Weed programme snuffing out purple loosestrife in Canterbury

Craig Alexander
DOC ranger Craig Alexander
preparing to spray purple
loosestrife.
Photo: I Hankin

While the best way to control weeds is to stop them getting here in the first place, effective programmes have been established to deal with pest plants that have made it past our border defences. The Department of Conservation (DOC) plays an important role in managing pest plants that are already established here.

DOC's role is to protect natural resources throughout New Zealand, so weed species that are already present and that could spread to new parts of the country are of particular concern.

The department administers a wide range of land types, with a variety of weeds issues. Most of DOC's weed control work is either to protect sites, species and ecosystems of high ecological value, or to deliver programmes of work under pest management strategies. A strategic plan lays out priorities and guides decision-making about invasive weed control. The plan also identifies the need for weed surveillance and early action.

Weed surveillance procedure updated

Weed surveillance is a system to encourage both active surveillance for new weeds on high-value sites, and the reporting of new weed sightings. The Weed Surveillance Standard Operating Procedure (SOP) is now being revised to include new developments like the Weedbusters initiative (see Biosecurity 65:9, 1 February 2006). The existing SOP predates the establishment of Biosecurity New Zealand (BNZ), and a revised SOP will have a new section on biosecurity and unwanted organisms. This section has been developed with input from BNZ, and will ensure DOC weeds staff are aware of how and when to contact BNZ.

New weed species of concern are targeted at an early stage, even before they threaten to invade high-value sites administered by DOC. To do this successfully, it must be possible to eradicate the weed from a large area, across lands of all tenure, with little risk of re-invasion.

Purple loosestrife targeted

One such programme is aimed at purple loosestrife (Lythrum salicaria) in Canterbury. Purple loosestrife is known to be a serious problem in wetlands in North America and also in the North Island of New Zealand (see 100 of the worst invaders: how do we rate? in Biosecurity 59:4, 1 May 2005). If allowed to spread, it forms dense stands and can completely alter wetlands, destroying the habitat of native species. Purple loosestrife had been sold in New Zealand, but now it is classed as an Unwanted Organism under the Biosecurity Act 1993, so it can no longer be sold.

Fortunately, there was very little purple loosestrife in Canterbury. However, the existing infestations were starting to spread and would become a serious threat, particularly to the region's wetlands. The known infestations were mainly in water margins, reserves and gardens administered by Environment Canterbury and three local councils. All of these organisations were prepared to join an eradication campaign. There had also been a few sightings of purple loosestrife on private property.

A group to lead the campaign was set up, mainly comprising representatives from DOC, Environment Canterbury, Christchurch City Council and Ngai Tahu. The first step was a local publicity campaign in the summer of 2002/03. As a result, around 200 new sites were found, ranging from a single plant in a garden through to an infestation running across several rural properties. In most cases, the land owners were more than happy for the organisations to control the purple loosestrife for them, and all agreed to it. This meant that a control programme was feasible, as purple loosestrife could be controlled wherever it was found in Canterbury.

Control has now been underway for four years, and has been very successful. Within a few years, the sites should just require checking annually to ensure no seeds have survived, and Canterbury will be free of one potentially serious weed.

For more information about weed control:

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Page last updated: 30 April 2008