|Vetiver Grass: A Thin Green Line against Erosion (BOSTID, 1993, 157 p.)|
To rely on a single species for hedges to control erosion throughout an area as large as the tropics is so unwise that a search for alternatives should be started quickly. This appendix identifies plants that might be employed either along with vetiver or as "safety-nets" in the event that vetiver develops problems in widespread practice. It should be understood that none of these species, as far as we know, has all of vetiver's properties. It should be understood also that additional species (those we haven't thought of) might be as good or even better.
With 10,000 different species of grasses worldwide, it seems logical to assume that vetiver is not the only one with the exact combination of properties required for erosion-control hedgerows. In the main, the grasses mentioned below are tall, vigorous perennials, adapted to many soils and site conditions.
It is important to realize that the most readily available strains of these grasses may be the least valuable in erosion control. They are likely, for example, to have been selected for features such as palatability to animals, high seed yield, or soft stems. The types best for erosion control, on the other hand, may be the rough, tough, "unproductive" and highly unpalatable types that were previously rejected at first glance.
Also, it is worth remembering that related grass species can often be hybridized. This can produce seedless hybrids that previously may have been considered worthless. But in erosion-halting hedges, such a reproductive flaw would be an asset.
VETIVER'S CLOSE RELATIVES
Perhaps the most promising alternatives to vetiver will be found among its own wild relatives. As has been noted already, vetiver has a dozen or so close relatives (see sidebar, page 116). These species belong to the same genus, but so far none has been explored for its erosion-controlling capabilities. Nothing suggests that any of them can match (let alone surpass) vetiver itself, but they deserve research attention nonetheless.
All these vetiver relatives are wild plants. Presumably, they are all fertile and spread by seed. However, none is known to be a weed or nuisance of any moment.
These plants are widely scattered. Four are native to Queensland, Australia, and some of those can also be found in the neighboring areas of New South Wales and Northern Territory, as well as in New Guinea. One is native to India-not to the northern plains and swamps like vetiver itself, but to the southwestern region in Karnataka. One is native to the islands of Mauritius and Rodrigues in the Indian Ocean. And two are native to the African continent.
The fact that these grasses mostly occur in riverine basins and other wet areas would seem to argue against their usefulness on hill slopes. However, vetiver itself also comes from a soggy background. By and large, they are robust plants with stout root stock and erect stems. Such features might make them good for erosion control, but the roots on at least one are said to grow horizontally, which would pose problems in hedges running across farms or forests.
VETIVER'S DISTANT RELATIVES
Apart from vetiver itself, lemongrass (Cymbopogon citratus) has probably been used in hedges against erosion more than any other grass species. It is planted on bunds for soil conservation as well as on hillsides and road cuts in Central America and elsewhere in the tropics.
It has a number of vetiver's vital features. For example, its large and strong stems can hold back soil, even when it is planted in a single line. It has wide adaptability and the capacity to survive where terrain is difficult and conditions terrible. It tillers strongly, forming large tussocks. Because it seldom flowers, it does not spread by seedlings.
All of these are features favorable for erosion-control hedges, of course. But in practice, lemongrass does not seem to work as well as vetiver. In Costa Rica, for instance, farmers who use both are slowly abandoning lemongrass. They say that compared with vetiver, it requires more labor. They have to replant their lemongrass hedges every four years or so, and separating the planting materials from the clumps is difficult. The hedges also are less dense, less resistant to stem borers, and more unruly. Whereas vetiver rows are regular, compact, and erect, lemongrass rows are irregular, patchy, and often extremely unkempt, with leaves hanging in all directions.
Like vetiver, lemongrass is widely distributed throughout the tropics. Its oil is one of the most important perfumery ingredients, widely used for scenting soaps, detergents, and other consumer products. People throughout Southeast and South Asia flavor popular drinks, soups, curries, and other foods with its leaves.
It seems possible that amongst the germplasm of this well-known grass, forms will be discovered that are well suited for use as hedges against erosion.
Citronella grass (Cymbopogon nardus) is another vetiver relative that has some of the vital properties for a barrier hedge. It, too, is a robust perennial. It, too, is cultivated for its essential oil and has thus been widely distributed throughout the tropics. And it, too, is sterile.
