Gunning for Gunnera

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Ross Nolly

The spectacular escapee known as Chilean rhubarb or giant rhubarb (Gunnera tinctoria or chilensis) is rapidly becoming a major weed problem on South Taranaki’s coastal cliff faces. Despite its common name, it is unrelated to the domestic rhubarb, although both have large leaves.Taranaki has the greatest density of wild Chilean rhu­barb in New Zealand, with approximately 40 kilometres of coastline from Pihama to Mokoia badly infested.

The plant is a native of South America and is found in the wilds of central and southern Chile, in the Argentine Andes and in Pat­agonia. It favours humid con­ditions and inhabits marshy lands, stream and river sides. It is extremely adaptable, and has naturalised in parts of northern Europe, the Azores, North America and Ireland. There are heavy infestations in the Irish counties of Mayo and Galway, where large stands of native vegetation have been smothered.

Department of Conserva­tion officer Jim Clarkson is escapee known as Chilean rhubarb or giant rhubarb (Gunnera tinctoria or chilensis) is rapidly becoming a major weed problem on South Taranaki’s coastal cliff faces. Despite its common name, it is unrelated to the domestic rhubarb, although both have large leaves.Taranaki has the greatest density of wild Chilean rhu­barb in New Zealand, with approximately 40 kilometres of coastline from Pihama to Mokoia badly infested.

The plant is a native of South America and is found in the wilds of central and southern Chile, in the Argentine Andes and in Pat­agonia. It favours humid con­ditions and inhabits marshy lands, stream and river sides. It is extremely adaptable, and has naturalised in parts of northern Europe, the Azores, North America and Ireland. There are heavy infestations in the Irish counties of Mayo and Galway, where large stands of native vegetation have been smothered.

Department of Conserva­tion officer Jim Clarkson is lobed and covered with small hairs, making them unpalat­able to stock.Up to five one-metre-tall flower stems appear on each plant between October and December. The stems are covered with small flowers and arise from the centre of the plant. The top part of the stem contains male flow­ers, the bottom has female flowers and those in the middle are hermaphrodite. The fruits ripen between

January and February, and each seed head can produce up to 80,000 sub-millime­tre-sized seeds. These seeds are spread by water and, to a lesser extent, by birds.Studies in Ireland have shown that only 0.1 per cent of the seeds germinate, and no more than 3 per cent of these over-winter successfully. It is not known whether these statistics have any relevance for New Zea­land conditions, but there is no doubt that Gunnera is aggressively colonising new areas here.“We think the seed stays viable for about two years, as it has taken two to three years to eradicate seed­lings on garden sites,” says Clarkson.

Once established, the plants are very invasive and form dense colonies that inhibit the growth of native plants. The huge leaves shade out almost all veg­etation, often leaving bare ground underneath. Over winter, the plants die down, lose most of their leaves and become semi-quiescent. In this state they are unaf­fected by frost.

Gunnera is a particularly hardy plant, able to survive just a few metres above the high tide line or perched on steep cliffs. Salt spray and howling westerly winds cause only a slight browning around the leaf edges.What can be done to stop the invader?Under a new strategy, Taranaki landowners will be required to control Chilean rhubarb on their properties as of July 1, 2003, but one of the hurdles in eradicating the pest is the terrain that it inhabits. Taranaki’s cliff faces are highly unstable, and continually crumble and collapse, making it difficult for weed-control workers to reach the plants.

“Last year we did a helicopter trial, broadcast spraying with glyphosate [the active ingredient in Roundup] at 1 and 1.5 per cent, but the results weren’t too flash,” says Clarkson. “Kill rates varied from 30 to 50 per cent.“This year we decided to use abseillers to cut off the leaf stalks and growing tips, and sprayed them with a 25 per cent mixture of glypho­sate.”The results of this year’s efforts won’t be known until next season, when resprout­ing growing points will be counted to determine the effectiveness of the control.

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