Iris pseudacorus

Common Name(s): Yellow flag

Non-Native to Florida

yellow flag, pale yellow iris, water flag
Iridaceae/Iris Family

pronounced: ee-ris sood-a-ko-rus
from: iris (G.): Greek goddess of the rainbow and messenger to the gods
pseud (G.): false
acorus (G.): sweet flag genus of plants
Referring to similarity with the genus Acorus

None known

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.


  • herbaceous perennial
  • there is little information on response to light, water, temperature, nutrients or factors affecting photosynthesis and respiration (Sutherland 1990)
  • growing into thickets; production in southern Poland estimated to be 7-8 tons per hectare (Sutherland 1990)
  • clumped distribution in grasslands, more linear growth in woodlands (Sutherland 1990)
  • spreading by underground rhizomes and seeds
  • its leaves sometimes die back over winter, but persist if winters are mild
  • is tolerant of drought; “excavated rhizomes continue growing after three months without water” (Sutherland 1990)
  • seeds germinate and grow well after being burnt in late summer (Ellis 1965)
  • flowering in early spring in the south (Florida) and in summer in the north (Canada)
  • in British studies, germination was not light dependent; seeds did not germinate at temperatures of 15o C and below; optimal germination was at fairly high temps of 20/30o C; germination was increased by scarification; submerged seeds failed to germinate (Gedebo and Froud-Williams 1998); seeds will germinate after being kept in seawater for 31 days (Jessen 1955)
  • age, growth, flowering and mortality of buds can be determined from looking at bulges on the rhizomes (Sutherland and Walton 1990)
  • no hybrids have been recorded for yellow flag; however new cultivars have been produced with variegated leaves of other yellowish colors (Dykes 1974)


  • in temperate climates
  • grows in water to 25 cm deep (Sutherland 1990), or very near water, such as lakeside muds
  • has escaped mainly into freshwater wetlands
  • grows in salt marshes in Scotland (Gimingham 1964)
  • occurs from sea level to 300 m in Ireland
  • is pollinated by bumble-bees and long-tongued flies
  • tolerates high soil acidity, occurs from pH 3.6 to pH 7.7
  • has a high nitrogen requirement (Ellenberg 1979)
  • yellow flag rhizomes can withstand long periods of anoxia (low soil oxygen) (Hetherington, et al. 1983)

Iris pseudacorus L.
Original description

  • 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


  • there are about many species of Iris in the world, including several species native to the United States
  • Iris pseudacorus is native to Europe and the British Isles, North Africa and the Mediterranean region (Cody 1961)
  • occurs in all European countries except Iceland, occurs in Scandinavia to 68o N
  • it is a weed in New Zealand (Sutherland 1990)

Distribution in the U.S.

  • Iris pseudacorus is already found throughout most of the US and Canada, except the Rockies

The best way to track the spread of invasive aquatic plants may be to identify the drainage basins (watersheds) they have been discovered in. Drainage maps give useful information to eco-managers because drainage maps show precisely where the plants are, making it easier for managers to infer where the plants might go next, and thus where to take preventive measures.

How it got here

yellow iris for sale invasive plants for sale

  • the unfortunately attractive Iris pseudacorus continues to be sold through garden and plant dealers and over the Internet
  • Iris pseudacorus was brought to Canada and the U.S. as an ornamental plant in the early 1900s; it is also used as an erosion control plant, is used in sewage treatment cells (Gedebo & Froud-Williams, 1998), and is reportedly used as a dye plant and as a fiber plant (Kartesz, 1999)
  • the earliest New World record of this plant was made by Fernald who collected it in the wild in Newfoundland in 1911; it was established in British Columbia by 1931; by 1950, Gray’s Manual reported its distribution as “Newfoundland to Minnesota”; by 1961 yellow flag was reported to be so plentiful in Canadian swamps as to “have the appearance of a native plant” (Cody 1961)
  • in 1958, large populations of yellow flag were discovered in Ninepipes National Waterfowl Refuge, Montana (Preece 1964)
  • first reported in California in 1957, where in one river it excluded all other plants including Typha (Raven & Thomas 1970)

Potential to spread elsewhere in U.S.

  • So long as it is sold and transplanted without regard, Iris pseudacorus will continue to spread into the wild areas of the U.S.
  • It also spreads downstream by broken rhizomes and possibly by seed


  • 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).


aquatic plant harvesting machineaquatic plant chopping machine

The action of mechanical harvestors and chopping machines

No biological control work has been done for this species. Damage by invertebrate and vertebrate grazers is negligible; deer eat it sparingly; hay containing yellow flag causes gastroenteritis in cattle (Sutherland 1990).

man applies aquatic herbicidehelicopter applies aquatic herbicide

Iris pseudacorus is susceptible to many registered herbicides, but is resistant to terbutryne (Thomas 1982) provide
yellow flag colonized areas of open water created by herbicide spraying of Phragmites australis (Axell 1982).

