Yellow Flag Iris

Yellow flag iris is a tall, rhizomatous perennial native to Europe, western Asia, and North Africa that has become invasive in much of North America. It grows in wetlands, ditches, and along shorelines, forming dense stands with sword-like leaves and showy yellow flowers that bloom in late spring. While visually striking, yellow flag iris spreads aggressively by rhizomes and seed, displacing native wetland plants and altering hydrology and habitat structure along streams and waterways. It is toxic to livestock and wildlife and is illegal to sell or move in Oregon due to its ecological impacts on aquatic and riparian systems.

Life Forms	Forb
ODA Listing	B
Soil and Moisture Conditions	Moist Wet
Suggested Actions	Report to Oregon Invasives Hotline Use BMPs to control on private property
Special Uses	Tolerates constant flooding
Shade Preference	Full sun Part shade
Mature Height	3'-6'
Distribution	In Oregon, yellow flag iris is widely established west of the Cascade Range, particularly in the Willamette Valley, along the coast, and in low-elevation wetlands, ditches, and slow-moving waterways. It is less common but present in parts of central and eastern Oregon where suitable wet habitats occur.
Control	Small infestations can be managed by digging out the entire rhizome system, taking care to remove all fragments, which readily resprout. Cutting or mowing alone is ineffective unless repeated multiple times per growing season and combined with rhizome removal. In larger or established infestations, targeted aquatic-approved herbicide treatments may be necessary, followed by monitoring and re-treatment. All plant material must be disposed of properly, and disturbed sites should be replanted with native wetland species to reduce reinvasion.
Disposal Methods	Bag seeds and rhizomes and dispose of them in the landfill.
Reproduction and Spread	Seeds and rhizomes float and move around and downstream with flowing water.
Introduced	Native to Europe, North Africa, the Mediterranean
Look Alikes	Cattails look similar to yellow flag iris when not in bloom, but have narrower, strap-like leaves that are round at the base. Ornamental yellow irises may look similar, but are generally smaller in stature and don't grow along waterways.
Impact	Yellow flag iris has significant negative impacts on wetland and riparian ecosystems where it becomes established. It spreads rapidly by rhizomes and seed, forming dense monocultures that displace native plants and reduce plant diversity. These dense stands alter hydrology by trapping sediment and slowing water flow, which can change channel structure and degrade habitat for fish and amphibians. Yellow flag iris is also toxic, limiting its value to wildlife, and its presence complicates wetland management and restoration efforts.
More Info	https://www.nwcb.wa.gov/weeds/yellow-flag-iris https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/yellow-flag-iris-control.pdf

Life Forms

Forb

ODA Listing

Soil and Moisture Conditions

Moist

Wet

Suggested Actions

Report to Oregon Invasives Hotline

Use BMPs to control on private property

Special Uses

Tolerates constant flooding

Shade Preference

Full sun

Part shade

Mature Height

3'-6'

Distribution

In Oregon, yellow flag iris is widely established west of the Cascade Range, particularly in the Willamette Valley, along the coast, and in low-elevation wetlands, ditches, and slow-moving waterways. It is less common but present in parts of central and eastern Oregon where suitable wet habitats occur.

Control

Small infestations can be managed by digging out the entire rhizome system, taking care to remove all fragments, which readily resprout. Cutting or mowing alone is ineffective unless repeated multiple times per growing season and combined with rhizome removal. In larger or established infestations, targeted aquatic-approved herbicide treatments may be necessary, followed by monitoring and re-treatment. All plant material must be disposed of properly, and disturbed sites should be replanted with native wetland species to reduce reinvasion.

Disposal Methods

Bag seeds and rhizomes and dispose of them in the landfill.

Reproduction and Spread

Seeds and rhizomes float and move around and downstream with flowing water.

Introduced

Native to Europe, North Africa, the Mediterranean

Look Alikes

Cattails look similar to yellow flag iris when not in bloom, but have narrower, strap-like leaves that are round at the base. Ornamental yellow irises may look similar, but are generally smaller in stature and don't grow along waterways.

Impact

Yellow flag iris has significant negative impacts on wetland and riparian ecosystems where it becomes established. It spreads rapidly by rhizomes and seed, forming dense monocultures that displace native plants and reduce plant diversity. These dense stands alter hydrology by trapping sediment and slowing water flow, which can change channel structure and degrade habitat for fish and amphibians. Yellow flag iris is also toxic, limiting its value to wildlife, and its presence complicates wetland management and restoration efforts.

More Info

https://www.nwcb.wa.gov/weeds/yellow-flag-iris

https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/yellow-flag-iris-control.pdf

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