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Yellow flag iris

Yellow flag iris
Iris pseudacorus

Progressive containment
Reduce the amount of yellow flag and limit the locations that have it.

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Why it is a pest plant

Progressive containment
Reduce the amount of yellow flag and limit the locations that have it.

Production threat Environmental threat Public threat

Identifying features

Yellow flag iris is native to Europe, Asia and North Africa. Introduced to New Zealand as an ornamental garden plant, it has since spread to sites throughout New Zealand.

Yellow flag iris forms dense stands that can displace native species and restrict access for recreational activities. Some infestations are the result of deliberate plantings, but most spread is by prolific seeding or by fragmentation of root rhizomes (horizontal stems that send out roots and shoots). It can also invade and displace low lying pasture and is toxic to livestock. The largest established stands grow on both sides of the Waikato River, particularly downstream of Hamilton, but the species is also establishing elsewhere. It typically inhabits the margins of lakes, rivers or drains.

Related species

Stinking iris (Iris foetidissima) is related but unlike yellow flag iris, it is not aquatic. It is a smaller plant with dull yellow-green flowers and its crushed leaves have a foul smell.

Responsibility for control

All landowners/occupiers in the Waikato are responsible for controlling yellow flag iris on their properties and are required to work with Waikato Regional Council in areas where control programmes are in place. Yellow flag iris is also banned from being sold, propagated, distributed or included in commercial displays.

How to control yellow flag iris

Physical control

Plants identified in gardens should be dug out. Ensure all root fragments are removed and disposed of at a refuse transfer station.

Herbicide control

Cut and inject

Smaller infestations may be controlled by ‘injecting’ glyphosate or metsulfuron herbicide into the rhizome (fleshy root).

Spray application

'Weed wipe' or spray with glyphosate or metsulfuron plus a penetrant.

Safety when using herbicides

  • Follow the instructions on the manufacturer’s label.
  • Always wear protective clothing.
  • Always minimise the risk to your other plants.
  • Contact the supplier for further advice.

Summary of herbicides and application methods for control



Glyphosate or metsulfuron plus a penetrant Spray application - weed wipe
Glyphosate or metsulfuron Cut and inject
Herbicide rules will apply. You may need to notify neighbours if spraying. The Waikato Regional Plan explains the agrichemical (herbicides) use rule in section 6.2
If applying herbicide over water, a resource consent may be required. Please check with Waikato Regional Council before you begin.



After initial control, it’s important to:

  • clean out the site again at least annually to control regrowth
  • stop weeds invading by replanting with non-pest plants (preferably native plants) once regrowth is no longer a problem.

More information


  • For advice and additional information on control methods, call our pest plant staff on freephone 0800 BIOSEC (0800 246 732).
  • Chemical company representatives, farm supply stores and garden centres can also be good sources for advice.


View, download or order the following publications or call our freephone 0800 800 401.

  • National Pest Plant Accord (Manual of plants banned from sale, propagation and distribution) ($10.00 plus GST)
  • Plant Me Instead! (Plants to use in place of common pest plants) (free)
  • Waikato Regional Pest Management Plan (RPMP) (free) (Section 5.69, page 163)
  • Waikato Regional Council pest guide (free)

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