Iris pseudacorus - Invasive Aquatic Plants of the United States

Non-Native Invasive Aquatic Plants in the United States

Center for Aquatic and Invasive Plants, University of Florida
and
Sea Grant

Habit
Habitat
Identification
Might be confused with...
Origin
U.S. distribution
How it got here...
Spread potential
Problems/Effects
Control
What can you do?
Laws and lists
Want to know more?
The Literature
Other web sites
Page author

IRIS PSEUDACORUS

pronounced: ee-ris sood-a-ko-rus (long/short marks) (audio clip)

from:

Referring to similarity with the genus Acorus

Synonymy:

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.

Habit:

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 15^o C and below; optimal germination was at fairly high temps of 20/30^o 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)

Habitat:

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)

Quick ID:

Iris pseudacorus

More complete identification:

Iris pseudacorus

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

Origin:

there are about ___ 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 68^o 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

How it got here:

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

Problems/Effects:

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).

Control:

Iris pseudacorus is difficult to control in much the same way that cat-tails are difficult to control. Once an infestation, with its extensive rhizomes, has taken hold, machines and possibly fire are the only possibilities for management.

mechanical harvestors and chopping machines

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

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)

From the University of Florida Aquatic Weed Management Guide, Vandiver 1999:

Restoration:

What can you do?

There's plenty

you can do to help.

Laws and lists:

Iris pseudacorus

is "state-listed" only by Vermont and Oregon, though it occurs in most U.S.

Want to know more?

The information contained on this web 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: http://www.usf.edu/~isb/projects/atlas/atlas.html

Other web sites that treat Iris pseudacorus:

http://www.univ-lehavre.fr/cybernat/pages/irispseu.htm

USGS Nonindigenous Aquatic Species Fact Sheets

http://nas.er.usgs.gov/plants/docs/ir_pseud.htm

This web page was authored in July, 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, the world's largest collection of scientific literature about aquatic, wetland and invasive plants. For this project, Jessica Munsch serves as the library research assistant; Mary Langeland searches the literature; Laura Line is the illustrator; and Ann Murray is the photographer and web image specialist.

Anecdotal information about Iris pseudacorus would be appreciated by users of this web site. Please submit anecdotal information, additional reference citations and corrections for this page to varamey@nersp.nerdc.ufl.edu