Yellow Flag Iris
Yellow flag is non-native in the U.S., and is spreading throughout the country.
Yellow flag (Iris pseudacorusis) a wetland plant that is especially showy during its short blooming period. This good-looking plant has been transplanted into well-watered gardens all over the world and has widely escaped; it is also used in sewage treatment, and is known to be able to remove metals from wastewaters. Like cat-tails, yellow iris colonizes into large numbers, forming very dense monotypic stands, outcompeting other plants.
Yellow flag dies back in harsh winter conditions, but the rhizomes will overwinter. In spring the long leaves and flower stalks regrow from the rhizomes and flower by 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. The leaves are mostly basal and are folded and clasp the stem at the base in a fan-like fashion. Yellow flag iris is perennial, and will remain green during winter where the weather is mild. It has stout rhizomes and long, spreading roots. Seeds form in large, glossy green, triangular capsules. The seeds are corky. The plants spread rhizomatously and grow tightly bunched together.
This is the only yellow iris found in Washington’s wet areas, but when not flowering it 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
If pulling or digging yellow flag care should be used to protect the skin as resins in the leaves and rhizomes can cause irritation. Because rhizome fragments can grow to form new plants, care must be taken to collect all fragments.
In addition to pulling or digging, a systemic chemical called Glyphosate can be applied to the iris which is very effective.