Yellow Flag Iris

What is yellow flag iris?

Yellow flag iris is a European iris sold for water gardens and once planted for erosion control and in stormwater ponds. It has since escaped into wetlands across most of the country. In bloom, yellow flag iris identification is easy, since it is the classic big yellow iris standing in the water; out of bloom it takes a closer look, because the leaves resemble cattail and native irises.

Its rhizomes spread into dense stands and even floating mats that shut out native wetland plants, including tough natives like cattail. Thick clumps trap sediment, change how water moves along a shoreline and leave less food for wildlife, since every part of the plant is poisonous. It also handles a lot: drying, low-oxygen mud, mildly salty water and acidic soil.

Yellow Flag Iris identification: what to look for

Check plants in late spring for the yellow flowers, then look at the base of the leaves and, in summer, for seed pods. If you dig, wear gloves; the sap can irritate skin.

• Leaves: Broad, flat, stiff and sword-shaped, bluish-green, folded and overlapping in a fan where they clasp the base. They often stay green through mild winters.
• Flowers: Classic iris shape, 3 to 4 inches across, pale to deep yellow. Three large sepals droop outward, often with brown or purple veining, and three smaller petals stand upright.
• Seed pods: Glossy green, angled capsules 2 to 4 inches long in clusters below the spent flowers, packed with flat, corky brown seeds.
• Rhizomes: Thick, fleshy and pinkish, forming a dense mat. Several hundred flowering shoots can be linked by one rhizome network.
• Habit: Dense clumps along shorelines and in shallow water up to about 10 inches deep, sometimes as floating mats.

Where it grows in the United States

Yellow flag iris now grows across most of the United States except the Rocky Mountain region. It was first collected in North America in Newfoundland in 1911 and has grown in Washington since at least 1948, where it now turns up in wet areas statewide. Wisconsin restricts it, and several eastern states list it as an invasive plant.

It does best in very shallow water and wet mud along ponds, lakes and slow rivers, but it also grows in marshes, swamps, bogs, wet woods and on drier ground nearby. It takes full sun or part shade and likes fertile soil, and in Washington it is most common near developed areas, where it has been planted.

Yellow Flag Iris look-alikes

A few plants get mixed up with it:

• Blue flag iris (Iris versicolor): A native iris that looks very similar out of bloom, but it is smaller, does not form dense clumps or floating mats, and its flowers are blue rather than yellow.
• Broadleaf cattail (Typha latifolia): Can be mistaken for non-flowering iris. Check the leaf base: yellow flag iris leaves fold and overlap in a flat fan, and in summer it carries plump seed pods.
• Bur-reed and sweet flag (Sparganium eurycarpum, Acorus spp.): Other strap-leaved wetland plants that can be confused with iris when it is not flowering. Use the same fan-shaped leaf base and seed-pod checks.

How to get rid of yellow flag iris

Pulling, digging and mowing: Small clumps can be dug out, but get every piece of rhizome, because broken bits resprout and can float to new spots. Wear gloves and long sleeves. The most important step is removing flower stalks and seed pods, which you can do any time of year; bag ripe pods. For new, small patches, cut the plants down and cover them with heavy landscape fabric or tarps for several years. Covering long stretches of shoreline usually requires permits. Burning a stand in place is not a fix: seeds sprout well after a late-summer burn and rhizomes resprout.

Cultural control matters most, because the right conditions keep it from coming back.

• Do not plant it in water gardens or ponds, and remove any you already have.
• Replant cleared shorelines with native wetland plants so the iris has competition.
• Bag rhizomes and seed pods for the landfill rather than leaving them near water.
• There are no biological control insects available for this plant.

Herbicides: Glyphosate and imazapyr are the herbicides commonly used on yellow flag iris. Both are non-selective and will injure other plants they touch, and imazapyr can leave bare ground for a while. One low-drift method is cutting the plants and then wiping glyphosate onto the cut leaves with a wick applicator. Apply to actively growing foliage in summer or fall before a killing frost, with fall treatments generally more effective, and treat before bloom when possible to protect pollinators. Use aquatic-labeled products anywhere near water, follow the label exactly (the label is the law), and keep people and pets away until the spray dries. Permits may be needed for treating wetlands and shorelines, and in Washington aquatic herbicides can only be bought and applied by licensed applicators. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Do not buy it for water features, and check that any yellow iris you plant near water is not this species. Pull seedlings early, cut flower heads before they set seed, and watch the shoreline downstream of known patches, since seeds and rhizome pieces travel on the water.

Questions we get about yellow flag iris

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