Purple Loosestrife is a highly invasive perennial herb that forms dense stands in wetlands, along riverbanks, and in other aquatic habitats. It outcompetes native vegetation, disrupts water flow, alters nutrient cycling, and can degrade habitats for wildlife, particularly wetland birds. The plant can block access to waterways, impact irrigation systems, and reduce the quality of hay and pasture due to its unpalatable nature. Purple Loosestrife is a prolific seed producer, with each plant capable of producing up to 2.5 million seeds annually, which can be dispersed by wind, water, and contaminated machinery, making it difficult to control once established.

Purple Loosestrife is locally established in Oamaru, with scattered populations in Tapanui and Dunedin. The plant is found in low incidence across the region, primarily in wetland and riparian habitats. 

• Size: Purple Loosestrife is an erect perennial herb that can grow up to 3 metres tall. 
• Leaves:  The leaves are narrow, opposite, less than 12 cm long, and clasp the stem. 
• Flowers: It has tall, dense spikes of purple or pink flowers that bloom from December to February. The plant produces numerous small, blackish seed capsules in autumn. 
• Habitat: It thrives in wet or damp soils, often in still or slow-moving water bodies, wetlands, riparian margins, riverbeds, and lake margins. 

Purple loosestrife is most visible when it flowers from December to February. 

Purple loosestrife is declared an Unwanted Organism in the National Pest Plant Accord (NPPA). This means it cannot be sold or be in a place where plants are being sold. It cannot be propagated, bred, multiplied, communicated, released, caused to be released, or otherwise spread. 

Physical control

You can dig out small plants, being careful to collect the whole plant including any seed heads. 

Contact your local council for appropriate disposal locations.

Chemical control

• You can spray leaves with 100ml glyphosate per 10L of water.
• For users with Certified handler/experienced agrichemical, you can foliar spray with 100ml glyphosate per 10L of water and 20ml penetrant. 

Caution: When using any herbicide or pesticide PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.

Disclaimer: Mention of product trade names does not endorse these products nor imply criticism of similar products not mentioned. The Otago Regional Council does not give any warranty that the information is accurate or complete or that it is suitable for all circumstances.

Site management 

• Follow up treated areas three times per year. 
• Encourage natural regeneration of native plants or replant treated areas where possible after two to three treatments to establish dense ground cover and minimise reinvasion.