Biosecurity factsheet 23 Yellow flag iris indd by gjmpzlaezgx


Biosecurity series – pest plant factsheet                                                                                   FACTSHEET

Yellow flag iris                                                                                                            23
Iris pseudacorus
Plant type: Perennial semi-aquatic
                                                                                                                           Plant type
Other common names: Yellow iris
                                                                                                                            Aquatic
Yellow flag iris is native to Europe, Asia and north Africa.
Introduced to New Zealand as an ornamental garden                                                                            Fern
plant, it was first recorded growing wild in the Hutt Valley
in 1938. It is present at isolated sites throughout New                                                                      Grass
Zealand, but is currently banned from sale, propagation
and distribution.                                                                                                            Ground
Why it is a pest plant
Yellow flag iris forms dense stands that can displace                                                                        Shrub
native species and restrict access for recreational
activities. Some infestations are the result of deliberate                                                                   Tree
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              Identifying features
low-lying pasture, and is toxic to livestock. The largest                Yellow flag iris is a leafy wetland-terrestrial    Toxic
established stands grow on both sides of the Waikato                     iris that grows up to 2 m. Forms dense floating
River, particularly downstream of Ngaruawahia, but the                   mats in shallow water. It has thick creeping
species is also establishing elsewhere. It typically inhabits            rhizomes with many rootlets. Plants die back in
the margins of lakes, rivers or drains.                                  winter.                                           Disposal

Related species                                                          Flowers                                           Compost
Stinking iris (Iris foetidissima) is not aquatic. It is a smaller        It produces large yellow flowers from
plant with dull yellow-green flowers and its crushed leaves              September to December. Flowers are up to 120       Herbicide
have a foul smell.                                                       mm in diameter and are typically iris-like in
                                                                         form.                                              Bury
                                                     Yellow flag iris.   Margins of still, fresh or slightly salty water    Report it
                                                                         such as rivers, swamps, salt marshes, pond
                                                                         edges. Tolerates frost, flooding, drought, high
                                                                         or low water fertility and many soil types.

                                                                         Seed capsules contain many smooth, flattened
                                                                         seeds which float on water.

                                                                         Broad flat sword shaped leaves, in fans from
                                                                         reddish base. Leaves are 20–30 mm wide, with
                                                                         a distinct mid-rib.
                                                                                                                                                          Yellow flag iris seedlings.

                                                                                                        Photo: Trevor James
                                                                                                                              Important contacts
                                                                                                                              Biosecurity pest plant contractors in the
                                                                                                                              Waikato region – 0800 BIOSECURITY
                                                                                                                              (0800 246 732).
                                                                              Yellow flag iris infestation.

                                                                                                                              For information phone the Waikato Regional
Responsiblity for control
                                                                                                                              Council’s freephone 0800 800 401 or check
In all parts of the Waikato region, land occupiers are responsible for the total
control of yellow flag iris on their property. If the land is less than 20 m from a
waterway, control may be undertaken subject to an approved Waikato Regional
                                                                                                                              For more information on
Council programme.
                                                                                                                              pest plants, check out
Yellow flag iris is banned from sale, propagation, distribution and commercial
                                                                                                                              More information
                                                                                                                              Regional Pest Management Strategy (RPMS).
How to control yellow flag
Herbicide control
                                                                                                                              What makes a pest a pest? – A guide to the
‘Weed wipe’ or spray with glyphosate (such as roundup) or metsulfuron (such as
                                                                                                                              Waikato Regional Pest Management Strategy
escort) plus a penetrant (such as Pulse).

Smaller infestations may be controlled by ‘injecting’ herbicide into the rhizome.
                                                                                                                              Plant Me Instead – Plants to Use in Place of
                                                                                                                              Common Pest Plants (free).
Follow-up control work is likely to be required.

                                                                                                                              The New Zealand Pest Plant Manual (National
Note: If applying herbicide over water, a resource consent may be required. Please
                                                                                                                              Pest Plant Accord Identification Guide) –
check with the Waikato Regional Council before proceeding.
                                                                                                                              $10.00 incl. GST.

When using herbicides:
                                                                                                                              Poisonous Plants and Fungi in New Zealand –
• read the instructions on the manufacturer’s label closely
                                                                                                                              A Guide for Parents, Schools and Child Minders
• always wear protective clothing
                                                                                                                              ($15.00 incl. GST).
• always minimise the risk to desirable plants
• contact the supplier for further advice.

Waikato Regional Council, 401 Grey Street, PO Box 4010, Hamilton East 3247.

For more information call the Waikato Regional Council’s freephone 0800 800 401 or visit

Waikato Regional Council biosecurity factsheet series no. 23. Updated September 2011 (S2775-0811).

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