Great willowherb is a highly invasive flowering weed that rapidly establishes in wet or damp environments, including wetlands, lakesides, riverbeds, and roadside ditches. It forms dense stands that crowd out native vegetation, disrupt natural ecosystems, and alter water flow. The plant spreads mainly by seed, which can be dispersed by wind, water, contaminated machinery, and footwear. Its thick rhizomes (thick underground stems) can also regenerate when disturbed, making it difficult to control. If left unmanaged, Great willowherb can cause significant damage to wetland habitats and biodiversity. 

Currently, Great willowherb has not been detected in Otago, but it is established in Canterbury, where it has been found in wetlands and lakesides. Surveillance in Otago is crucial to prevent its spread, particularly in high-risk areas like wetlands and other damp environments where it could establish and outcompete native species. 

• Size: Great Willowherb grows up to 2 meters tall with upright, branched stems.  
• Leaves: The leaves are willow-like, slightly hairy, arranged in pairs, and have toothed edges. The plant produces long, narrow seed pods that split open to release small seeds with long white hairs. 
• Flowers: It features obvious pink flowers, 2-3 cm in diameter, with a white centre and notched petals. 
• Habitat: Found in wet or damp environments, including wetlands, lakesides, riverbeds, and roadside ditches. 

Great willowherb is most visible when flowering from December through April. 

There are no specific rules associated with Great willowherb in the Otago Regional Council’s Regional Pest Management Plan (2019-2029).  

It is an offence to breed, knowingly communicate, exhibit, multiply, propagate, release, or sell, an Unwanted Organism (Biosecurity Act 1993). 

Do not attempt to undertake control of great willowherb yourself. 

www.orc.govt.nz/greatwillowherb