Aquatic Weeds

LINZ is actively working to help manage invasive aquatic weeds such as Lagarosiphon major (oxygen weed) and Ceratophyllum demersum (hornwort).

Learn here about current LINZ programmes for aquatic weeds, the control methods used and the implementation of control programmes.

Control programmes

LINZ's lake weed control programme for 2009-2010 targets the Rotorua Lakes, Lake Wanaka, Lake Karapiro, Lake Dunstan and Lake Benmore.

You can download LINZ's Lake Weed Spray Programme 2009-2010 (pdf 42KB).

Lake Wanaka Lagarosiphon Management Plan

In 2005 a 10-year Lagarosiphon Management Plan was launched to stop the spread of Lagarosiphon major in Lake Wanaka and to other nearby lakes.

You can download the plan Lake Wanaka Lagarosiphon Management Team. Approved 23 August 2005. A 10 Year Lagarosiphon Management Plan for Lake Wanaka: 2005-2015 (pdf 514KB).

Lake Karapiro hornwort control

LINZ has developed a programme to manage the invasive weed hornwort (Ceratophyllum demersum) at Lake Karapiro in the lead up to the 2010 World Rowing Championships.

Find more information about aquatic weed control programmes in reports and reviews.

Control methods

LINZ uses a variety of methods such as mechanical weed control by suction dredging, hand weeding or surface spraying by boat or aerial application using a contact herbicide.

Suction dredging

Suction dredging removes large quantities of weed using a powerful pumping device. However, it is impractical in some situations. Therefore this control method is targeted at areas with high use such as boat ramps. When an area is suction dredged, the area is also checked by a diver who removes any remaining weeds by hand.

Use of herbicide

The chemical Reglone® is the herbicide used to control invasive aquatic weeds. Reglone® is a contact herbicide which contains an active ingredient, diquat. Diquat is recognised as the most effective and environmentally responsible herbicide to control hornwort and lagarosiphon.

Reglone® is applied to the weed in gel form. The use of gel ensures the herbicide is not affected by underwater and air currents, causing unintentional drift. With diquat, there is no toxic residue and targeted weeds fall to the lakebed and decompose. Local and overseas studies show that diquat does not pose a risk to humans, animals, fish and most native plant life.

You can download an information sheet about diquat - NIWA, n.d. Diquat Summary - Use and Safety (pdf 41KB).

Implementation of control programmes

Public consultation

The control method used for lake weed control is decided after extensive consultation with interested parties such as Fish and Game, local iwi, regional councils and local interest and user groups.

Public notices

Before a control programme takes place, the public are informed via local newspapers and radio notices, and signs by waterways.

Read the latest biosecurity public notices.