Whanganui’s Lake Wiritoa treated for hornwort, swimming restricted

The swimming area and water-skiing lane will be sprayed with herbicide. Photo / NZME

Lake Wiritoa near Whanganui will be treated with herbicide to tackle the invasive hornwort plant.

Whanganui District Council contractors undertake the treatment every two years, with the next round on Thursday, January 23, at the lake’s public swimming area and water skiing lane.

A council spokesperson said for 24 hours afterwards there should be no swimming in the lake, using or drawing water for human or animal consumption, or fish farming.

“For 10 days, avoid using the water for overhead irrigation,” they said.

“Warning signs will also be placed at the public swimming area gate and the water ski club for visibility and affected properties have been advised.”

The council uses Reglone (Diquat) herbicide, which is applied as a gel and sinks to target the weeds.

The spokesperson said it was safe for fish and most other aquatic life, and broke down quickly in water.

Speaking to the Chronicle in 2022, Horizons Regional Council biodiversity, biosecurity and partnerships manager Craig Davey said hornwort had a simple survival mechanism, allowing it to be spread between lakes when carried on fishing equipment, boats, trailers and even dogs.

The plant could “massively dominate a lake”, he said.

“It will grow from the beach all the way down to 14 metres, down to the deep dark depths.”

High toxic algae levels caused the lake to be closed for swimming for much of December and the start of January.

The closure was lifted on January 9 after the alert level for toxic algae dropped from red to amber.

Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.