The Environment
Purple loosestrife guide
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Purple loosestrife (Lythrum salicaria)
Purple loosestrife is a serious weed of wetlands and waterway margins. Purple loosestrife has now become established in several locations in New Zealand, including Canterbury.
In an attempt to determine how widespread this species is, field staff have searched extensively for purple loosestrife in Canterbury, and there has been a publicity campaign to elicit reports from the public. The results suggest that it is possible to eradicate this species from Canterbury, and that is currently our aim.
Why get rid of it?
Purple loosestrife is a serious threat to New Zealand's unique wetland biodiversity. It can spread and form dense stands which block out other plants, some of which are found nowhere else in the world. The tall plants can change the habitat of wetland and waterway sites so that the species that used to grow there are no longer able to survive.
If allowed to spread, it will colonise water margins, ponds and shallow lakes, affecting wetland birds. Purple loosestrife also has the potential to clog drainage and irrigation ditches.
Each plant is capable of producing millions of seed, which are mainly dispersed by water. Pieces of plant will also take root. Its ability to reproduce prolifically and spread also contribute to its weediness. This potential for rapid spread and increase in population size mean that if we are to have a realistic chance of getting on top of the problem, we have to act quickly.
Until relatively recently, purple loosestrife was available through garden centers and nurseries. However it has now been declared an 'unwanted organism' under the Biosecurity Act. This means that it can't be propagated or sold. Purple loosestrife is also currently the subject of a small-scale management programme, under Section 100 of the Biosecurity Act.
Why focus on purple loosestrife when there are so many other weeds?
Purple loosestrife has the potential to be as serious a problem as weeds like old mans beard, gorse and broom, but at this stage it is manageable. We believe it may be realistic to achieve eradication if we act quickly.
How do I recognise it?
The plants die right back in winter. The easiest time to identify it is when the plants are flowering - typically between December and February.
- 1-2 m tall with stems that are square in cross-section
- Tall stalks with clusters of pink-purple flowers
- Leaves are in pairs, opposite each other
The species that it is most likely to be confused with in a wet situation (along a waterway or in a wetland) are willowherbs (Epilobium spp.). These have small flowers and long pods.
In a garden situation it is sometimes confused with Linaria purpurea. These plants are much smaller and have flowers like snapdragons.
If you would like the identification confirmed, collect a plant, place it in a plastic bag and send it in (see contact details below).
Where is it already recorded?
To date purple loosestrife has been recorded at the following locations in Canterbury:
- Avon River, Christchurch
- Cockayne Reserve
- Travis Swamp
- Halswell River and drainage system
- Drain near Leeston
- Tutaepatu Lagoon
- Several gardens
What is being done about the existing sites?
We are currently controlling purple loosestrife at the sites where it has been found on public land, and especially by the Avon and Halswell Rivers. We are particularly concerned to prevent it from spreading into Te Waihora (Lake Ellesmere).
In some cases removal has been offered to private land owners, although currently there is no obligation that they remove existing plants.
I have some, what should I do?
Please let us know where you have seen it (contact details are below). If it is on your property, its removal would be appreciated.
For any locations found, the following information would be helpful:
- A description of the location (an address or a grid reference).
- What, if any, action has been taken to remove the plant(s).
- A contact name and phone number would be helpful (in case we need to clarify anything with you).
- If you are not quite sure if it is purple loosestrife, send in a plant, in a plastic bag, for us to confirm.
- Even if you remove the plant(s) we would appreciate being notified about the location. This information will help us in determining likely areas of concentration and also assist if we decide to attempt eradication.
Plant Removal
Small plants can be pulled up or dug out. However older plants can be quite woody so cutting the plant off at the base and then painting the stump with herbicide is most effective.
Any parts that are removed should be put out in your rubbish rather than composted, as fragments of plant are able to re-sprout.
Ideally, plant something else in the vacant spot. Suggested alternatives for damp sites are kiahaha (Libertia ixioides), bush lily (Astelia fragrans), swamp kiokio (Blechnum minus), or rush (e.g. Juncus edgariae). The Streamside Planting Guide has other suggestions.
In drier situations (e.g. a home garden) a Hebe might be suitable. Replacement planting will reduce the likelihood of re-invasion of the site with other weed species.
Contact
Helen Braithwaite
Department of Conservation
Email: purpleperil@doc.govt.nz
Phone: (03) 371 3751
Who is running the purple peril programme?
Three agencies are combining their efforts in Canterbury. They are the Department of Conservation, Environment Canterbury and Christchurch City Council.
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