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Yellow flag iris (Iris pseudacorus)
ODA rating: B
 
USDA Symbol: IRPS
Oregon yellow flag iris distribution

Other common names
water flag, yellow flag and yellow iris

Click on image to view larger photo.
 
Image courtesy of Jefferson County Weed Control
 
Image courtesy of J.S. Peterson, USDA-NRCS Database

Image courtesy of Jefferson County Weed Control
 
If images are downloaded and used from the ODA web site please be sure to credit the photographer.

Description
Yellow flag is a very showy species growing 3-4 feet in height with the most vigorous growth attained in the wettest environments. The leaves are long, flattened and sword-like, typical of most iris. Large plant clumps are formed from the lateral growth of rhizomes sometimes attaining 20 feet in width. It has erect plant stalks with multiple flowers produced on each. Fruit capsules are large, 3-angled and up to 4 inches in length. Disk-like seeds are shed from the capsules throughout the fall and winter. Floating mats of seed can be observed in backwaters and marshes aiding dispersal.  Reproduction can occur asexually through rhizome fragmentation or by seed production. Food storage in this species is unique. Fructan is the main storage compound held in the cells as opposed to starch. This allows the plants to metabolize energy under very low oxygen conditions and may provide cells with a natural anti-freeze for the winter. 

Impacts
An infestation of yellow flag iris presents a dual impact on both human interests and native environments. This plant displaces native plants including sedges and rushes. This can reduce the carrying-capacity of wetlands for waterfowl and disrupt other ecological relationships.  Irrigation canals and flood control ditches can be severely restricted by the physical nature of the plant clumps. Removal can be costly requiring large excavation equipment or herbicides. Control of heavily infested waterways can be cost prohibitive due to the huge volume of plant material needing to be removed. Any rhizome fragments that remain quickly reestablish a population.  Invaded marshes in some eastern states are experiencing a significant displacement of native sedges and rushes with monocultures of iris. Many over-wintering waterfowl species are dependent on sedge and rush seeds as a high-energy food source. Replacement of this food source with yellow flag iris would reduce the carrying capacity of these marshes to sustain waterfowl populations.  A small volume of yellow flag still exists in the nursery trade. A variegated variety is popular with aquatic gardeners and can be found in several catalogs and web sites. The ease with which this plant can be established using rhizome fragments has led to extensive trading among gardeners and aquatic plant enthusiasts. 

Introduction
Yellow flag is native to most European countries with the exception of Iceland. It is also known to occur in North Africa and the Mediterranean regions. It has significant freeze tolerance and can be found in Scandinavia to 68 degrees North.   Most states in the U.S. report escaped populations.  Yellow flag iris is a popular ornamental in North America that is planted in natural and artificial wet areas within urban and rural landscapes.  It is particularly popular as a large and colorful flowering element in ponds and has been planted in wastewater ponds where it is used to remove heavy metals.  Unfortunately, this plant commonly escapes from cultivation.  The species has naturalized extensively and is currently distributed across the United States.  Once established, it is an aggressive invader in most wet habitats.  Along the Pacific, Atlantic and Gulf Coasts it has invaded estuarine and wetland habitats.  Inland, I. psuedacorus has invaded riparian, open water features and irrigation ditches.  In these habitats it has displaced native plants, disrupted ecological relationships and interfered with water movement.  It is reported weedy in New Zealand and Canada. Most states in the U.S. report escaped populations but the extent of these populations are not well documented.
 
Distribution in Oregon
In Oregon, significant populations can be found in the Willamette valley, in coastal lakes and streams, in the Columbia River and in Central Oregon on irrigation canals. Smaller populations are also known to exist in northeast Oregon.


 
Biological controls
There are no approved biological control agents for this plant.
 
Informational links
Yellow flag iris printable trifold(pdf)