The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

Iris pseudacorus
Iris pseudacorus
(yellow iris)
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Iris pseudacorus L.

Common name: yellow iris

Synonyms and Other Names: Flag iris, paleyellow iris, pale-yellow iris, yellow flag, yellow flag iris, tall yellow iris, water flag, water iris, European yellow iris, Iris pseudoacorus, Iris pseudocorus, Iris pseudacoris

Taxonomy: available through www.itis.govITIS logo

Identification: Iris pseudacorus is a perennial, emergent aquatic plant ranging from 0.5–1.5 m in height (Campbell et al. 2010, Forest Health Staff 2006). Its inflorescence units consist of 4–12 flowers per stem; 6–9 cm spathes are green with brown margins. The outer spathe is strongly keeled, while the inner is without keel; they are subequal and the margins are not dry or membranous. Bright yellow flowers are approximately 7–9 cm wide and occasionally have brown/purple veins at the base of lanceolate to spatulate petals (Lui et al. 2010, Noxious Weed Control Program 2009). Each flower has three downward sepals (5–7.5 cm by 3–4 cm) and three upward petals (2–3 cm) and a floral tube 0.6–0.8 cm (Lui et al. 2010). Flowers typically bloom from April-June (Forest Health Staff 2006). Fruit are prismatic, 6-angled, glossy green capsules (3.5–8.5 cm); individual plants may produce up to 6 pods (Campbell et al. 2010, Jacobs et al 2011). Each capsule may release up to 120 lustrous brown, flattened, D-shaped seeds (6–7 mm), but a small fraction of these are actually viable (Campbell et al. 2010, Jacobs et al. 2011). The corky seeds are buoyant, with 95% of them able to float for up to 2 months (Forest Health Staff 2006, Jacobs et al. 2011, Lui et al. 2010, Noxious Weed Control Program 2009).
The basal deciduous leaves are smooth, stiff, broad, dark green with a gray/blue cast and have a central ridge (40–100 cm by 2–3 cm) (Forest Health Staff 2006, Lui et al. 2010). Stems are usually solid, unbranched, and 70–150 cm in length (Lui et al. 2010). The plant remains green during mild winters (Noxious Weed Control Program 2009).

The fleshy roots are about 10–30 cm long (Lui et al. 2010). This species also has numerous, thick, pink tuberous rhizomes (2–3 cm in diameter) that are freely branching and may form extensive clumps (Noxious Weed Control Program 2009). If broken, rhizomes release black sap (Jacobs et al. 2011).

When not in bloom, it can be difficult to distinguish I. pseudacorus from native irises (Lui et al. 2010, Sarver et al. 2008). It can be distinguished from Northern blue flag iris, which has a three-angled seed capsule (yellow iris has a six-angled capsule) (Campbell et al. 2010). When in bloom, it is easy to distinguish because it is the only iris that grows completely yellow in natural environments (Goodridge et al. 2011).

Size: 0.5-1.5m tall

Native Range: Eurasia.

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Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Nonindigenous Occurrences:

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Iris pseudacorus are found here.

