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Popular boating and swimming spots have been added to the control programme tackling lake weed in Lake Dunstan.

More than $200,000 is due to be spent this financial year to tackle the invasive plant pest lagarosiphon.

The funding comes from Land Information New Zealand (LINZ), Otago Regional Council and Contact Energy.

“We are focussing our efforts this season on all of the popular recreation spots such as the Old Cromwell Town beach and Bannockburn,” says Marcus Girvan, Biosecurity Programme Manager at Boffa Miskell, working on behalf of LINZ.

Lagarosiphon at old Cromwell town

Lagarosiphon at old Cromwell town.


“We also plan to place more emphasis on boat launching ramps so that boaties don’t snag weed on the way out of the lake.”

Work around the lake will involve divers pulling out the weed by hand, herbicide spraying from boats and helicopters and using a cutter to slice the weeds away from the surface.

“This is good news for users of the lake,” says Glen Christiansen, Chair of the Guardians of Lake Dunstan.

“Whilst there is a significant amount of lagarosiphon in Lake Dunstan, focussing on high use areas will improve the experience of people using the lake for swimming and boating.”

Lagarosiphon spreads quickly and can grow up to five metres tall. It’s hoped that by clearing areas such as the boat ramps the invasion of the plant may be kept in check.

“With so much weed in Lake Dunstan, preventing the spread of lagarosiphon to other waterways is absolutely critical,” says Richard Lord, Team Leader Biosecurity Compliance at Otago Regional Council.

“I’d like to urge anyone using a boat, jet ski or kayak on the lake to check for weeds when they leave the lake and clean everything thoroughly.”

A recent inspection by NIWA showed where treatments for lagaroshiphon have had an impact in the lake. At Northburn, Lowburn Boat Harbour and Lowburn Inlet lagarosiphon has been pushed back and native plants are starting to regrow.

“Last year we saw really positive results in most areas where work was carried out to treat lagarosiphon. The main area where there wasn’t as much impact was around Bendigo”, adds Marcus.

Herbicide spraying at Old Cromwell had to be delayed due to heavy sediment in the water. Instead of spraying, Boffa Miskell cut the weeds back in November.

“It’s disappointing we couldn’t use herbicide at Old Cromwell,” says Marcus.

“The results from NIWA show that the herbicide diquat has been very effective in most parts of the lake. It’s a safe treatment to use as it doesn’t affect swimmers or others using the lake water, and doesn’t harm native plants. Although we do recommend that people don’t use the lake in the treated area until 24 hours after the treatment has been completed.”

The work programme for Lake Dunstan is being led by the Lake Dunstan Aquatic Weed Management Group which includes LINZ, the Ministry for Primary Industries, NIWA, Contact Energy, Boffa Miskell, Otago Regional Council, Fish and Game, the Guardians of Lake Dunstan, Central Otago District Council, the Clutha Fisheries Trust and the Cromwell and Districts Community Trust.

Watch this video that shows LINZ working with others to tackle weeds in some of our most popular lakes.

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