decided to give vetiver a try—it would be more of a survival test than an erosion-control experiment, he thought.

Accordingly, Materne grew vetiver slips in large pots in a greenhouse, and in the spring of 1990 planted them side by side on the bare and barren slopes immediately above each check dam. He wanted to establish the hedges quickly, and so he dropped a tablet of slow-release fertilizer beside each plant. His hope was that any hedges that formed might filter the turbid waters and thereby stop dirt from ever defiling the dams.

Because of the siting of the dams, some of the vetivers had to be planted into waterlogged soil (owing to a recent downpour, they were standing in water when Materne left the site). Others had to be planted into pure sand, described by Materne as "drier than popcorn." To make matters worse, a gully-washer barrelled through before the plants had a chance to establish deep roots. The speeding water knocked out some and scoured out the soil around others.

Despite all these hazards, however, most of the plants in each of the four sites, from the wettest to the driest, survived. Moreover, a few of them withstood yet another adversity when a freak fire swept through one of the plantings. It scorched and even killed surrounding pine trees, but the vetivers all survived.

In fact they did more than survive—they thrived. In 8 weeks some were almost 2 m tall. In 10 weeks they had grown together into hedges. And on one site, more than 20 cm of sediment had built up behind the thin green line of grass. Some plants held back such a load that after a storm they were temporarily bent over, almost hidden beneath a "sandbar."

By that time the hedges were so effectively filtering the runoff that the old flow of mud and silt down the streams was largely cut off. The check dams were receiving mostly clear water and were functioning as designed: temporarily holding back surplus runoff for later release into the streams.

When first hearing of Materne's proposal to plant vetiver, the local county agent vehemently disapproved, arguing that introducing an exotic plant to the watersheds might create an uncontrollable weed problem. But he was mollified—even overjoyed—when native grasses, wildflowers, shrubs, trees, and vines came crowding in behind the hedges and grew to revegetate the site. He even declared that nothing like it had been seen in the area before.

By that time it was clear that vetiver was acting as much more than an erosion trap; it was a "nurse plant" that was protecting other species and thereby giving these devastated watersheds a chance to heal themselves. Whether because of better soil moisture or the captured silt, the combination of hedges and revegetated slopes solved what had seemed an intractable erosion problem little more than a year before.

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