Yellow Flag Iris (Iris pseudacorus) also known as the Yellow Flag, Yellow Iris, or Water Flag

Yellow Flag Iris is a species of flowering plant in the family Iridaceae that has invaded wetlands and other aquatic habitats. It is native to Europe, western Asia and northwest Africa. Its specific epithet pseudacorus means "false acorus", referring to the similarity of its leaves to those of Acorus calamus (sweet flag), as they have a prominently veined mid-rib and sword-like shape. However, the two plants are not closely related.  The sword-like leaves are flat, erect and linear with a raised midrib. The dark to blue-green blades are 25-90 cm long and have sharply pointed tips. The flowering stems are usually similar in size to the leaves (50 to 100 cm in length). Flowers are pale to bright yellow or cream colored and seven to nine centimeters wide. The large seed pod is three -sided and angular and turns from glossy green to brown as it ripens. Each pod contains dozens of seeds densely arranged in 3 rows. Roots are 10-30 cm in length, and the fleshy rhizomes are one to four centimeters diameter.

Small clumps can be dug out, though this is only effective if the rhizomes are entirely removed. Mowed plants will regenerate from the rhizomes, so plants must be cut multiple times to exhaust their energy reserves. The sap may cause skin irritation, so gloves should be worn when handling cut or otherwise damaged stems.

To learn more about it, and to see additional photographs, go to New York Invasive Species (IS) Information.