For all that, however, citronella grass has not been used for erosion control-as far as we know. Now it should be tried. The type that is cultivated, the so-called "Java type," may not be the best for the purpose. A "Sri Lanka type" that had been rejected because its oil is inferior is possibly more suitable. It is exceptionally robust and resilient.
Other members of the lemongrass and citronella genus might also work in erosion control. Indeed, about 60 Cymbopogon species are found in the tropics and subtropics of Africa and Asia. In general, they are coarse perennials, with aromatic leaves. None has been seriously assessed for erosion-hedge purposes, but in Senegal, C. schoenanthus is used more widely than vetiver for erosion control.
Vetiver's Close Relatives
Perhaps the most promising alternatives to vetiver will be found among its own wild relatives, discussed below.
This Australian grass is found around (and even within) freshwater lagoons, damp depressions, and rivers in the Northern Territory and Queensland. It also occurs in New Guinea.
Another Australian species, this plant is found in Queensland and New South Wales. It is characteristic of the vegetation often growing on riverbanks, sandbanks, riverine plains, high ground near creeks, Melaleuca swamps, dry creek beds, and depressions in open forests. It is said to be eaten freely by cattle.
This species is of special botanical interest because its morphology is intermediate between vetiver and lemongrass (see page 114). One accession was found to have a chromosome number (2n=40) twice that normally found in the genus.
Yet another Queensland species, this one also likes the sandy banks of channels, growing either in the open or in partial shade.
This, the fourth Australian species, is found in the Northern Territory and Queensland. It, too, occurs on sandbanks and riverbanks.
This Australian species has only recently been described.
Like vetiver itself, this species is from India. However, it is native to the southwestern region. It is, for example, common in the district around Dharwar in Karnataka State. The root stock is stout; the stem is erect, simple, and slender; and the internodes are very long. Such features might make the plant good for erosion control, but the root is said to grow horizontally, which would compete with nearby crops or trees.
This little-known species apparently occurs only in Mauritius and the neighboring island of Rodrigues in the Indian Ocean.
A West African member of the genus, this grass is found from Senegal to Cameroon, including Ghana, Togo, Mali, and Nigeria. It occurs mainly on flood plains and is a robust perennial that grows up to 2 m tall. Almost nothing has been reported about the plant, but it can be quite common. For instance, in Ghana it can be found on the plains between Accra and the Volta River.
Its roots and stems are (apparently) scented. People in Mali use the roots to perfume drinking water and the stems to weave mat-like wall panels called "seccos." Both roots and stems can be bought in the markets of Bamako, for example.
This grass is the main African species. It is found in most sub-Saharan areas from Senegal to Mozambique. It is a tufted perennial, as tall as 2.5 m. It flourishes on wet soils and grows in water or at least in wet, usually swampy, ground. It occurs, for example, on alluvial floodplains in Ghana, extending along seasonal streams and channels in areas of marsh grass and tree savanna. It also remains as a relict in the low, wet spots left unused in heavily farmed areas. It is said to tolerate slightly saline soils.
Although native to tropical Africa, this species apparently also occurs in scattered locations in Sri Lanka, Thailand, Malaysia, and the Philippines.
Yet a fourth distant relative of vetiver, gamba grass (Andropogon gayanus), is planted on bunds as an anti-erosion measure in parts of Africa. Farmers in Niger also use grassy hedges of this species for windbreaks.
An erect tufted perennial up to 2 m high, gamba grass is native to tropical Africa and has been introduced into other tropical areas, such as Brazil, India, and Queensland, Australia. It is adapted to many types of soil, can tolerate a long dry season, and has produced good results in northern regions of Nigeria and Ghana. It persists well under grazing but becomes unpalatable after it flowers.
Those are virtues for a vetiverlike hedge, but gamba grass has severe limitations. It is "clumpy," and gaps tend to form between the plants. Also, the individual clumps tend to die in the center. In addition, the plant spreads from seed and has shallow, horizontal roots.
There are about 100 other members of the genus Andropogon, and some of those may prove better than gamba grass as erosion-hedge plants. They tend to be big and brawny. Some are severe weeds, but sterile forms might be found or created by hybridization.
