'Weed mat' for lagarosiphon control promising

Ecologist Scott Hooson from the environmental planning agency Boffa Miskell inspects a hessian mat (left of picture) on the lakebed in Lake Wanaka's Paddock Bay. The matting has proved successful at suppressing the growth of lagarosiphon by starving the invasive weed of sunlight. Photo / Supplied

Slowly but surely progress is being made on eradicating the lakeweed lagarosiphon from Lake Wanaka, with about two-thirds of the lake now clear of the invasive weed.

A new method of controlling the weed, using hessian mats to starve the weeds of sunlight, has also proved effective in removing it from parts of the lake.

Lake Wanaka aquatic weed project manager Marcus Girvan said there had been a long history of fighting the weed in Lake Wanaka and more gains were made recently by the introduction of the hessian mats.

"It's a bit like what you would put down in your garden for a weed mat; we're pinning that down on the lakebed to shade out the lagarosiphon and it's been working really well."

A trial of the mat method in Lake Wanaka's Paddock Bay last year proved so successful it was expanded to other parts of the lake as well as Lake Aviemore and the Kawarau River.

It took about five months for the weed to die off after the mat was placed but the big benefit was native plant species were able to grow through it, Mr Girvan said.

"Once you remove those exotic plants you want the natives to start replacing them as soon as possible and that's what we're seeing."

When combined with the usual control method of herbicide spray the mats were very effective at stopping the weed growing back.

While Lake Wanaka was nowhere near being free of lagarosiphon, major gains against the weed had been made in isolated parts of the lake such as Glendhu Bay and Paddock Bay.

"In more isolated cases, large parts of Glendhu Bay are clear of the weed now and by this time next year we hope to be clear of it in Paddock Bay, which is a huge effort."

About $380,000 had been put towards the management of lagarosiphon in Lake Wanaka through a number of different agencies but more money was always needed, he said.

"Some years we get a little more and others a little less. We could always do with more but we are restricted by the technologies available to us."

Queenstown Lakes deputy mayor Calum MacLeod said the eradication effort against the invasive weed was a "good news story" and showed what could be done when local and national groups worked together.

"This is in no way going to happen overnight but it does show what can be done and possibly gives some hope in the fight against other problems like lake snow."