Vetiver Grass for Erosion Control and Land Stabilisation in the Wet Tropics
Vetiver Grass (Vetiveria zizanioides) was first introduced to the Wet Tropics of far north Queensland in 1988 when it was grown on the South Johnstone Research Station to build up stocks for use in other parts of Queensland. When enough material became available small plantings were established throughout the district in various locations to observe the growth habits and performance as an erosion control measure.
These locations were chosen so that the growth and erosion control potential could be observed on a variety of soil types with a range of physical, nutritional and drainage characteristics. Some of the locations included a railway cutting in alluvial soil, an old quarry site in gravel and some gully heads in Krasnozem soil.
During these first few years a large number of seed heads were picked and germination tests conducted to see if any fertile seed was produced. As no fertile seed was found the risk that vetiver would be a potential weed if it was used in the sugar industry was greatly reduced.
Use in the Sugar Industry
1. Contour Bank Replacement
In the wet tropics where sugar cane is grown on some fairly steep slopes the use of conventional soil conservation measures such as contour banks presents some safety hazards for machinery operators. The possibility of being able to replace an earthern contour bank with a vegetative one was very attractive.
In March 1992 the first planting was arranged on a cane farm to assess the compatibility of vetiver with sugar cane. A short contour bank was levelled with a back blade and the vertiver slips planted. Even though the cane was more than 2 metres high good growth was achieved in the vetiver plants.
National Landcare Program funding was applied for in 1993 and in October of that year two long rows of vetiver were planted in a plant cane crop as a replacement for contour banks. The land slope of the paddock ranged from about 10% to about 18%. These rows had a gradient along them as would the normal contour banks in sugar cane, but no waterway was constructed in the depression as would normally be done with a conventional soil conservation layout. Conditions were very dry and the vetiver had to be watered once a week for about a month before sufficient rainfall was received to ensure continued growth. The erosion suffered in the plant cane in this paddock during the first wet season was quite acceptable to the landholder but was more than optimal. The contribution made by the vetiver hedges was minimal because although the vetiver was reasonably well established when the wet season commenced the hedges were not thick enough to provide a proper barrier to runoff water. The timing of the vetiver planting operation is one of the problems yet to be resolved in sugar cane in the wet tropics. The length of time required for hedge development probably means that it can't be used in ploughout and replant situations.
2. Waterway Stabilisation
In the broken basalt country around Innisfail farmers have in the past used rock walls across depressions to trap silt. These are a problem for mechanical harvesting as well as being a rat and weed harbourage. The implementation of a conventional soil conservation layout would require the construction of grassed waterways in many of these depressions resulting in the loss of productive land. Most farmers are landlocked in this area and are not keen to lose any land as this means lost production. Stabilisation of waterways is also a problem and grass establishment is often poor.
In other situations the use of vetiver has encouraged the establishment of other stabilising vegetation. To see if stabilisation could be improved in waterways short rows of vetiver were planted across two waterways at about 1 metre vertical interval. In these waterways the vetiver acted as a silt trap but in some places the strips were outflanked. The hedges were also not thick enough to be as effective as they should have been.
As an alternative to the construction of waterways several short rows of vetiver were planted across a hollow in the plant cane crop. In an adjacent hollow the cane was planted through the hollow as is normal practice on many farms. This was designed to see if vetiver strips were more effective than sugar cane in controlling erosion in these depressions.
The results were inconclusive in the plant cane because again the hedges were not as dense as they could have been. In some places the flow was strong enough to almost flatten the vetiver hedge while in other sections the vetiver was outflanked.
3. Drain Batter Stabilisation
Two rows of vetiver were planted along the top of a section of a major drain where the batter was eroding from the runoff from the adjacent cane paddock. This planting took place in January 1993 and the soil was so hard that a crow bar had to be used to dig holes for the vetiver slips. Parts of the soil in the drain had the appearance of laterite. Fertility was low and growing conditions in general were very difficult. Even though a good strike was achieved and fertilizer applied the vetiver struggled for about the first year before achieving acceptable growth. It was noted on this drain that there was much better establishment of other vegetation on the side where the vetiver was planted.
