Twenty years of dedicated effort and community support is finally paying off in the fight to clear Lake Wānaka of the
invasive lakeweed lagorosiphon.
This photo of Paddock Bay taken in 2005 shows dense weed beds of lagarosiphon reaching the surface. Image credit NIWA
Lagarosiphon is one of New Zealand’s worst submerged weeds and it was first recorded in Lake Wānaka in the 1970s. It can
form tall dense surface reaching weed beds that can interfere with recreational activities and smother out native
plants.
Clearing Lake Wānaka of the invasive weed, lagarosiphon is the ultimate goal of agencies of the Lake Wānaka Lagarosiphon
Management Committee led by Toitū Te Whenua Land Information New Zealand (LINZ).
This autumn, the management committee’s control programme is on track to remove weed beds remaining in the western part
of the lake, which stretches over 13 kilometres of shoreline. This would allow the containment line to be moved further
south to include the Paddock and Parkins Bays area, a long-awaited milestone.
“The containment line represents the divide between cleared sections of the lake, which we continue to monitor, and
where we need to actively fight lagarosiphon,” says LINZ Biosecurity Manager Tracey Burton.
“The containment line in Lake Wānaka currently stretches from the southern point of the peninsula to the southern point
of Bishops Bay. The idea is to progressively push the line further down the lake towards Wānaka township and eventually
eradicate lagarosiphon from the lake.
“Clearing Lake Wānaka is a priority for us, as it will help prevent the pest weed from spreading to other waterways in
the region. If we can shut off the flow of weed entering through these pathways, we can move to clear downstream
waterways too,” says Ms Burton.
NIWA Freshwater Ecologist Mary de Winton has been engaged by the management committee to independently inspect the weed
control works in Lake Wānaka for over 10 years.
Over the past few years, Ms de Winton says she has seen “amazing progress” clearing weed beds and preventing reinvasion
of cleared areas – particularly in Glendhu and Paddock Bays.
“It’s really exciting seeing native plants returning to the cleared areas and providing great habitat for other lake
life,” she says.
Ms de Winton says that the innovative use of hessian matting has been a gamechanger for eradicating lagarosiphon.
“Hessian matting was first trialed in New Zealand in 2016 at Lake Wānaka by LINZ’s biosecurity partner Boffa Miskell.
The matting is biodegradable, so it disappears after it’s done the job of controlling the invasive lake weed. Best of
all, native plant regrowth is fast because native seedlings can grow through the weave of the hessian.
“One super hard area to control was Paddock Bay. Lagarosiphon grew really fast here. Aquatic herbicide was used
successfully to reduce the amount of weed. But it was hessian matting that finally smothered the weed beds. Now divers
are removing the last scattered plants by hand.”
Jeff Donaldson from the Guardians of Lake Wānaka says the group is very supportive of the control programme led by LINZ.
“The development of hessian matting has allowed us to not just control the weed but eradicate it from areas throughout
the western bays including Glendhu Bay - which is a very high use recreational area,” he says.
“Recreational activities like swimming and boating, in Paddock Bay have also been able to increase in recent years as
the bay has been so much clearer of weed.”
Lagarosiphon was introduced to New Zealand as an aquarium plant, but it has spread rapidly throughout the country’s
waterways. A fragment as small as a fingernail is enough to start an infestation.
Lagarosiphon can grow several metres tall, disrupting activities like swimming, fishing and boating and smothering
native ecosystems.
“The Wānaka community and lake users are playing their part too - checking that their boats and other gear are free of
plant fragments to prevent the weed from spreading” says Ms Burton.
“Boaties have also been asked to take care around areas where hessian has been laid to prevent damaging it.
“It is important lake users continue to Check Clean Dry to ensure pest species, like lagarosiphon and didymo don’t hitch
a ride on their gear.”
Please remember to:
Check your watercraft and gear for weeds, mud or pests and remove them at site or put them in the bin.
Clean your watercraft and gear with tap water, and then treat absorbent materials with hot water and detergent.
Dry your gear for at least 48 hours before moving to another waterway.
Lake Wānaka Lagarosiphon Management Committee membership:Toitū Te Whenua Land Information New ZealandOtago Regional CouncilQueenstown Lakes District CouncilGuardians of Lake WānakaDepartment of ConservationFish & Game OtagoClutha Fisheries TrustWānaka Marina LimitedWai Wānaka