Yellow flag iris

Iris pseudacorus (Iridaceae) is a vigorous rhizomatous iris native to Europe, northern Asia, the Middle East, and northern Africa. It grows 3 to 5 feet tall, forming large colonies via thick rhizomes and prolific seed production. The bright yellow flowers measure 3 to 4 inches across, featuring darker yellow zones with brown or violet veining on the falls, and appear on rigid upright branching stalks in late spring to early summer.

Yellow flag iris thrives in full sun in saturated soil or standing water up to 12 inches deep, though it also grows in average garden soil with consistent moisture. It favors acidic, moisture-retentive conditions. This species spreads aggressively and has naturalized across much of North America, posing significant invasion risk near streams, ponds, and wetlands. Seed pods are valued for dried floral arrangements. Bloom begins around 601 GDD (base 50 F). All parts are mildly toxic.