StateYear of earliest observationYear of last observationTotal HUCs with observations†HUCs with observations†
Alabama198220082Cahaba; South Atlantic-Gulf Region
Arkansas198519975Arkansas-White-Red Region; Bayou Bartholomew; Lower Arkansas-Maumelle; Lower Saline; Upper Ouachita
California1948201850Aliso-San Onofre; Big-Navarro-Garcia; Butte Creek; Central Coastal; Clear Creek-Sacramento River; Cottonwood Creek; Coyote; Gualala-Salmon; Honcut Headwaters-Lower Feather; Lake Tahoe; Lower American; Lower Eel; Lower Pit; Lower Sacramento; Mad-Redwood; Mattole; Middle Kern-Upper Tehachapi-Grapevine; Middle San Joaquin-Lower Chowchilla; Mojave; Monterey Bay; Newport Bay; North Fork American; Russian; Sacramento Headwaters; Salinas; San Diego; San Francisco Bay; San Francisco Coastal South; San Gabriel; San Joaquin; San Joaquin Delta; San Luis Rey-Escondido; San Pablo Bay; San Pedro Channel Islands; Santa Ana; Santa Barbara Channel Islands; Santa Barbara Coastal; Santa Clara; Santa Margarita; Santa Monica Bay; Santa Ynez; Shasta; South Fork Kern; Suisun Bay; Tomales-Drake Bays; Upper Merced; Upper Mokelumne; Upper San Joaquin; Ventura; Whitewater River
Colorado201120187Arkansas Headwaters; Cache La Poudre; Clear; Colorado Headwaters-Plateau; Middle South Platte-Cherry Creek; Upper Arkansas; Upper South Platte
Connecticut190320133Lower Connecticut; New England Region; Shetucket
District of Columbia195120171Middle Potomac-Anacostia-Occoquan
Florida194320194Apalachee Bay-St. Marks; Chipola; Lower Ochlockonee; Oklawaha
Georgia196120165Conasauga; Coosa-Tallapoosa; Middle Chattahoochee-Lake Harding; Upper Coosa; Withlacoochee
Idaho197420188Brownlee Reservoir; Coeur d'Alene Lake; Hells Canyon; Lower Boise; Lower Clark Fork; Middle Snake-Succor; Pend Oreille Lake; Upper Spokane
Illinois195520159Chicago; Des Plaines; Lower Illinois; Lower Illinois-Senachwine Lake; Rock; Upper Illinois; Upper Mississippi Region; Upper Mississippi-Meramec; Upper Sangamon
Indiana198220186Chicago; Kankakee; Little Calumet-Galien; Ohio Region; Upper Wabash; Upper White
Kansas195920103Middle Kansas; South Fork Beaver; Upper Marais Des Cygnes
Kentucky198619904Lower Ohio; Lower Ohio-Salt; Ohio Brush-Whiteoak; Upper Green
Louisiana197219937Calcasieu-Mermentau; Eastern Louisiana Coastal; Lake Maurepas; Lower Mississippi; Lower Mississippi Region; Lower Red-Ouachita; Red-Saline
Maine196720125Lower Androscoggin; Maine Coastal; Piscataqua-Salmon Falls; Presumpscot; Saco
Maryland196920177Chester-Sassafras; Choptank; Middle Potomac-Anacostia-Occoquan; Monocacy; Nanticoke; Severn; Tangier
Massachusetts1884201112Blackstone; Cape Cod; Charles; Chicopee; Concord; Deerfield; Housatonic; Merrimack River; Middle Connecticut; Narragansett; Nashua; New England Region
Michigan1932201940Au Gres-Rifle; Au Sable; Betsie-Platte; Black-Macatawa; Boardman-Charlevoix; Brule; Carp-Pine; Cheboygan; Clinton; Detroit; Flint; Great Lakes Region; Huron; Kalamazoo; Kawkawlin-Pine; Lake Erie; Lake Huron; Lake Michigan; Lake St. Clair; Little Calumet-Galien; Lower Grand; Manistee; Manistique; Maple; Michigamme; Muskegon; Ontonagon; Ottawa-Stony; Pere Marquette-White; Pigeon-Wiscoggin; Raisin; Saginaw; Saginaw; Shiawassee; Southeastern Lake Michigan; St. Joseph; Tahquamenon; Thunder Bay; Tittabawassee; Upper Grand
Minnesota1991201821Beaver-Lester; Big Fork; Buffalo; Buffalo-Whitewater; Chippewa; Clearwater; Cloquet; Crow; Elk-Nokasippi; Kettle; Little Fork; Lower Minnesota; Lower Rainy; Lower St. Croix; Otter Tail; Prairie-Willow; South Fork Crow; St. Louis; Twin Cities; Upper Mississippi-Crow-Rum; Upper St. Croix
Missouri193620157Cahokia-Joachim; Lower Missouri-Moreau; Lower Osage; Meramec; Osage; Spring; Upper Mississippi-Meramec
Montana195820138Bitterroot; Blackfoot; Clarks Fork Yellowstone; Flathead Lake; Gallatin; Lower Clark Fork; Lower Flathead; Pend Oreille
Nebraska197720133Big Blue; Niobrara Headwaters; Salt
Nevada200820181Las Vegas Wash
New Hampshire196420119Black-Ottauquechee; Contoocook; Merrimack River; Piscataqua-Salmon Falls; Saco; Upper Connecticut; Upper Connecticut-Mascoma; Waits; West
