 m between plants within the row.

Direct planting suckers
Suckers should be planted at a depth of 5–8 cm. If the rainfall is above 700 mm, planting density should be 1 m x 750 mm.  If rainfall is 500–700, density should be 1 m x 1 m.  If it is 300–500 mm, they should be planted at 1.5 m x 1 m (DAFF, 2015).

Fertilizers
Fertilizers are not usually used when growing A. sisalana on new land. They may be used on old soils and include: urea, lime-ammonium nitrate and superphosphate. It is suggested that waste material from the crop is ploughed back into the soil. Where pH is below 6, lime should be applied (DAFF, 2015). The required nutrients can partially be obtained from the waste material after fibre extraction and from mineralization of soil components. This may be sufficient; however, applying some extra nutrients is usually profitable. It can be based on the removal of nutrients, which per tonne of fibre is about 27–33 kg N, 5–7 kg P, 60–80 kg K, 42–70 kg Ca and 34–40 kg Mg (Elzebroek and Wind, 2008).

Irrigation
This is not required as the plant is drought resistant and cultivated as a rainfed crop (DAFF, 2015).

Weed control
This is important in the first two years after transplanting to the field. Control can be physical or chemical. After the first two years weeds can be cut down at the start of the dry season to conserve moisture and to provide a mulch. Cover crops can be grown between rows in the first three years to control weeds.

Common weeds in sisal plantations include: couch grass (Cynodon dactylon (L.) Pers.), nut grass (Cyperus spp.), African couch (Digitaria abyssinica (Hochst. ex A. Rich.) Stapf), Lalang (Imperata cylindrica (L.) P.Beauv.), cow-itch (Mucuna pruriens (L.) DC.) and Guinea grass (Panicum maximum Jacq.) (DAFF, 2015).

Suggested cultivation schedule in South Africa (DAFF, 2015):

Young, tender leaves are harvested for use as a vegetable. Harvesting for fibre production can usually begin 3–4 years after establishment of the crop. At this stage the plants will have 120–125 leaves that are at least 60 cm long, and the plant will be 1.5 m tall. Leaves are ripe and ready for harvest when the spine at the leaf tip changes from dark to light brown in colour. Leaves are harvested at regular intervals throughout the crop life cycle at a rate of 25 per plant per year. If harvesting is delayed, leaf losses occur through withering. Overcutting can reduce yields, and selective cutting may be required when the field is uneven to avoid cutting immature plants. Harvesting leaves can be repeated yearly for many years, until the emergence of the inflorescence. By removing leaves regularly, the emergence of the inflorescence will be postponed.

A. sisalana is usually harvested by hand at 2.5–5.0 cm from the bole. In the first cutting, 20–25 leaves are left on the plant. In subsequent harvests, this number is decreased to 15–20 leaves. Spines are removed before or after the leaves are cut.

The number of chromosomes for this species is 150 with a ploidy level = 5x (Cavallini et al., 1996).

There are many varieties of sisal throughout the tropics and subtropics, especially in Central America. The most important for commercial fibre production are A. sisalana 4 and its hybrids. The most common of these is Hybrid 11648 and A. fourcroydes (henequen) (DAFF, 2015).

A. sisalana does not produce much viable seed, and so it is usually propagated from vegetative bulbils that develop in the inflorescence.

Bulbils are established in a nursery by planting in beds at 10 cm x 10 cm apart. They are grown for 6 months, after which time they are transplanted into secondary beds at 30 cm x 30 cm apart. After 12–18 months they are ready to be planted in the field. Suckers can be used to propagate the crop, and these are planted directly into the field (DAFF, 2015).

Tissue culture is also used for obtaining excellent planting material (Elzebroek and Wind, 2008).

Toxicity includes raw sap which is highly irritating to the eyes and skin. Mattresses may cause allergic reactions in sensitive individuals (EOL, 2018).

South Africa produces 2000 tons of sisal per annum (DAFF, 2015).

World production of sisal and a similar agave fibre, henequen, is estimated at around 300,000 tonnes, valued at $75 million. The major producers are Brazil (120,000 tonnes), Tanzania (30,000) and Kenya (25,000). Brazil exports around 100,000 tonnes of raw fibre and manufactured goods, particularly rope to the USA. Kenya exports around 20,000 tonnes and Tanzania 15,000 tonnes. China is also a major producer and consumer (FAO, 2018).