Yellow Flag

(Iris pseudacorus)

Regional Surveillance Pest

Yellow Flag is an evergreen semi-aquatic iris often found on the edge of waterbodies and drains, growing up to 1m tall, producing conspicuous yellow flowers in spring. It forms dense floating mats of rhizomes that smother native plant and exclude animal communities. A native of Europe and Western Asia, it was introduced as an attractive garden plant and become widely distributed, but quickly became naturalised.

Reasons for the Strategy

Yellow flowers atop tall green stalks.

Yellow Flag is an internationally renowned weed of wetlands, where it forms dense sprawling stands up to 1 m tall that smother and impede the regeneration of other vegetation. The dense mats impede access to water bodies for fishing and other recreational activities, as well as being toxic to stock. It can replace other aquatic margin vegetation and degrade habitat for aquatic fauna. It can also adversely affect estuarine and saltmarsh vegetation. It has the potential to take over wetland margins and the margins of other waterbodies and drains and represents a significant threat to wetlands in the Tasman-Nelson region. It can be spread by seed or rhizome fragments.

Yellow Flag is tentatively assessed at “4” on the infestation curve. Given its limited distribution, the importance of remnant wetlands, and its potential impact on wetlands and stock, placing it into Progressive Control will provide benefits that outweigh the costs.


To gather information on the distribution of Yellow Flag in the Tasman-Nelson region and prevent the human spread of this pest during the term of the Strategy.

Alternative Measures

The inclusion of Yellow Flag in the Strategy will ensure the Management Agency is actively involved in surveillance and monitoring, as well as undertaking education and providing advice to landowners. The principal alternative measure is to adopt a greater level of regional intervention but this is dependent on having better information on its distribution. The other alternative is to “do nothing” and this will allow spread to continue, increasing future treatment costs and increasing the risk of further loss of wetland and estuarine habitat.

Biosecurity Act Requirement

No person shall knowingly sell, propagate, breed, release or display commercially, Yellow Flag, under Sections 52 and 53 of the Act.