Science Centres: Aquatic Biodiversity and Biosecurity

Waikato weeds assessed

NIWA scientist Dr Paul Champion surveys a plot that was smothered with reed canary grass before treatment (inset).

NIWA has recently assessed the impacts and control of three weeds in Waikato wetlands for the Department of Conservation.

Royal fern is well established in the Lower Waikato/Hauraki Plains area, reaching heights of up to 2.5 m, with no current options for eradication. Our control trials found none of the herbicides used gave selective control of this fern, but a range of products could potentially be used where pure stands of this species grow. We found slashing is a safe method of controlling the weed, with no regrowth a year after treatment in our trials.

A re-survey of trial willow control plots in the Kopuatai Wetland Reserve showed little regeneration of willow three years after spraying with glyphosate herbicide. Control of willow had led to the invasion of the wetland grasses reed canary grass and reed sweet grass, both of which smothered other plants in willow treated areas.

Reed canary grass is an internationally renowned weed of wetlands, where it forms dense sprawling stands up to 1 m tall. It’s a major problem in Waikato wetlands, where it threatens the regeneration of kahikatea and other native species. Our trials with a grass-specific herbicide found that it reduced average reed canary grass cover from about 61% to less than 1%, while increasing the cover of native plants, especially sedges.