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More Herbicide To Fight River Weed

Another application of the herbicide Diquat is be used in Kerrs Reach next week to control the weed, Egeria densa.

Initial feasibility trials held in mid-January have shown good promise. Areas of application were inspected by NIWA scientists in late February.

Christchurch City Council staff has now evaluated the post-survey information and plan a second application in Kerrs Reach on Wednesday 14 March or the next fine week.

The second application will cover areas not treated in the first herbicide application and this will provide the chance to refine details of environmental impact and water quality.

The process on 14 March will be a repeat of the first application with a 24-hour river closure, a leaflet drop to properties fronting the river where it will be closed, and mobile patrols and signs as needed to inform river users of the application.

As other control methods had previously failed to eradicate the weed from the Avon, the Council decided on its January trial.

The weed grows quickly and forms a tangled mass on the surface of water. The dense canopy also blocks light and stops other plants living in the same area. It also decreases oxygen levels, especially at night, and this can cause problems to other aquatic life.

The plant impedes drainage and affects the recreational use of the water.

Harvesting the weed is costly and if Egeria densa becomes widespread maintenance costs will increase.

Egeria can grow from very small pieces and using a weed cutter potentially increases its rate of spread from the fragments.

The weed is originally from South America but is widespread in the North Island waterways, such as Lake Karapiro where it effects recreational use and power generation efficiency, and in the Opawa River in Blenheim.

The Christchurch City Council’s Parks and Waterways Manager, Anne Greenup, says the Council is using Diquat following investigations into the impacts of the weed if it is left uncontrolled.

Other control methods tested are less effective and Diquat is the next tool available to the Council to evaluate, she says.

A resource consent has been granted by the Regional Council, Environment Canterbury, to apply the herbicide and this includes a requirement to close the affected area of the river to all use for 24 hours.

The closing will extend from the Dudley Creek confluence, Banks Avenue and McBratneys Road corner, to the Wainoni Road Bridge.

River users will be asked to keep clear of that stretch of the river in the 24-hour closure period. Any boats, trailers or nets used in the area should be cleaned thoroughly.

The herbicide will pose no risk to pets, fish or wildlife and the closure is a purely precautionary procedure.

Diquat is not harmful to humans when applied at the recommended dilution rates.

The herbicide application is being closely monitored by NIWA scientists and other groups to record water quality, impacts on fish and fauna, and the effect on the Egeria weed.


Further information: Chris Rance, Greenspace Maintenance Team Manager: 371 1391 or 025 232 7468.

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