PIJ #54, March – May 1995
Soil erosion is perhaps the world’s most chronic environmental problem that is literally costing the earth. The soil it carries off now totals 20 billion tons a year and this loss is not only severely degrading the environment, it is eroding the economic viability of countries. Despite enormous effort, standard soil conservation methods have been largely unsuccessful. However, a remarkable tropical grass may hold the key to a cheap, practical solution for controlling soil erosion on a huge scale in tropical and semi-arid regions. It also has many attributes that make it useful to farmers.
Vetiver (Vetiveria zizanoides) is a densely tufted, perennial clump grass with stiff leaf bases which overlap.
It forms narrow, dense hedges when planted along the contours of sloping land, slowing down run-off and helping the water soak into the soil rather than washing off the slope. The stiff foliage also blocks the passage of soil and debris which gradually builds up a soil terrace.
This deeply rooted, persistent grass has restrained erodable soils for decades in India, the Caribbean and in Fiji, where its use was discovered by John Greenfield in the late 1950s.
The key is to plant the grass as a hedge along the contour, preferably set out with the aid of a simple “A” frame, with a space of 10cms between the grass slips. Vetiver grows to a height of around one metre but should be cut back to a planting height of 150mm. The thatch can then be placed behind the newly planted slips to provide an instant filter to control run-off. We did this with amazing results on an urban-fringe forestry programme to control erosion on bare, decomposed granite soil on the hills above Hong Kong. The night after planting saw a very heavy tropical rainstorm. We went next morning and found that two inches of soil had been held back behind the intact grass hedge.
Vetiver is a live system, rather than engineered (bulldozers, graders, bunds, contour drains), which grows with the deposition of sediment. In Fiji, vetiver hedges which have been in position on 20 percent slopes for thirty years have built up terraces two metres high. Experimental plots in Colombia showed significant soil loss reductions using vetiver hedges, where in one year, soil losses were reduced to 1.4 tons per hectare compared to 143 tons per hectare for a bare fallow control.
Best results arise from a two metre vertical interval between hedges, but this depends on the degree of slope and the friability of the soil. On our friable soils in Hong Kong a double staggered row had the best effect. The edges of unsealed roads and drainage ditches can also be protected by vetiver.
Because of their basal density, vetiver hedges are far more effective in controlling erosion than lemongrass or hedges of trees or shrubs such as Leucanea.
Vetiver is a native of northern India and southern China, growing where the annual rainfall is more than 300mm, or 600mm where there is a six month dry season. It can also survive more than a month of submergence. Its growth is limited by frost. Vetiver grows in soils from pH 4.5 in Ethiopia and China to pH 10.5 in India and in saline soils up to an electrical conductivity of 4.0.
Minimal Space Requirements
Vetiver has an upright growth habit, although hedges perform best when kept 500mm high and wide. This means that vetiver hedges can be introduced on farms with minimum changes to the existing farm layout. Because vetiver roots grow vertically for at least three metres, not only do they bind the soil, but they do not compete with neighbouring crops for water and nutrients, unlike agroforestry alley cropping systems. Vetiver can be planted along the edge of existing terraces to reinforce the banks against collapse. The oil in vetiver roots also appears to be a deterrent to burrowing rats.
Low Maintenance and Many Uses
An annual trim is all that is needed to keep a vetiver hedge in good shape. It is important to keep the grass from flowering otherwise the stem will die back, which inhibits tillering and slows the growth of the clump (although the clump grows from the plant’s centre so there is no disintegration). The cut material makes excellent thatch or mulch for trees. It may be used as an animal feed supplement and can be fed with a high protein fodder such as Amaranthus, while the Chinese feed it to grass carp. Vetiver also regrows rapidly after fire, although it is very fire-resistant when green and may be used as fire breaks.
Ease of Propagation
Most varieties of vetiver are naturally sterile hybrids and do not set seed, nor does vetiver produce stolons, so there is no danger of the grass spreading from where it is planted. Propagation could not be simpler. Large clumps are split up to give around five to six slips which may be planted bare rooted with a little slow release fertiliser in the same way as forestry seedlings. If a nursery bed is sited on a sandy soil the digging up of the stock plants will be easier. Gaps in an existing hedge may be filled by layering a flowering stem which will root out from the nodes. The grass can also be propagated from nodal cuttings for growing in polythene tubes as is done with forestry seedlings.
Where’s the Catch?
If vetiver is so wonderful, what is the catch? So far there doesn’t seem to be one despite much research.
In parts of India, vetiver hedges have been in place for two hundred years. The grass is non-invasive, does not appear to have any significant pests or diseases and does not harbour vermin. The denseness of the root and basal leaves also forms a barrier to the spread of invasive stoloniferous grasses such as couch and kikuyu and can be used as a border around gardens. We have found that tree planting by itself does not prevent soil erosion for some time, but by planting vetiver grass we are able to have an immediate impact on erosion. The other important function of vetiver is that it forms a windbreak to slow the force of winds sweeping up the bare hillside, so that native grass and herb seeds are not blown away and have a rapid growth of native ground covers that provides the second line of defence against erosion. The trees provide the third line. After about the fifth year, our trees close canopy and the vetiver is affected by shade. However, after thinning the trees or after a fire the vetiver rapidly grows again to keep erosion in check.
There are five other vetiver species which are native to Australia, including Vetivera filipes, usually found in damp areas and on river banks in Queensland and New South Wales. Vetivera nigritana is the main African species and is found from Senegal to Mozambique. It would be interesting for someone to carry out trials of these native types to see if they perform as well as V. Zizanoides in their home areas.
Ideal for Keyline
Vetiver grows with the land, and so appeals to those who actually use the land, the farmers and foresters who are more concerned with increased production from crops and trees than stopping erosion itself. Vetiver is ideal for use in Keyline systems.
Given the vast areas of bare, eroded hillsides in the tropics, or those areas of south east Asia that are covered with unproductive Imperata grass, in which vetiver grows with no problem, I see great potential for its use. I am convinced that the combination of vetiver with Acacia mangium or Peltophorum pterocarpum, which can shade out Imperata, can bring these lands into effective production and put forests back onto the hills.
Extra Design Notes: If there is the possibility of zero annual rainfall, don’t use vetiver alone on contour banks as the roots could die and collapse the soil. Vetiver grows better in semi-arid regions if cut regularly and should be planted in summer, and watered until established, otherwise it will struggle.