Yellow Flag Iris
A class “C” noxious weed in Benton County
Yellow Flag Iris spreads aggressively. It blocks water flow, damages bird nesting sites, and fish habitat. It also threatens native wetland vegetation such as cattails, sedges and rushes.
Yellow flag Iris is a perennial that has sword-like leaves and it looks like cattails until flowering. Flowers resemble a common garden iris.
In late spring and early summer, it develops large yellow flowers that have three larger and downward facing yellow sepals and three smaller upward pointing petals.
After the flowers are gone in the summer and early fall, large drooping green seed pods remain. Leaves may remain green during a mild winter but the leaves will usually die back due to drought or below-freezing temperatures. The plant spreads by these seed pods and by thick brown rhizomes that clump together to form a massive root base.
Some control may be obtained for plants in standing water by cutting all leaves and stems below the waterline. Removing the seed pods will prevent some dispersal, but will not harm the plants or prevent them from continuing to sprout from the rhizomes. Covering plants with a heavily weighted tarp that extends well beyond the edges of the patch for several years can control small patches as well. Small stands can be controlled by hand removal but please wear gloves and use caution because the sap can cause skin irritation. Plant parts should be disposed of responsibly, as rhizomes can re-sprout if left on the ground. This is also why the area should be monitored for regrowth because it is easy to miss rhizomes during removal.
Yellow flag iris can be controlled using specific herbicides. Herbicide information can be taken from the Pacific Northwest Weed Management Handbook. Keep in mind that herbicide applications that are made to a body of water must be made by a licensed herbicide applicator according to the label. Also permits may be required to control aquatic plants. Larger sites will need a plan that combines several control methods.