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Christchurch City Council Media Release 10 January 2001

Herbicide To Fight Egeria Densa

As other control methods have failed to eradicate the weed, Egeria densa, from the Avon River, the Christchurch City Council is to trial the use of the herbicide Diquat from next week.

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 will use 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 herbicide will be applied in areas of Kerrs Reach on Monday 15 January or on the first fine day after this, but not at weekends.

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 hours 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 trials are 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 and Utility Maintenance Team Leader, 941 8391 or 025 232 7468.

Egeria guide

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