Iris pseudacorus

Yellow flag

Nonnative to Florida

Species Overview

Yellow flag is non-native in the U.S., and is spreading throughout the country.

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.

Species Characteristics

  • monocot, forb, perennial, forming dense stands of robust plants
  • stout rhizomes, 1-4 cm in diameter; roots10-30 cm long.
  • leaves erect with upper part arching; leaves flattened, arising in a fan from the soil; raised midrib; sword-like, fine-pointed; 3-4 feet in height
  • flowers on erect stalks (peduncles) 3-4 feet in height; bisexual; large, showy, pale to deep yellow; the only yellow iris in the U.S.; several flowers on each stem; flowers having 6 clawed perianth segments including 3 large downward-spreading sepals and 3 smaller erect petals; on each flower sepal (yellow, large and petal-looking) are patterns of delicate light-brownish to purple veins or flecks
  • fruit a capsule (seed pod); large (4-8 cm, (to 4 in.)), 3-angled cylindrical, glossy green; many flattened brown seeds

Iris pseudacorus especially when not in bloom, might be confused with

  • native irises, which have more-or-less identical leaf structure and size
  • the larger Typha (cat-tail) species, which look similar in structure and height


  • Iris pseudacorus is a fast-growing and fast-spreading invasive plant that can outcompete other wetland plants, forming almost impenetrable thickets, in much the same was as cat-tails (Typha) do. “Individuals produce from several dozen to several hundred rooted rosettes and flowering shoots connected by durable rhizomes” (Falinska 1986).