Vetiver Grass

How vetiver holds a slope

Roots as reinforcement

What makes vetiver special lies underground. Unlike most grasses, which spread a fine mat of roots in the upper soil, vetiver drives a dense curtain of roots almost straight down. Field reports from Thailand and Australia describe depths of two to four metres within the first year; three metres is the most frequently cited value. That is a reference point, not a promise: in shallow, stony soil or over bedrock no root grows that deep.

These roots are also remarkably tough. In tensile tests Hengchaovanich and Nilaweera measured strengths of about 40 to 180 megapascals, depending on diameter. The thinner the root, the stronger it is per square millimetre. For the most common roots, 0.7 to 0.8 millimetres thick, they gave a mean of about 75 megapascals, often described as roughly a sixth of the strength of mild steel. Other researchers arrived at somewhat higher values with a different measuring method.

For a slope this means: the roots pass through the upper soil layer like reinforcing bars and pin it to the layers below. This acts mainly against shallow slips, the kind that tear away a terrace edge or the face of a road cutting after heavy rain. Against deep landslides with a failure surface many metres down, no grass can help.

The hedge on the contour

The second effect is above ground. Vetiver is not planted as single clumps but as a closed row along the contour line. The slips, each a few tillers split off from a clump, are set about 10 to 15 centimetres apart, often in a double row. Within one or two growing seasons the row closes into a stiff, dense hedge.

When water runs down the slope, the hedge does not stop it but brakes it. Water backs up, spreads along the row and seeps through slowly; the soil it carries settles in front of the hedge. Over the years a step builds up there, and on a slope with several rows a sequence of living terraces develops without anyone moving earth. The vertical distance between rows depends on slope, soil and rainfall; the Vetiver Network gives about two metres of height difference as the usual guide value.

This is where the method meets older techniques. A vetiver hedge works on the same principle as terracing or the contour lines of keyline design: it breaks a long slope into short sections so that water never gathers enough speed to carry soil away.

Sterile clone or seed-bearing wild form

No question about vetiver is more important than this one, and it has a clear answer. There are two groups. The wild types of northern India flower profusely, set viable seed and can spread on their own; the documented cases in which vetiver became a weed relate to these fertile types. The cultivated types of southern India, on the other hand, flower late, produce largely sterile pollen and rarely set seed. They are propagated only by dividing clumps.

A DNA study by Adams and colleagues in 1998 showed that almost all vetiver planted outside South Asia for erosion control belongs to a single clone, known as Sunshine, from this sterile cultivated group. The Australian cultivar Monto is of the same type. A weed risk assessment for the Pacific islands rated these sterile cultivars as low risk. "Largely sterile" does not mean absolutely sterile: Indian trials reported that south Indian plants grown under northern Indian conditions did set some seed that germinated.

Vetiver is not on the EU list of invasive alien species of Union concern, in the version applicable since August 2025. That answers the question of the list, not the practical one. For any planting in Europe, the origin of the material is what counts: only certified sterile clones, propagated vegetatively, and never seed. The 1998 study also points to a different risk: because nearly the whole world plants one single clone, a pest or disease that attacks it could affect hedges everywhere at once.

Frost, drought and the Sardinian climate

Vetiver is a tropical grass and grows in warmth. Frost kills the leaves, which turn purple and dry out, while the growing points underground survive. The Vetiver Network reports that plants in Australia survived ground frosts of minus 14 degrees, and in Georgia in the United States plants survived soil temperatures of minus 10 degrees but not minus 15. The plant dies when the ground around its base freezes through. Root growth was observed down to 13 degrees and is estimated to stop at around 5 degrees.

Frost of that kind is rare on Sardinia's coast and lower slopes. The real question for a Mediterranean island lies elsewhere. Here the rain falls in the cool half of the year, exactly when vetiver barely grows, and the warm half of the year is dry. Established plants tolerate long drought well, but young slips need moisture to root, so a planting would require water through its first summer. Whether a hedge closes quickly enough under these conditions, and how it behaves during the winter rains when the leaves are dormant, has not been investigated for Sa Medada. It would have to be tested in a small trial before anything larger.

How Terra Humana sees it

We include vetiver in the TerraWiki because it is one of the best-researched plant tools for holding slopes, and because it shows how much a single living hedge on the contour line can achieve against erosion.

For Sa Medada it would be an option to examine, with clear conditions: only sterile clones, a small trial first, and an honest look at whether a tropical grass fits a dry Mediterranean summer, or whether native shrubs and the methods of soil bioengineering do the same job with less effort. We follow the principle we apply to all vigorous pioneer plants: use their strength deliberately and in a controlled way, never let them run unchecked.