Tongariro Alpine Crossing’s Emerald Lakes are weed-free again

The Emerald Lakes after being cleared of the invasive Juncus bulbosus weeds on the Tongariro Alpine Crossing.

A weed that was smothering the shorelines of the Emerald Lakes on the Tongariro Alpine Crossing has been exterminated.

The distinctive lakes were named for their jewel-like colours caused by dissolved minerals, but the shorelines and shallows have also been home to the invasive Juncus bulbosus for some years.

A type of rush, it was native to Eurasia and North Africa and showed up in New Zealand more than a century ago.

It is commonly found in swampy places, especially in areas with high rainfall, such as Tongariro.

Department of Conservation supervisor Danial Van der Lubbe said the lakes were an iconic part of the landscape.

“We know they are important to so many people – hapū, local communities and those who complete the Tongariro Alpine Crossing – so it’s for more than just biodiversity that we’ve tackled these weeds.”

The invasive Juncus bulbosus weeds growing on the shore and shallows of the Emerald Lakes, on the Tongariro Alpine Crossing.

Twice a year since 2019, rangers have conducted weed control around the shorelines and in the lakes.

This year, for the first time, Juncus density was down to undetectable levels, which Van der Lubbe said was a positive milestone for the health of a site considered internationally significant.

“We still have some work to go around the edges and will keep checking to make sure it doesn’t pop up again in the lakes themselves.

“The next big step for us is eDNA testing of the larger Blue Lakes for Juncus – we haven’t detected anything visually, but the eDNA check will tell us for sure.”

The Blue Lakes are about 1km north of the Emerald Lakes.

The eDNA testing was being conducted as part of the larger Tongariro Alpine Crossing sustainability project, which included a range of changes to better manage environmental and cultural outcomes.

Van der Lubbe said DoC and Ngāti Hikairo ki Tongariro wanted to ensure the experience of walking the Tongariro Alpine Crossing was safe, protected the fragile environment and respected the cultural significance of the area.

Tongariro water bodies are considered tapu, or sacred, and walkers are asked not to swim in them. By staying clear of the water, people will also prevent further weed incursions into the lakes.