What can you do?

Laws and lists

Iris pseudacorus

  • Connecticut: Invasive, banned
  • Massachusetts: Prohibited
  • Montana: Category 3 noxious weed
  • New Hampshire: Prohibited invasive species
  • Oregon: “B” designated weed, quarantine
  • Washington: Class C noxious weed

Want to know more?

The information contained on this wep page was extracted from published scientific literature and agency reports. It is important to know that plant research, like most areas of scientific research, is still relatively young and incomplete–much may have been published about the physiology of one plant but not about its management; much may have been published about how to culture and grow another plant but not about its natural ecology. Thousands of research articles may have been published about one invasive plant, but perhaps only a dozen about another.

If you want to read the research yourself, perhaps to clarify or expand an area of information contained here, or to help determine your own line of research, you are welcome to query the world’s largest collection of international scientific literature about aquatic, wetland and invasive plants, the APIRS bibliographic database, which contains more than 54,000 citations and their content keywords. Or you might want to ask us to do it for you and mail or e-mail the search results to you.

This is the literature about Iris pseudacorus that was used to develop this web page. More research items about this plant may be found at APIRS

  • Caffrey J, Monahan C. 1997. Natural aquatic plant colonisation in a newly constructed Irish canal. Internationale Revue der Gesamten Hydrobiologie 82(4):479-486
  • Cody WJ. 1961. Iris pseudacorus L. escaped from cultivation in Canada. Canadian Field Nat., 75: 139-142
  • Dykes WR. 1974. The Genus Iris. Cambridge University Press, Cambridge
  • Falinska K. 1986. Demography of Iris pseudacorus L. populations in abandoned meadows. Ekol. Polska 34(4):583-613
  • Gedebo A, Froud-Williams RJ. 1998. Seed biology of Impatiens glandulifera, Typha latifolia and Iris pseudacorus. In: Proceedings, 10th EWRS Symp. Aquatic Weeds, 1998, Lisbon, A. Monteiro et al., editors. pp. 55-58
  • Hanhijarvi AM, Fagerstedt KV. 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
  • Hetherington AM, Hunter MIS, Crawford RMM. 1983. Short communications–Survival of Iris species under anoxic conditions. Ann. Bot. 51:131-133
  • Jessen K. 1955. Is Iris pseudacorus thalassochorous? Acta Societatis pro Fauna et Flora Fennica 72:1-7
  • Judd WW. 1953. Iris pseudacorus L. established in the vicinity of London, Ontario. Rhodora 55:244
  • Kartesz
  • Kelly DL, Iremonger SF. 1997. Irish wetland woods: the plant communities and their ecology. Biology and Environment: Proceedings of the Royal Irish Academy 97B(1):1-32
  • Laublin G, Cappadocia M. 1992. In vitro ovary culture of some Apogon Garden Irises (Iris pseudacorus L., I. setosa Pall., I. versicolor L.) Botanica Acta 105(4):319-322
  • Mulqueen J, Gleeson TN. 1988. Association of the yellow flag (Iris pseudacorus L.) with ground water seepage and its possible use as an indicator plant. Irish J. Agric. Res. 27(1):106-110
  • O Criodain C, Doyle GJ. 1994. An overview of Irish small-sedge vegetation: syntaxonomy and a key to communities belonging to the Scheuchzerio-Caricetea Nigrae (Nordh. 1936) Tx. 1937. Biology and Environment 94B(2):127-144
  • Piccardi EB, Clauser M. 1983. Absorption of copper by Iris pseudacorus. Water, Air and Soil Pollution 19:185-192
  • Preece SJ. 1964. Iris pseudacorus in Montana. Proc. Montana Acad. Sci., 24:1-4
  • Raven PH, Thomas JH. 1970. Iris pseudacorus in western North America. Madrono. 20:390-391
  • Rubtzoff P. 1959. Iris pseudacorus and Caltha palustris in California. L. West. Bot., 9:31-32
  • THE BEST LITERATURE REVIEW — Sutherland WJ. 1990. Biological flora of the British Isles. Iris pseudacorus L. J. Ecology 78(3):833-848
  • Sutherland WJ, Walton D. 1990. The changes in morphology and demography of Iris pseudacorus L. at different heights on a saltmarsh. Functional Ecology 4(5):655-660
  • Wunderlin RP, Hansen BF, Bridges EL. 1995 (updated May 1996). Atlas of Florida vascular plants. Website:

yellow flagView the herbarium specimen image from the University of Florida Herbarium Digital Imaging Projects.

Sea Grant
This web page was authored in June, 2001, by Victor Ramey (Center for Aquatic and Invasive Plants, University of Florida), with significant contribution from Barbara Peichel (Sea Grant, University of Minnesota). The information contained herein is based on the literature found in the APIRS database.