In parts of India grasses are commonly used for erosion control- not in the single-line hedge like vetiver but in blocks or bands on bunds and berms. Among the various species employed is Saccharum moonja. This weedy sugarcane relative is extensively used along the bunds and in gullies to check wind and water erosion in northern India. It is unpalatable to animals and is very hardy. This and other Saccharum species might be useful as eventual back-ups for vetiver.
The 12 known Tripsacum species all have stout stalks that might make them useful in hedges. Two are already showing promise.
Guatemala grass (T. laxum) is used to form bench terraces in the Nilgiri Hills of Tamil Nadu, India. It is said to check erosion on steep slopes very effectively.
In the United States, the U.S. Department of Agriculture is testing game grass (T. dactyloides) as a vegetative contour strip for field agriculture. This native, cold-tolerant species is already widely planted as a forage, and many different genotypes have been collected. These are now under evaluation for use in erosion hedges. Plants with upright habit and other desirable characteristics (notably, disease resistance) are being examined. An especially interesting possibility is the use of game grass as a perennial grain. Even the unimproved cultivars have yielded about 25 percent as much as wheat, without the annual costs of replanting or the erosion caused by tilling bare ground.
Sorghum and Its Relatives
Certain little-known types of sorghum have such strong stems that they are used as building materials and as living stakes to support climbing crops. They are so strong that West African farmers employ them to hold up even the massive weight of yam plants. Also, these sorghums are very resistant to rot-even when dead. For months after setting seed, they continue holding up heavy yam plants, despite the heat and humidity beneath the tentlike curtain of yam foliage.
The sorghums used this way today are annuals, but perennial sorghums are known, and to combine the strong stalks and perennial habit seems well within the realm of possibility.
TROPICAL GRASSES UNRELATED TO VETIVER
As noted in earlier chapters, skeptics commonly criticize vetiver on the grounds that it is not "useful" enough for farmers to want to plant it. Usually, these skeptics claim that farmers will only plant a species that could feed animals. Often, they propose napier grass as a better alternative.
Napier grass (Pennisetum purpureum) is native to tropical Africa but is now available throughout the tropics. It is a tall, clumped perennial that in some places is planted on bunds as a windbreak and to conserve the soil. Although the dry stems or canes are used for fencing and for house walls and ceilings, it is primarily grown for fodder.
Despite the fact that it is useful for erosion control, napier
grass (also known as elephant grass) cannot be used in a single line like
Its stalks are too weak and the gaps between them too wide for it to stem the onrush of soil and water after tropical deluges. Moreover, its shallow, spreading roots compete with any nearby crops.
It is likely, however, that certain genotypes (possibly those rejected by the forage developers because of coarseness, woodiness, and lack of palatability) might prove to be good erosion-hedge species. Also, there is a sterile dwarf form that might prove applicable.
Napier grass has been crossed with a wild relative (P. typhoides) to give a sterile triploid. This is said to be produce more fodder than its parents, but it might be even more useful as a hedge for erosion control.
Other Pennisetum Grasses
Even if napier grass never works out, perhaps some of the other 80 species in the genus Pennisetum might have the right combination of properties. These tend to be particularly well suited for the tropics and are already in use as food, fodder, and papermaking crops. A few have been used for erosion control (for example, moya grass, P. hohenackeri, which is used for controlling erosion in parts of India).
Rhodes grass (Chloris gayana) is sometimes proposed as a vetiver substitute. This native of southern Africa is widely cultivated for fodder. It comes in many forms, some of which withstand fires well. However, it spreads by stolons and by seed and can become a weed. It may be useful as a sod-forming species to cover and "clamp down" eroding sites, but as an erosion hedge it is unlikely to match vetiver.
Tropical Panic Grasses
In Kenya, a panic grass (Panicum sp.) called "kisosi" is successfully used as an erosion hedge on gentle slopes. The stalks are laid out flat in shallow ditches dug on the contour lines and covered with soil. The internodes (spaced 10 cm apart along the stalks) produce roots and shoots. Farmers regularly cut back the shoots for fodder, and this stimulates tittering and produces a living hedge sufficiently strong to hold back an accumulation of soil about 25 cm high. In this sense, the plant operates like vetiver. However, it is killed by fire and can be used only where burning is never practiced.
