CORVALLIS, Ore. – There's a gorgeous, large yellow iris called "yellow flag" or "yellow water iris" (Iris pseudacorus) found in wetlands, along riverbanks and near ponds in Oregon. It is sold in local nurseries and garden stores as well.
But there's a big problem with this lovely perennial iris. It is invasive and out-competes native riparian vegetation, including cattails, sedges and rushes, and it degrades native fish habitat, as well as bird nesting and rearing sites. Native to Europe, Great Britain, North Africa and the Mediterranean region, yellow flag iris has been introduced in temperate areas nearly worldwide and occurs throughout the United States except in the Rocky Mountains. It appears to be most common near developed areas.
You wouldn't expect widely available, familiar plants like yellow flag iris to be out-of-control noxious weeds. But yellow flag iris, native to Europe, has escaped cultivation and moved into rivers of the Pacific Northwest.
With large showy yellow blossoms, yellow flag, also known as water flag, can grow in large clumps and has been know to reduce the carrying capacity of wetlands for water storage, and block irrigation canal flow and flood control ditches. Difficult to eradicate, it is common in mid-Willamette Valley riparian areas and has been found along rivers in central Oregon.
"If you see a yellow-flowered iris growing directly in water in Oregon, it is most likely yellow flag," said Andy Hulting, a weed specialist with the Oregon State University Extension Service.
In Oregon, yellow flag blooms in late spring or early summer. Several flowers can occur on each stem, along with one or two leafy bracts. Each flower resembles a common garden iris with three large (1.5- to 3-inch) downward facing yellow sepals and three smaller upward pointing petals. The yellow sepals are often streaked with brown to purple lines. Flower color ranges from cream to bright yellow. Some horticultural varieties have been developed with variegated leaf color. The plants may grow to almost five feet in height. The leaves are mostly basal and are folded and clasp the stem at the base in a fan-like fashion.
A perennial, yellow flag iris will remain green during the winter where the weather is mild, but leaves will die back during periods of prolonged drought or below-freezing temperatures. It spreads both by seed and by stout underground stems called rhizomes from which its roots can grow to a foot in length.
After flowering, the large seed capsules of yellow iris are up to 2.5 inches long and contain many dark to reddish-brown seeds. When not flowering, yellow flag iris may be confused with cattail (Typha latifolia) or broad-fruited bur-reed (Sparganium eurycarpum). Look for the fruits in the summer, or the fan-shaped plant-base at other times of year.
Up to several hundred flowering plants may be connected by rhizomes. Fragments of rhizome can form new plants if they break off and drift to suitable habitat. The flowers are pollinated by bumblebees and long-tongued flies. Seeds germinate and grow well after being burned in late summer. Yellow flag readily resprouts from rhizomes after burning.
As a popular ornamental plant for wet areas or well-mulched soil, yellow flag is widely sold in nurseries and on the Web. It has often been planted in wastewater or storm water treatment ponds, been used to control erosion and is known to take up metals and nutrients in wastewater treatment facilities. It is a popular garden plant for wet or well-mulched soil, and has been introduced as an ornamental throughout the world.
"Yellow flag is being widely distributed by water garden enthusiasts," said Hulting. "This likely leads to unintentional releases in urban and suburban wetlands and eventually wetlands across the landscape, spread by seed and rhizome fragments during high water events. We'd like to help gardeners become more aware of the ecological effects invasive plants like yellow iris might have on the environment.”
The best control is prevention, said Hulting. "Learn to identify it and don't plant it. Encourage other gardeners not to plant it."
Yellow flag is listed on the Oregon Department of Agriculture's Noxious B list, meaning it is locally invasive and not yet widespread. The Pacific Northwest Exotic Pest Plant Council lists it as 'A-2 Most Invasive-Regional' (highly to moderately invasive but still with a potential to spread).
The OSU Extension Service recommends the following wetland native plants as being more ecologically appropriate alternatives to yellow flag: monkey flower, Rocky Mountain iris, Douglas iris and skunk cabbage. Also, these non-native ornamentals are less invasive: Japanese iris, Siberian iris and blue flag.
To help home gardeners and landscape designers make sound ecological choices about what to plant in their gardens, the Oregon State University Extension Service has published a 52-page booklet called GardenSmart Oregon (EC 1620), in cooperation with City of Portland, The Nature Conservancy, Oregon Association of Nurseries, Clackamas Community College, Oregon Public Broadcasting, OSU Extension Service and OSU Sea Grant. GardenSmart Oregon is available online; or, call 1-800-561-6719 to request a printed copy of GardenSmart Oregon ($3 per copy shipping and handling fee). Local county offices of the OSU Extension Service have copies available for no charge.
Pulling can control isolated plants of yellow flag iris or digging, but use care and protect your skin as resins in the leaves and rhizomes can cause irritation. Because rhizome fragments can grow to form new plants, be sure to clean up all fragments. Large-scale tillage to control yellow flag should be avoided because of the likelihood of spreading rhizome fragments, warned Hulting.
"Limiting soil disturbance can help native plants survive and make the site more resilient to reinvasion by yellow flag," he said.
If you can do nothing else, remove and destroy the seed heads and flowers of yellow flag, he advised.
Large-scale infestations of yellow flag most likely need to be managed by chemical means to achieve complete control and limit the spread. Formulations of glyphosate labeled for aquatic uses (examples: Rodeo or Aquamaster) have been effective on yellow flag iris when applied as a spot treatment. Take care to not overspray onto desirable plants when making glyphosate applications. Follow the mixing directions and application instructions on each label.
"Cut stump" treatments, or cutting or mowing foliage and applying glyphosate directly to the cut surface of individual plants have also been effective for managing smaller infestations of yellow flag iris and can negate herbicide injury to non-target plants. Because this plant is a rhizomatous perennial multiple applications over multiple years will likely be needed for complete control, said Hulting.
No biological control organisms have been approved for yellow flag iris.