Yellow flag (Iris pseudacorus L.)

Family: Iridaceae

Description and Variation: When flowering, yellow flag is unmistakable with its showy yellow flowers colorfully displayed along the edge of water and in wetlands. In Washington, the flowers occur 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 3 large (3 to 8 cm) downward facing yellow sepals and three smaller upward pointing petals.  The sepals are often streaked with brown to purple lines.  The plant, including flower stalk, is up to 1.5 m tall.  The leaves are mostly basal and are folded and clasp the stem at the base in a fan-like fashion.  They stand erect or bent at the top, with shorter leaves toward the outside of the plant. Yellow flag iris is perennial, and will remain green during winter where the weather is mild.  It has stout rhizomes 1 to 4 cm in diameter and roots to 30 cm long.  The fruits are a large capsule to 8 cm long.  It is 3-angled, glossy green and contains many flattened brown seeds.  The seeds are corky and about 7 mm across.  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.

There is little variation in the appearance of yellow flag, aside from flowers which may range from cream to bright yellow.  Some horticultural varieties have been developed with variegated leaf color.

 Economic Importance: Yellow flag is a popular ornamental plant for wet areas or well-mulched soil.  It is widely sold in nurseries and on the internet.  It has often been planted in wastewater or stormwater treatment ponds.

Yellow flag has been used medicinally.  The roots have been used for several ailments, but all parts of the plant can also causes vomiting and diarrhea.  Flowers have been used to make a yellow dye, and the roots a black or brown dye.

 It will sicken livestock if ingested, and is generally avoided by herbivores (although muskrats will eat the rhizomes).  Contact with the resins can cause skin irritation in humans.

Yellow flag is listed on invasive species lists in Vermont, Virginia, Connecticut, and Massachusetts.   It is also considered invasive in New Zealand and Australia. 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).

Geographic Distribution: Yellow flag is native to Europe, Great Britain, North Africa and the Mediterranean region.  It has been introduced in temperate areas nearly world wide and occurs throughout the United States except in the Rocky Mountains.  It is found in wet areas throughout Washington, though it appears to be most common near developed areas.

Habitat: Yellow flag grows in temperate wetlands (to 68° N in Scandinavia).  It is found on both sides of the Cascades in wetlands and along the margins of lakes and slow-moving rivers.  It will grow in water to .25 meters deep, though is most common in very shallow water or mud.  It will tolerate drying and anoxic sediment and is also tolerant of at least some salinity, as it is found in brackish marshes in its native range.  It is also tolerant of high soil acidity, occurring from pH 3.6 to 7.7.  It does well in nutrient rich conditions, and has a high nitrogen requirement.  It prefers part shade or full sun exposure.