What does it look like?
Yellow flag iris is an aquatic plant that grows as leafy clumps up to 2m tall. The sword like leaves emerge in fans from a reddish purplish base. From October to December, it produces pale yellow to golden orange flowers that are up to 12cm in diameter. These are followed by seed capsules containing a large number of brown, flattened, three-sided, disc-like seeds.
Yellow flag iris grows in still and slow flowing water bodies and wetlands. It can also invade flood prone pasture. In other countries it has been recorded in salt marshes.
Why is it a problem?
Yellow flag iris is a fast-growing, rapidly spreading invasive plant that outcompetes other vegetation, forming dense, nearly impenetrable thickets that overtop and displace native species.
It can also grow out across the water, forming floating mats that are strong enough to support the weight of a person. It’s poisonous to grazing animals and is tolerant of saline conditions, frost, flooding, drought, physical damage and many soil types.
Yellow flag iris reproduces from both rhizomes (roots) and seeds. Each seed capsule contains many viable seeds but the longevity of yellow flag iris seeds in the seed bank is unknown. It has been found to facilitate its own spread by contributing to soil compaction and hardpan development, which prevents other desired species from establishing on a site.
Control methods
Herbicide control
- Stem injection (all year round) with 5ml glyphosate (undiluted) into each stem at base. Or,
- Spray (spring-autumn) with glyphosate (100ml/10L + penetrant) or metsulfuron-methyl 600g/kg (0.5g/10L).
- Begin control work at margins, working upstream to downstream where possible. Plant tall, shading species adjacent to sites, and exclude livestock. Prevent seeding, and ensure regular follow-up of sites that have been controlled, as rhizomes resprout and seed bank can reinfest bared sites.
- CAUTION: When using any herbicide or pesticide, PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.
Related links
Management Programme
National Pest Plant Accord
All plants on MPI's National Pest Plant Accord (NPPA) are unwanted organisms under the Biosecurity Act 1993 whose formal or casual trade contributes significantly to their spread.
Plants on the Accord cannot legally be propagated, distributed, or sold in New Zealand. If allowed to spread further, these pest plants could seriously damage our economy and environment.
Unwanted Organism
An unwanted organism is any organism that's capable of causing harm to natural or physical resources (like forests and waterways) or human health. A number of introduced pests in New Zealand are classed as unwanted.
More informationRules
Section 26ZM of the Conservation Act (1987)
In accordance with the Conservation Act 1987 section 26ZM, no person shall transfer live aquatic life or release live aquatic life into any freshwater. This includes native or introduced fish, plants, and invertebrates. The taking and holding of some organisms requires a special permit from MPI.
More informationTaxonomies
FamilyIridaceae
TypePlants
GroupGroundcover
HabitatFreshwaterLand
Management ProgrammeNational Pest Plant AccordUnwanted Organism
