Yellow flag iris (Iris pseudacorus)

Yellow flag iris is a clumping herb up to 1.6 m tall with bright yellow flowers. It can out compete native plants and invade wet areas.

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How does this weed affect you?

Yellow flag iris is an invasive plant that:

• outcompetes native plants especially in waterways and areas with wet soils
• has rhizomes that form dense underground mats 
• reduces habitat for native animals
• invades pastures in wet areas 
• is poisonous to people and animals and can cause skin irritation
• changes water flows and streambank width.

 Human health

Plants are poisonous if eaten. Handling the plant, especially the sap, can cause skin irritation, including blisters.

Livestock poisoning

Yellow flag iris contains glycosides which have caused diarrhea in cattle. Livestock usually avoid eating plants.

What does it look like?

Yellow flag iris is a clumping, perennial herb, usually 75 - 100 cm tall though it can grow up to 1.6 m.

Leaves are:

• dark green with veins visible when held up to the light
• 50-100 cm long and 1-3 cm wide
• sword shaped with a pointed tip
• raised along the midrib
• hairless.

Flowers are: 

• bright yellow
• usually 7-10 cm in diameter but can be up to 12 cm
• a short funnel shaped tube with:
  • 3 larger, drooping, outer petals, up to 7.5 cm long and 3 cm wide, that have brown or violet veins
  • 3 smaller, upright, yellow petals, which are up to 3 cm long and 8 mm wide
•  present from spring to early summer.

Fruit are: 

• capsules which contain over 30 seeds 
• yellow green to green at first, ripening to dark brown
• about 5 cm long and 2 cm wide
• oblong or oval shaped with a beak.

Seeds are:

• white at first then turn brown
• disc shaped or three sided. 

Rhizomes are:

• reddish brown or pink
• up to 100 cm long and 5 cm in diameter
• branched.

Roots are:

• usually 10-20 cm long but occasionally up to 30 cm long.

Similar looking plants

Yellow flag iris looks similar to other introduced plants:

• Tall bearded iris (Iris germanica), which has fringed hairs (a beard) at the base of the outer petals, its leaves are shorter, only up to 50 cm long. The flowers may be in a range of colours including yellow. It does not grow in water.
• Copper iris (Iris fulva) can grow on the water’s edge. It usually has orange or reddish flowers (occasionally yellow). All 6 petals droop downwards and are similar in size.
• Montbretia (Crocosmia crocosmiiflora), which can also grow on the water’s edge. Its flowers are usually orange red. The petals are only up to 2 cm long and all 6 are a similar shape and size.

Where is it found?

Yellow flag iris mostly grows in humid temperate and Mediterranean climates, though it can also grow in subtropical climates. Plants thrive when growing in water (up to 1 m deep) but can also grow well in areas with moist soils. Plants are both flood and drought tolerant and can tolerate brackish water. 

Yellow flag iris can tolerate partial shade but it grows best and produces more flowers in full sun. This weed thrives in soils with high nitrogen and it can tolerate a variety of soil types including acidic, low oxygen and saline soils.

Yellow flag iris grows:

• in wetlands, swamps and salt marshes
• along the edges of dams, lakes, streams and waterways
• in pastures in wet areas
• in woodlands and forest edges with moist soil
• in roadside drains and irrigation channels.

How does it spread?

Yellow flag iris is an ornamental garden plant and has been spread by people to new areas in Australia.

By seed

Plants start producing seeds when they are 2 to 3 years old. Each plant can produce up to 1500 seeds. Seeds are buoyant and are spread by moving water. They can survive in seawater.

By plant parts

New plants can grow from rhizome fragments. These are spread by water, especially after flooding events. The can also be spread by people dumping garden waste.

References

DiTomaso, J. M. & Kyser, GB. (2013). Weed Control in Natural Areas in the Western United States. Weed Research and Information Centre, University of California.

Gardenia (2025). Iris pseudacorus (Yellow Iris). Retrieved 11 November 2025 from: https://www.gardenia.net/plant/iris-pseudacorus-yellow-iris

New Zealand Weed busters: Yellow flag iris retrieved 10 October 2025 from: https://www.weedbusters.org.nz/what-are-weeds/weed-list/yellow-flag-iris/

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 10 October 2025 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Iris~pseudacorus

Sandenbergh, E., Gervazoni, P., Grewell, B., Franceschini, C., Minuti, G., McGrannachan, C., ... & Coetzee, J. (2025). Biology of Invasive Plants 7. Iris pseudacorus L.(Iridaceae). Invasive Plant Science and Management, 18, e6.

Sandenbergh, E., Petruzzella, A., & Coetzee, J. A. (2024). Distribution and sexual reproductive potential of Iris pseudacorus L.(Iridaceae) in South Africa. South African Journal of Botany, 167, 31-39.

Sutherland, W. J. (1990). Iris pseudacorus L. Journal of Ecology, 78(3), 833-848.

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Control

For large infestations along waterways, start upstream where possible to limit reinfestation when seeds are washed down stream.

Clearing any vegetation along waterways may cause erosion and may require consent before any work starts. It is the landholder’s responsibility to obtain any approvals that may be required prior to undertaking clearing. Contact NSW Local Land Services for advice on the required approvals for your site.

Wear protective clothing when handling yellow flag iris as it can cause skin irritations.

By hand

Small numbers of plants can be dug out by hand. All parts of the plants including the rhizomes need to be removed.

By machine

The risk of spread by fragmentation is very high when using machinery. All the rhizomes will need to be dug out to prevent regrowth.

Chemical control

Spraying

Spray actively growing plants. Ensure all the foliage is covered.

Only use herbicides registered for aquatic use for plants near water.

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2030
Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: 100 mL per 10 L of water
Comments: Spray actively growing plants. See permit for critical comments.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2030
Metsulfuron-methyl 600 g/kg (Only products registered for aquatic use)
Rate: 5 g per 100 L of water + a non-ionic surfactant.
Comments: Spray actively growing plants. See permit for critical comments.
Withholding period: Nil (recommended not to graze for 7 days before treatment and for 7 days after treatment to allow adequate chemical uptake in target weeds).
Herbicide group: 2 (previously group B), Inhibition of acetolactate and/or acetohydroxyacid synthase (ALS, AHAS inhibitors)
Resistance risk: High

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Biosecurity duty

The content provided here is for information purposes only and is taken from the NSW Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.

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Reviewed 2025