'Weedy wasteland': Whanganui woman concerned by effects of riverbank bamboo spraying

Saph the dog waits while Rachel Rose investigates weeds by the Matarawa Stream. Photo / Bevan Conley

A "weedy wasteland" on the riverside where she walks her dogs is concerning a Whanganui woman.

But a Horizons Regional Council engineer says the weeds have to stay until the bamboo that threatens the integrity of river stopbanks is completely gone.

Rachel Rose and her partner take their dogs along the riverside near the James McGregor Arboretum, an off-lead dog exercise area, every day. She has watched as areas repeatedly sprayed to kill bamboo sprout thick layers of weeds - until they are resprayed.

The weeds that worry her most are convolvulus and tradescantia (wandering Jew). Both are invasive and hard to control.

"The running roots of the convolvulus will twine through the rootballs of young trees and shrubs, and the climbing tops will smother them," she said.

Tradescantia (wandering Jew) can cause itchy rashes for dogs. Photo / Bevan Conley

Tradescantia has an another attribute.

"It's well known for causing nasty, itchy rashes in dogs."

The council wanted to kill bamboo in two riverside areas, Horizons northern area engineer Wayne Spencer said.

Its invasive roots weaken stopbanks. When they rot and collapse, they leave little tunnels that water moves through.

Some of the bamboo areas were first cut and sprayed 10 years ago. In other places, where it does not threaten stopbanks, bamboo has been left because some people like it.

It was very hard to kill, Spencer said. It "comes back and back and back" and a cocktail of sprays had been tried.

"There's no way we would even contemplate any other plantings in there until the bamboo is gone."

Horizons doesn't control the weeds that grow after the bamboo is sprayed.

"Our focus is to make sure that our flood protection structures are safe and secure."

Any amenity planting would only happen when the bamboo was completely gone, and would be done by Horizons or Whanganui District Council, Spencer said.

Giant thistles and rampant convolvulus grow in an area where bamboo was sprayed. Photo / Bevan Conley

Rose thinks the sides of the stopbanks would be better planted with other trees and shrubs that would tolerate periodic flooding, protect banks from eroding, slow the current and trap sediment.

"I just don't understand why there seems to be no follow-up plan, and no one taking responsibility for what happens next," she said.