Vegetative Soil Stabilization - a General Guide

Saturday, November 1, 2008

As seen in the picture from Thailand at left, the basic technique of soil stabilization using Vetiver consists of one or more hedgerows planted on the contour. Nursery plants or slips (clumps) of about 3 tillers each, are typically planted 6 inches (15 cm) apart on the contour to create, when mature, a barrier of stiff grass that acts as a buffer and spreader of down slope water flow, and a filter to sediment. Multiple hedgerows may be required to stabilize a slope.

Vetiver grass does not have stolons or rhizomes. Its massive, finely structured root system can grow very fast - in some applications, rooting depth can reach 10-12 ft (3-4 m) in the first year. This deep root system makes the Vetiver plant extremely drought tolerant and difficult to dislodge by strong current. It also has stiff and erect stems, which can stand up to relatively deep water flow. New shoots will develop from the underground crown, making Vetiver resistant to fire, frosts, traffic and heavy grazing pressure. Vetiver Grass is not affected to any significant extent by pests and diseases, nor does it act as a host for pests or diseases that might attack crop or garden plants.

The development of strong plants and a deep root system requires full sun. Partial shading stunts its growth, and significant shading can eliminate it in the long term by reducing its ability to compete with more shade-tolerant species.

When multiple hedgerows are required, each row must be planted with the same separation of 6 inches
(15 cm) between plants. The separation between rows will depend on the slope, soil condition and composition, and the severity of the problem. As a rule of thumb, typical distances in domestic environment should range between three and six feet. Some published guidelines recommend a distance between rows of about 5.7 ft. (1.7 m) for a 30° slope, and about 3 ft. (1 m) for a 45° slope.

A good hedge will reduce rainfall run off by as much as 70% and sediment by as much as 90%. A hedgerow will stay where it is planted and the sediment that is spread out behind the hedgerow gradually accumulates to form a long lasting terrace with Vetiver protection. It is a very low cost, labor intensive technology with very high benefit/cost ratio. When used for civil works protection, its cost is about 1/20 of traditional engineered systems and designs. Engineers compare the Vetiver root to a "Living Soil Nail" with an average tensile strength of 1/6 of mild steel. Vetiver hedges are fully effective only when plants form closed hedgerows. Gaps between clumps should be timely re-planted.

A very important characteristic of the "domesticated" Vetiver varieties from southern India used in The Vetiver System is that it does not produce seed and stays where it was planted. In some countries Vetiver has even been used to define property lines and hedgerows will not invade other areas of the property. Vetiver hedges are a natural, soft bioengineering technique - an eco-friendly alternative to rigid or hard structures. Being vegetative, it is also visually acceptable in any neighborhood.
A very comprehensive publication from The Vetiver Network International can be found here: Vetiver System for erosion and sediment control

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