4. Cooling Cascades
To try to achieve a reduction in the temperature of their waste water the South Johnstone Mill constructed a cooling cascade beside the South Jolinstone River. This consisted of a plastic and bidum lined channel approximately 200 metres long and about 10 metres wide with a series of rock filled drops to provide agitation and cooling to the waste water. The main structure was constructed out of local river bank and bed material and consisted of a silty loam containing a large percentage of river gravel and stone. The structure was constructed in a flood prone area which would be at least partially flooded on an annual basis.
Vetiver hedges were used as part of the stabilizing vegetation. The vetiver was planted in October and November of 1994. The area was very dry at the time of planting and supplementary irrigation was used for a time until the cascade became operative and provided enough seepage water to satisf~r the needs of the vetiver. The hedges were located along the edge of the containing bank of the cascade and at the foot of the batter and then about 1 metre vertically below the batter. Some vertical rows were established to provide resistance to flow along the batter. A mixture of millet and couch grass was used to provide cover between the rows of vetiver. Vetiver growth was quite good although there was severe competition from Guinea grass in some areas. Some weed and grass control was undertaken by the mill. The first flood occured at the end of February 1995 and there was no erosion on the site. In March 1996 there was a much bigger flood and again no erosion has resulted. The vetiver has trapped some silt on the area suggesting that there is a significant retarding effect on the velocity of the flood flows.
Other Areas of Interest
Recently there have been enquiries regarding the use of vetiver in river stabilisation by the local river improvement trust and local farmers. Vetiver is going to be used to provide some additional protection to a rock chute designed to prevent the South Johhstone River from cutting through the neck of a large loop.
There have also been quite a number of enquiries for various types of stabilisation ranging from gully heads to drain batters and creek and river banks. Often when told that there is no seed and that the vetiver will have to be planted by hand and is not going to provide an instant cover there is a loss of interest.
There are many areas of potential use for vetiver but there are some major drawbacks to the ready adoption of its use.
When vetiver is planted at the same time as sugar cane the growth tends to keep pace with the cane for quite a while and this results in fairly spindly growth because I suspect that the shade inhibits stooling. This means that when the cane is cut the vetiver should also be trimmed to encourage stooling. In one trial the cane harvester was run along the vetiver row and the vetiver successfully trimmed. This method probably has some potential in the sugar areas as the vetiver is given maximum time for growth before the shading effect from the ratoon cane becomes too great.
It would be important to ensure that the base cutter height was adjusted when cutting the vetiver as the level used to harvest the cane would probably kill the vetiver. Finding a planting time to fit in with the cultural practices presently used in sugar cane is a bit of a problem because the vetiver needs to be the most effective in the plant cane and this is the time when it is least effective using the present system. There has not yet been a crop of plant cane established in an area with a fully established vetiver hedge. Hopefully this will occur next planting season as the paddock with the best hedge in it should be ploughed out for replanting. The crop is either eighth or ninth ratoon this year.
Vetiver does not withstand strong weed competition. In the north there have been several plantings that have been well established and then overtaken and killed out by such grasses as Guinea grass (Panicum maximum) and Para grass (Brachiaria mutica). In the nursery at South Johnstone there is a problem with Signal grass (Brachiaria decumbens) taking over plantings if the management is not good.
It has not yet been proven that a vetiver hedge system is a suitable replacement for conventional contour banks in the steep sugar areas of the wet tropical coast of north Queensland. The planting method needs to be looked at with the aim of finding a system that will allow the establishment of a solid hedge in the least possible time. In the wet tropics this means a system which produces a solid hedge in time for the first wet season. Many of the potential uses involve works that need to be done fairly quickly because the planning and funding arrangements do not seem to allow sufficient lead time for good establishment. At present the cost of the planting material is fairly high and the planting method is very labour intensive and thus expensive also. Costs must be reduced significantly if the general farming community is going to be interested in using vetiver to its full potential.