Calamagrostis argentea is being compared to vetiver in field tests near Grenoble, France. A related species, C. festuca, was used as an erosion-control grass by the Incas. They planted bands of it above their terraces, not only to protect soil but to spread out the runoff so it would trickle evenly down the often massive terrace systems.
Erosion is not confined to the tropics, of course. But, as we have noted earlier, vetiver is. In terms of the worldwide erosion problem, therefore, one of the greatest discoveries would be a temperate-zone counterpart to vetiver. Several possibilities are discussed below.
The U.S. Department of Agriculture is paying special attention to switch grass, Panicum virgatum. This native, warm-season grass is already widely planted in the eastern half of the United States as a fodder and revegetation plant, and many selected genotypes have been characterized in detail. Of particular interest is a variety called "Shelter," so-called because it was selected to enhance wildlife habitat during winter. Somewhat like vetiver in form, Shelter has stiff, erect culms that remain upright under the weight of snow and ice.
A "vetiver for the cool zones" might be found among the wheatgrasses (Agropyron and Elymus spp.). One possibility is stream-bank wheatgrass (A. riparium). This tall turf-type grass is relatively low-growing, but in Montana and other parts of the United States single or double rows of it are planted across fields to reduce soil erosion. These hedges capture the blowing snow and greatly increase crop yields. Farmers employ it mainly in semiarid areas where they want the winter snow to accumulate to build up enough soil moisture for the subsequent spring and summer crops. Another species used in the same way is A. elongatum, a clump-type grass that can grow to over a meter high. It also retains its stems throughout the cold Montana winters.
Strips of these and of some of the other 15 wheatgrasses have also been established across expanses of rangeland to conserve snowfall and soil as well as to provide cover and food for wildlife. They work well, but if not spaced properly, they may funnel the wind, worsen the erosion, and cause the crops to ripen unevenly across the fields.
Pampas grass (Cortaderia argentea) is a temperate-zone species from the southern cone of South America. It looks for all the world like vetiver. It may also function in the same way. However, it is fertile, tends to spread, and has become a serious weed in some areas.
Several related species, commonly called "toe toe," are native to New Zealand. Attractive ornamentals, they are often used to beautify homes both in New Zealand and in parts of the Northern Hemisphere (the San Francisco Bay area, for instance). However, these plants are fertile and seem likely to spread slowly in fields. Perhaps sterile hybrids could be found within, or created between, these various species.
The genus Stipa comprises 300 species, some of which might make useful hedge species. Native to either temperate or tropical areas, they tend to be long-lived perennials whose clumps can survive two decades or more. They tolerate burning and grazing and often grow in dry areas. Their stems are so woody and strong that they have been used for making paper, ropes, sails, and mats. They are not considered weedy or aggressive, although they can spread by seed. Generally, they are considered to be free of pests and diseases.
Some examples follow:
· Esparto grass (S. tenacissima). Mediterranean. Used
locally and exported to make fine paper; also used to make cordage, sails, and
· Ichu grass (S. ichu). South America to Mexico. A good fodder grass in arid areas.
· Black oat grass (S. avenacea). North America.
· Needle-and-thread grass (S. comata). North American prairies. An important pasture species, it is also used for revegetating mine spoils in arid parts of Wyoming.
· Porcupine grass (S. vaseyi). Western United States and Mexico. Stems of this species are so strong they are used to make brushes.
This grass (Achnatherum splendens) is native to Siberia and Central Asia, notably Inner Mongolia. It is a long-lived perennial reaching 2 m in height and surviving in a clump for up to 20 years. Although its native habitat is wet saline marshlands, it thrives in well-drained upland soils. However, it always seems to be confined to neutral or alkaline (pH 7-9.5) sites.
So far, this species has been used primarily as a fodder; however, it has been used in erosion control. Its flowering stems are so stiff and strong that brooms are made from them. Like vetiver, it has a long, tough, and fibrous rooting system, more vertical than lateral. Amazingly resilient, it is resistant to fire, drought, extreme cold (for instance, below -30°C), and even livestock. In addition to all that, it does not seem to spread (except by seed in marshlands) and is not invasive.
