Pest water weed eradicated from South Island

Media contact: Lesley Patston

The Ministry for Primary Industries (MPI) today confirms that the invasive aquatic weed hornwort has been eradicated from the South Island.

“Hornwort is a significant threat to the ecology of fresh water ecosystems and can affect the functioning of hydro power generators and irrigation and drainage systems, with major economic consequences,” says MPI Senior Adviser Dr Liz Clayton.

“MPI made it a high priority to contain the hornwort and clear it from the South Island.”

MPI called on the National Institute of Water and Atmospheric Research (NIWA) to carry out the operation at Timaru’s Centennial Park Lake. NIWA had assisted with an earlier eradication of the plant from sites near Motueka.

Hornwort was found in the Timaru lake in 2006 and was treated with the aquatic herbicide endothall in 2008.

NIWA Aquatic Ecologist Rohan Wells says since that time his team has monitored Centennial Park Lake but there has been no re-appearance of hornwort in that time and MPI has now been able to declare the pest eradicated from the South Island.

“It was a difficult site and conventional methods of weed removal were not successful. We then tried the new herbicide endothall and only one treatment was needed to successfully eradicate it,” says Dr Wells.

Hornwort can grow to ten metres tall and is well established in North Island waterways. This does pose a risk of re-introduction to the South Island.

“For this reason, MPI is seeking pre-approval from South Island regional authorities to use endothall against hornwort, should it be found in their area in future,” Dr Clayton says.

Endothall is a very safe aquatic herbicide which is broken down naturally to carbon, hydrogen, oxygen and organic acids. Recent studies have shown it is safe to swim where the water has been treated and fish are safe to eat.

NIWA trials show it is very specific to certain targeted weeds and that native plant species present in the trial treatments are still flourishing one year later.

“However, prevention is better than cure and to stop the spread of freshwater pests people should always Check, Clean, Dry any equipment that has come into contact with river or lake water before moving to another waterway,” says Dr Clayton.

Photos to illustrate this story can be downloaded at:

Background Information

Fast facts about hornwort:

  • Scientific name: Ceratophyllum demersum.
  • Is an introduced plant and was first recorded in natural waters near Napier in 1961.
  • Can grow to a depth of 15 metres or more and up to 10 m tall.
  • Does not have roots but is well anchored to the lake or stream bottom via buried stems.
  • Is presently confined to the North Island only, with all five known South Island sites eradicated.

Facts about invasive aquatic weeds:

Over 70 freshwater aquatic plants introduced into New Zealand have now naturalised here and many have become problem weeds. Most New Zealand lakes, rivers and streams are affected by at least one of these species.

Invasive aquatic weeds smother native submerged plants, affecting food and shelter in the lake for fish and other wildlife, and often make recreational activities such as swimming and boating very difficult. Currently lake managers are fighting to control these types of weeds using a mixture of mechanical, chemical (herbicides) and biological control measures.  

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