Vetiver Grass Technology (VGT) is perhaps the world’s most versatile nature-based technology for improving soil and water and mitigating related problems. Its prime component is vetiver grass.

Vetiver is a tropical grass of the genus Chrysopogon (30 identified species). Two species, both called vetiver, matter to VGT:

• Chrysopogon zizanioides (L.) Roberty, formerly classified as Vetiveria zizanioides (L.) Nash, with its center of origin in south Asia; and
• Chrysopogon nigritanus (L.) Roberty, formerly Vetiveria nigritana (L.) Nash, known as black vetiver, originating in Africa.

The two species are rather similar. VGT, and the 40 years of research behind it, uses primarily Chrysopogon zizanioides, with a strong preference for the domesticated sterile cultivars from south India that have spread to most tropical and semi-arid countries.

Chrysopogon zizanioides (L.) Roberty is robust and complex, with many different applications relating to soil and water. Vetiver grass is a densely tufted, awnless, wiry, glabrous perennial grass that is a “shy breeder” outside its natural habitats (e.g., swamplands) in South Asia. Genetic testing has shown that it is the domesticated, sterile cultivars that are found today throughout tropical, sub-tropical and Mediterranean countries and regions.

It has no rhizomes or stolons and is propagated by root division. The plant grows in large clumps from a much-branched “spongy” rootstock, with erect culms 0.5–1.5 meters high. The leaf blades are relatively stiff, long and narrow (up to 75 centimeters long and no more than 8 millimeters wide) and, although glabrous, are “downward rough” along the edges. The lower glume is muriculated. The panicle is 15–40 centimeters long; joints and pedicels glabrous. Spikelets are narrow, acute, appressed and awnless. One spikelet is sessile, hermaphrodite and somewhat flattened laterally with short sharp spines; it has a glabrous callus, three stamens and two plumose stigmas. The other spikelet is pedicelled and staminate. Some cultivated forms rarely flower. Chromosomes: base number x = 5 or 10; the plant is typically diploid (2n = 20).

Vetiver roots are unique and complex, with cells that can modify to adapt to either waterlogged or dry conditions. They also have a special relationship with soil microbes and the exchange of soil nutrients. Paul Truong’s book “The Hidden Half” details these characteristics and shows how the roots influence the success of vetiver applications.

The root structure is critical for bio-engineering, giving soil a significant increase in shear strength. The roots also interact with soil micro-organisms, improving soil health, nutrient recycling, water infiltration and more.

• Vertical roots grow almost straight down, with less aggressive lateral roots (neither stolons nor rhizomes), a crucial trait that keeps the roots within a zone close to the point of planting.
• Deep anchoring: the root system is extensive, dense and deep:1–2 m in the first year and 3–5 m by years two to three where soils are deep and the hedge is well managed, with depths of up to 6 m reported. Roots are shallower in hard, shallow or very dry soils, and where hedges are overgrazed, cut hard in the dry season or watered only lightly.
• The root wall, a hydraulic engine: in a hedge, the roots form a continuous wall of vertical macropores that draws water down into the profile, works as a biological drain, and lowers pore pressure on slopes. See Vetiver and Water.
• Structural strength: in a dense hedgerow the roots form an underground curtain. Their tensile strength of 40–120 MPa (on average about 75 MPa, roughly one-sixth that of mild steel) and they can increase soil shear strength by up to 45%.
• Stopping soil loss: this deep vertical architecture can reduce the deep mass loss of soil associated with some landslips.

Growing interest in VGT in sub-Saharan Africa makes it more likely that C. nigritanus will be used for VGT applications under the umbrella of the Vetiver System (VS). Black vetiver originates in the Zambezi and Niger basins and is found in most of their tributaries, flood plains and swamps. It is already used in Malawi, Nigeria, Niger, Senegal and Mali for various purposes, including soil and water conservation.

A 2014 master’s thesis by Diedericks, Genetic diversity of Vetiver clones (Chrysopogon zizanioides and Chrysopogon nigritana) available in South Africa based on sequencing analyses and anatomical structure, found the two species indistinguishable using the limited set of markers in the study. It notes that Veldkamp (1999) treats them as separate species but found them morphologically very similar. There is, however, an important practical difference: C. nigritanus seeds freely, so it should only be used where it is endemic.