Of all the species for creating vetiver-like hedges in the cooler parts of the world, this seems the most promising at present.
Currently, Miscanthus sinensis is being tested by the U.S. Department of Agriculture as a temperate-zone counterpart to vetiver. This tall species, native to Japan and other parts of the Far East, is an attractive and increasingly popular ornamental. It looks much like vetiver and forms similar types of clumps, although they are neither as dense nor as strong. It appears to be sterile; however, the clumps spread slowly outwards, and this species is considered a pesky weed in parts of Japan.
It seems likely that among the 1,250 species of the woody grasses known as bamboos, good hedge species can be found or bred for both temperate and tropical regions. One species (Bambusa oldhamii) is already used in New Zealand. A local nurseryman, Dick Endt, points out that it performs impressively both in wind protection and erosion control. He writes: "One year in Kerikeri, I saw river-flooded land covered in silt and uprooted trees, yet the bamboo held. Not only that, but silt built up behind it, and did not kill it."
Giant Reed (Spanish Cane)
The "reed" of the Bible, Arundo donax, has been used for 5,000 years to make the vibrating tongues that give clarinets, organs, and other pipe instruments their voice. Its stems are so strong and fibrous that they are also used for walking sticks and fishing rods as well as for making rayon and paper. The plant has been used in erosion control in the United States and Guatemala. It would appear to have wider applicability as well. It is a perennial with stout stems mostly 2-6 m tall, growing from thick knotty rhizomes. It often occurs in dense colonies but makes poor forage and is only sparingly grazed or browsed, especially when anything else is available. Apparently, it produces some fertile seed.
Throughout much of Texas, this species is used in plantings along highways, culverts, stream banks, and ditches. It is considered extremely valuable for retarding erosion, and it also provides cover for wild birds and small animals.
Despite its qualities, great care must be taken when testing this species on new sites. Recently in California, it has become a severe weed that is almost impossible to control.
Phalaris arundinacea is used to shelter sheep in Australia. It is a clump grass that is reported to form hedges. A native of temperate Europe and Asia, it is unpalatable to livestock. Sheep, for example, will not graze it out.
SHRUBS AND TREES
As noted at the beginning of this chapter, the concept of using trees as hedges against erosion has been treated in an excellent book. However, here we present a few little-known favorites of our own.
Sea Buckthorn (Hippophae rhamnoides) is a small tree or shrub of sandy coasts and shingle banks in mountains from Europe to northern China. It is very adaptable, fast growing, and can take abuse. It is readily propagated both vegetatively (from cuttings) and from seed. Although not a legume, it is a nitrogen-fixer, and there are several reports that it is good for "restoring fertility" to degraded lands.
In southeastern Russia it has successfully stabilized gullies and ravines, for example. In Inner Mongolia it is used on soils containing carbonates, in loess, and in sand. In the Danube Delta it has also been used to stabilize dunes and to reclaim open-cast mining sites where the soils are heavy clays lacking organic matter. It is also excellent as a hedge to corral or control livestock.
Russians have been studying this plant for at least 50 years and have selected and developed more that 1,000 accessions-they have even hybridized it. In large measure, this has been for uses beyond erosion control. Almost everywhere, its berries are made into jellies; in France they are stewed into a sauce for meat and fish; whereas in Central Asia they are eaten with cheese and milk. The fruits are high in carotenes and vitamins, especially vitamin C. Annual production from good cultivars can run up to about 30 tons of fruit per hectare.
The prunings from the shrubs are a much appreciated fuel. They occur in abundance and have a high calorific value.
New Zealand foresters are using European alder (Aldus viridis) and some South American alders (A. jorullensis and A. acuminata) as hedges against erosion. These hardy, resilient, soil-improving trees have shown good growth on some dreadful sites-down to bare subsoil in several cases and on bare rock in others. This has been observed especially in the Craigieburn Range on the South Island, where the alders form healthy hedges that stabilize mountain screes (cascades of stones and rocky debris). These trees, too, fix nitrogen. Others among the 35 Alnus species are likely to be equally good for appropriate sites.
