Whanganui’s Lake Wiritoa sprayed with herbicide to tackle pest plant hornwort

If left unchecked, hornwort could make Lake Wiritoa unusable for the public. Photo / Bevan Conley

Lake Wiritoa was sprayed with herbicide last month in an effort to tackle the ongoing problem of hornwort.

Horizons Regional Council biodiversity, biosecurity and partnerships manager Craig Davey said it was designated as a pest plant and needed to be “progressively contained”.

“It doesn’t arrive on its own, it needs to spread by people.

“This happens inadvertently. No one in their right mind would chuck hornwort into a lake.”

Davey said the plant had a very simple survival mechanism which allowed it to be spread between lakes when carried on fishing equipment, boats, trailers and even dogs.

“One tiny fragment can partially dry out during the week, then next time you go to another water body it’ll rehydrate and float off.

“That fragment clones itself - bang, bang, bang.

“It has the ability to massively dominate a lake. It will grow from the beach all the way down to 14 metres, down to the deep dark depths.”

Whanganui District Council carried out the spraying on November 28.

Those who lived within 1km of Lake Wiritoa and used or drew water from the lake were advised not to bathe in it, use it for human consumption or use it for fish farming or livestock watering for 24 hours afterwards.

Swimming in the lake was prohibited for 24 hours.

Hornwort in a coastal dune lake in Santoft Forest, to the west of Bulls. Photo / NZME

The toxicity of the herbicide used, Reglone, was targeted at plants, Davey said.

“It’s doing the right thing in the right place for the right reasons.”

A different plan of attack would be introducing grass carp to the lake.

“They start small and get massive. They will eat hornwort but it’s not their preferred food,” Davey said.

“Hornwort is sort of like the parsnips and brussel sprouts on your plate. Grass carp will eat all the natives first.”

An example of the damage hornwort can do is at Lake Rotoaira near Taupō.

“The Tokaanu Power Station is fed by water from it and they have had to install a $5 million weed screen to run through large chunks of the year, particularly in the warmer months,” Davey said.

“There was 30 hectares of water area completely covered in hornwort. When that pushes against the screen it can break it because of the sheer volume.

“It’s a one-way street, you can only get more and more.”

Davey said ducks could bounce along certain coastal lakes in the Horizons region.

Horizons runs a “check, clean, dry” campaign to alert the public to the dangers of spreading the plant.

“There is signage and giveaways to try to drum in a behaviour change,” Davey said.

Horizons Regional Council biodiversity, biosecurity and partnerships manager Craig Davey. Photo / Supplied

Lakelands Holiday Park is next door to Lake Wiritoa.

Owner Daryl Dowman said if left unchecked, hornwort would “take over the place”.

“It will make the lake unswimmable, unboatable and unusable in all facets.

“The council generally sprays it annually to keep things in check.

“Waka ama train on here and you’ve got the waterski club as well. A lot of boat users have enjoyed the facility for decades.”

Dowman said he and his wife Jackie Dowman had owned Lakelands for the past four years.

In that time the water quality of Lake Wiritoa had been great.

“It’s checked by Horizons once a week during the summer season.

“The work both councils have done has meant the lake is in good condition. It’s very user-friendly.”

Hornwort had been in Lake Wiritoa since 1997 and spread to nearby Pauri and Kohata, Davey said.

It can also be found in lakes Herengawe and Mangawhio near Waverley.

“It only got there because someone has come from a hornwort lake with dirty gear.

“We need to look after our places and every action we make can be potentially positive or negative.

“Let’s make it positive, and give our kids the ability to do what we used to do - go jump in a lake and swim.”