florida Early Detection Network


Yellow iris, Yellow flag


Iris pseudacorus L.


Iris family


Iris pseudacorus

Flowers-bright yellow form


Flower - pale yellow form



Inside of cut rhizome





Synonyms: None


Botanical Glossary

Iris pseudacorus is a herbaceous perennial that can grow up to 0.9-1.2 m (3-4 ft.) in height. The broad, sword-shaped leaves are stiff, erect and glaucous. They measure between 0.5-1 m (1.6-3.3 ft.) long and 1-3 cm (0.4-1.2 in.) wide. The rhizomes are pink-fleshed and 1-4 cm (0.4-1.6 in.) in diameter. The showy flowers of Iris pseudacorus bloom from April to June. Most often they are yellow, but their color can also range from nearly white to cream. The flowers are 7-9 cm (2.75-3.5 in.) wide. They are borne on erect peduncles and there are several flowers on each stem. There are six perianth segments that are clawed. Three of these are upward-pointing petals and three are downward spreading sepals. These sepals often have purple, brown or red veins on their yellow surface. The fruits are 4-8 cm (1.6-2.7 in.) long capsules. These capsules are 6-angled and cylindric-prismatic to ellipsoid in shape. The average capsule contains around 120 seeds that start out white, then harden and turn brown as the season goes on. Page References Bailey 273, Crow & Hellquist 323, Fernald 462, Gleason & Cronquist 484, Holmgren 810, Magee & Ahles 357, Newcomb 120, Peterson & McKenny 100, Seymour 191. See reference section below for full citations.


Iris versicolor L. (Northern blue flag) Picture of I. versicolor, Typha spp. (cattails) Picture of Typha spp. When Iris pseudacorus is in bloom, its yellow flower distinguishes it from native irises. Iris versicolor has a purple/blue flower and its fruits are 3-angled prismatic-cylindric. While the leaves of Typha spp. are similar, if either plant is in flower they are easily told apart.


Iris pseudacorus's primary agent of dispersal is water. However, the rhizomes are often planted by gardeners, enabling its further spread into the environment.


Iris pseudacorus is native to all the countries of Europe except Iceland; it is also native to the Caucasus Mountains, Western Asia and North Africa. In North America, it has been reported in Canada from Newfoundland to British Columbia. It is present in the majority of the United States, with the exception of a handful of western and mid-western states. Iris pseudacorus has been reported from all New England States.


Iris pseudacorus was most likely introduced via garden plantings around the middle 1800's. After naturalization, it spread throughout New England. This plant was also used for erosion control and in sewage treatment cells. A very early record of escaped Iris pseudacorus comes from the Hudson River basin in 1868, near Poughkeepsie. An early New England record is from Concord, Massachusetts in 1884.


Floodplain Forest, Lake or Pond, River or Stream, Yard or Garden. Iris pseudacorus can be found along the edges of lakes, ponds, rivers and streams and immersed in water up to 25 cm (10 in.) deep. It grows well in freshwater wetlands and can tolerate high acidity. In its native habitat, Iris pseudacorus can tolerate living in the upper zones of salt marshes, where it may be surrounded by saline water. The plant also tends not to favor calcareous substrates, though there are exceptions to this, such as along the Housatonic River of Connecticut.


Iris pseudacorus forms large clonal populations that displace native species. The rhizomes of this plant are able to survive rather heavy droughts. Both the rhizomes and seeds of this plant can be transported downstream, allowing for further spread of the plant. The seeds of Iris pseudacorus can germinate even after a wetland area burns. In its native habitat, this plant is not widely grazed because of the glycosides it contains, making it poisonous to grazing animals. No birds are known to disperse the seeds of this plant. Iris pseudacorus is still sold and used for water gardens. Caution should be used when hand-pulling this plant, as it can cause skin irritation.


Center for Aquatic and Invasive Plants, University of Florida

The Nature Conservancy


Documentation required: Specific photograph or mounted snippet of the flowers. Best time for documentation: Early summer.


Center for Aquatic and Invasive Plants, University of Florida General information including control and photographs

The PLANTS Database General information and a map

The Nature Conservancy Photographs and link to extensive description and control information

Integrated Taxonomic Information System Has general taxonomic information about the species


Bailey, L.H. 1949. Manual of Cultivated Plants. Macmillan, New York.

Crow G.E. and C.B. Hellquist. 2000. Aquatic and Wetland Plants of Northeastern North America. Vol #2. University of Wisconsin Press, Madison.

Fagerstedt, K. 1991. Flooding tolerance mechanisms in the rhizomes of the yellow flag iris (Iris pseudacorus). Physiologia Plantarum 82 (3): B11.

Fernald, M.L. 1950. Gray's Manual of Botany 8th edition. American Book Company, New York.

Gleason H.A. and A.C. Cronquist. 1991. Manual of Vascular Plants of the Northeastern United States and Adjacent Canada. 2nd ed. New York Botanical Garden, Bronx, New York.

Hanhijarvi, A.M. and K.V. Fagerstedt. 1995. Comparison of carbohydrate utilization and energy-charge in the yellow flag iris (Iris pseudacorus) and garden iris (Iris germanica) under anoxia. Physiologia Plantarum 93 (3): 493-497.

Holmgren N.H. 1998. Illustrated Companion to Gleason and Cronquist's Manual. New York Botanical Garden, Bronx, New York.

Magee D.W and H.E. Ahles. 1999. Flora of the Northeast. University of Massachusetts Press, Amherst.

Mulqueen, J. and T.N. Gleeson. 1988. Association of the yellow flag Iris pseudacorus L. with ground water seepage and its possible use as an indicator plant. Irish Journal of Agricultural Research 27 (1): 106-110.

Newcomb N. 1977. Newcomb's Wildflower Guide. Little Brown, Boston.

Peterson R.T. and M. McKenny. 1968. A field Guide to Wildflowers of Northeastern and North-central North America. Houghton Mifflin, Boston.

Seymour, F.C. 1969. The Flora of New England. Charles E. Tuttle Company, Inc., Tokyo, Japan.

Sutherland, W.J. 1990. Biological flora of the British Isles: Iris pseudacorus L. Journal of Ecology 78:833-848.

USDA, NRCS. 2001. The PLANTS Database, Version 3.1. (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.