A companion volume already describes the value of this fast-growing nitrogen-fixing tree. Its use in hedgerows is increasingly common in tropical areas. Indeed, many people hearing about vetiver often claim that leucaena is better. However, we think that each species has its own place, and that on many sites they will perform exceptionally well together-the vetiver in the front blocking soil loss down-slope and the leucaena behind benefiting from the accumulated moisture and soil.
A Promising Alternative
J.J.P. van Wyk thinks he has a plant for erosion control that is even better than vetiver. It is an African grass called weeping lovegrass. This is not a big, rough-and-tough species like vetiver. It does not form thick, dense hedges that stand like sentinels across the slopes, blocking the passage of soil. Instead, weeping lovegrass (Eragrostis curvula) covers slopes with a perennial carpet of vegetation that acts en masse to protect the ground from rain and wind.
Weeping lovegrass is quite well known as a forage (in the dry Southwest of the United States, for instance), but van Wyk has searched through every nook and cranny of South Africa (the plant's native habitat) seeking out special genotypes with qualities for erosion control. The types he has found are truly remarkable- as amazing in their way as vetiver. They are, he says, "plants that don't know what good topsoil is. . tin fact, they don't know the difference between topsoil and no soil!"
Van Wyk has identified about 20 types that are proving excellent for reclamation. The South African government has adopted them for protecting roads, spoil dumps, and other eroding sites. The departments of Health, Water Affairs, Minerals. and Energy, and Agriculture all have initiated trials or field projects. The mining industry also got involved, and these love grasses are now stabilizing gold-mine spoil-material from a mile or more underground that is basically more sterile than the Sahara.
These are mat-forming grasses. They reduce runoff, increase infiltration, cool the land, and reduce temperature fluctuations. Some are cold tolerant and can withstand temperatures down to -10°F without getting frostbitten. Most are dwarf, erect, high seed-yielding types. Van Wyk has found types for use in the entire range of environments from 200 mm to 2,000 mm annual rainfall, from sea level to 3,000 m elevation, and from temperate to tropical areas, including both winter and summer rainfall patterns.
To keep up with the demand, van Wyk has established seven research farms and seven seed-producing farms. Weeping lovegrass stays in place and does its job for years, he says, but it is slow to establish. Van Wyk therefore uses tef (Eragrotis tef), an annual that is a close relative, to quickly generate grass cover. Tef acts as a mother crop for weeping lovegrass.
In some locations leucaena seems to have matched vetiver's soil-stopping abilities, but the shrubs had to be constantly cut back and maintained in the form of a thick, dense, contiguous hedge. In one trial in India where both were left almost untouched, soil losses through thick leucaena hedges were 15-16 tons per hectare, whereas the loss through (young) vetiver hedges was only 6 tons per hectare.
A decade ago, Thean Soo Tee began cultivating asparagus in the Mt. Kinabalu area of Sabah in his native Malaysia. He planted this Eurasian "shrub" some 1,200 m above sea level in hedges along the contours. The asparagus grew well and quickly on this irrigated land, maturing nine months after sowing. The plant's enormous root system held back the soil and saved the sloping fields from erosion.
Tee initiated this pioneering work years before the World Bank resuscitated the then moribund vetiver idea. His concern was to protect vegetable farms from erosion. With cabbage, peas, carrots, and other crops, the earth must be turned over after each harvest, exposing it to wind and water. Sabah's vegetable areas were turning into stone-strewn wastelands.
Tee recognized that because asparagus fetches a high price in the marketplace its cultivation would boost the income of local farmers. For his originality and endeavor, he earned a Rolex Award for Enterprise in 1984.
Siberian Pea Shrub (Caragana)
This rugged, dense, leguminous shrub (Caragana arborenscens) is used in low shelterbelts and for erosion control in some of the coldest, driest, and most desolate areas of the American Great Plains. A nitrogen-fixing species from Siberia, it grows almost anywhere, but is best adapted to sandy soils. It can be successfully grown where annual precipitation is no more than 350 mm and where winter temperatures plunge to -40°C.