Discovery of invasive weed in Canterbury

by Anonymous Author

The invasive weed, great willowherb, or Epilobium hirsutum, has been found growing in several areas in Canterbury.

Great willowherb has not previously been recorded as present in New Zealand.

Biosecurity New Zealand (MPI) said the initial discoveries occurred in and around the lake at Pegasus township in the Waimakariri District, and near the Kate Valley landfill, located approximately 40 kilometres north of Lake Pegasus.

A field team has been searching all likely places in the wider area to determine the extent of the incursion. To date, great willowherb has been positively identified at five sites.

Great willowherb is characterised by its aggressive growth, and there is concern it may crowd out native wetland plants.

It can form dense stands, impeding water flow in waterways and wetlands. It may also spread to undisturbed damp areas and invade existing vegetation.

However, Biosecurity New Zealand said at present there is no risk to freshwater fish or agriculture.

Biosecurity New Zealand Response team manager, John Brightwell, said once the extent of the incursion is understood, officials will be in a better position to determine what response actions can be carried out.

“We’re in the early stages of determining the severity of this incursion,” Brightwell said.

“Once we’ve determined this, we’ll work closely with Environment Canterbury and the Department of Conservation to develop an appropriate response.” In the meantime, Biosecurity New Zealand is urging all Cantabrians especially to be on the lookout for great willowherb and to report it to Biosecurity New Zealand’s pest and diseases hotline if they think they’ve spotted it anywhere on public or private property.

“If you think you’ve seen great willowherb, don’t remove it,” Brightwell said.

“Take a close-up photo and call 0800 80 99 66 to report it. It’s extremely important that members of the public do not attempt to remove this weed on their own. They may mistake it for other, similar-looking native plants and, just as importantly, they may risk spreading the seeds.”

Great willowherb is a flowering plant also commonly known as the hairy willowherb or great hairy willowherb.

It is a highly invasive weed in parts of North America and in Victoria, Australia. It is closely related to and sometimes mistaken for a rare, endangered native herb (Epilobium hirtigerum) that goes by the common name of hairy willowherb. Overseas, great willowherb typically grows in wet or damp places without dense tree cover, up to 2,500 metres above sea-level. Common habitats include marshland, ditches and the banks of rivers and streams. At this time of year, most of the flowers will be gone, and the plant will be partially covered in seed.

Great willowherb reproduces by wind-dispersed seeds and spreads by its thick rhizomes (underground stems). It tends to spread most rapidly in early autumn.  The rhizomes can grow submerged in water or water-saturated soils, but can also spread into meadows and other upland areas.

Other characteristics of great willowherb:

  • grows up to two metres tall
  • stems are erect and branched
  • large, showy pink-purple flowers (3cm diameter) with white centres and notched petals
  • leaves are opposite, lance-shaped, tooth-edged, and attach directly on the stem
  • long, narrow seed pods split open to release numerous seeds with long white hairs