New Jersey193820178Cohansey-Maurice; Hackensack-Passaic; Middle Delaware-Mongaup-Brodhead; Middle Delaware-Musconetcong; Mullica-Toms; Raritan; Rondout; Sandy Hook-Staten Island
New Mexico200720173Mora; Rio Grande-Albuquerque; Rio Grande-Santa Fe
New York1868201819Black; Great Lakes Region; Hudson-Hoosic; Hudson-Wappinger; Lower Hudson; Lower Hudson; Middle Hudson; Mohawk; Oak Orchard-Twelvemile; Oneida; Oswego; Owego-Wappasening; Sacandaga; Seneca; Southwestern Lake Ontario; St. Lawrence; Upper Allegheny; Upper Genesee; Upper Hudson
North Carolina1967201914Albemarle; Cape Fear; Haw; Lower Catawba; Lower Yadkin; Neuse-Pamlico; New River; Nolichucky; South Atlantic-Gulf Region; Upper Cape Fear; Upper Catawba; Upper French Broad; Upper Neuse; Watauga
Ohio190320133Lake Erie; Paint; Tuscarawas
Oregon1959201846Alsea; Brownlee Reservoir; Clackamas; Coast Fork Willamette; Coos; Coquille; Deschutes; Hells Canyon; Klamath; Lost; Lower Columbia; Lower Columbia-Clatskanie; Lower Columbia-Sandy; Lower Crooked; Lower Deschutes; Lower Owyhee; Lower Rogue; Lower Snake; Lower Willamette; Middle Columbia-Hood; Middle Columbia-Lake Wallula; Middle Fork Willamette; Middle Rogue; Middle Snake-Payette; Middle Willamette; Molalla-Pudding; Necanicum; North Santiam; Pacific Northwest; Pacific Northwest Region; Powder; Siletz-Yaquina; Siltcoos; Siuslaw; Sixes; South Santiam; Trout; Tualatin; Umpqua; Upper Deschutes; Upper Klamath Lake; Upper Willamette; Walla Walla; Willamette; Wilson-Trusk-Nestuccu; Yamhill
Pennsylvania1923201425Conewango; Connoquenessing; Crosswicks-Neshaminy; French; Lake Erie; Lehigh; Lower Allegheny; Lower Delaware; Lower Susquehanna; Lower Susquehanna-Swatara; Middle Allegheny-Redbank; Middle Allegheny-Tionesta; Middle Delaware-Musconetcong; Pine; Raystown; Schuylkill; Shenango; Susquehanna; Upper Delaware; Upper Juniata; Upper Ohio; Upper Susquehanna-Tunkhannock; Upper West Branch Susquehanna; West Branch Susquehanna; Youghiogheny
Rhode Island192720064Massachusetts-Rhode Island Coastal; Narragansett; Pawcatuck-Wood; Virginian
South Carolina193120194Coastal Carolina; Cooper; Enoree; Saluda
South Dakota198519851Snake
Tennessee1993200811French Broad-Holston; Lower Cumberland; Lower French Broad; Lower Little Tennessee; Middle Tennessee-Elk; Nolichucky; Red; South Fork Holston; Upper Tennessee; Upper Tennessee; Watauga
Texas19725Lower West Fork Trinity; Pine Island Bayou; San Marcos; Upper Frio; West Galveston Bay
Utah197420142Jordan; Utah Lake
Vermont190920077Black-Ottauquechee; Lake Champlain; Missiquoi River; Otter Creek; St. Francois River; West; Winooski River
Virginia1992200114French Broad-Holston; James; Lower Chesapeake; Lower James; Lower Potomac; Lower Rappahannock; Mid Atlantic Region; Middle New; Potomac; Upper Clinch; Upper James; Upper Levisa; Upper New; Upper Tennessee
Washington1971201853Banks Lake; Chief Joseph; Colville; Deschutes; Dungeness-Elwha; Duwamish; Franklin D. Roosevelt Lake; Grays Harbor; Hoh-Quillayute; Hood Canal; Kettle; Lake Washington; Lewis; Little Spokane; Lower Chehalis; Lower Columbia; Lower Columbia; Lower Columbia-Clatskanie; Lower Columbia-Sandy; Lower Cowlitz; Lower Crab; Lower Skagit; Lower Spokane; Lower Yakima; Middle Columbia-Hood; Middle Columbia-Lake Wallula; Naches; Nisqually; Nooksack; Okanogan; Pacific Northwest Region; Palouse; Pend Oreille; Puget Sound; Puget Sound; Puyallup; San Juan Islands; Similkameen; Skykomish; Snohomish; Snoqualmie; Stillaguamish; Strait of Georgia; Upper Chehalis; Upper Columbia-Entiat; Upper Columbia-Priest Rapids; Upper Cowlitz; Upper Spokane; Upper Yakima; Walla Walla; Wenatchee; Willapa Bay; Yakima
West Virginia197820115Cheat; Greenbrier; Kanawha; Monongahela; Upper Ohio
Wisconsin1983201834Bad-Montreal; Baraboo; Beartrap-Nemadji; Black-Presque Isle; Castle Rock; Door-Kewaunee; Flambeau; La Crosse-Pine; Lake Dubay; Lake Michigan; Lake Superior; Lower Chippewa; Lower Fox; Lower St. Croix; Lower Wisconsin; Manitowoc-Sheboygan; Middle Rock; Milwaukee; Namekagon; Northwestern Lake Michigan; Oconto; Peshtigo; Red Cedar; South Fork Flambeau; Southwestern Lake Michigan; St. Croix; St. Louis; Upper Chippewa; Upper Fox; Upper Fox; Upper Rock; Upper St. Croix; Upper Wisconsin; Wolf

Table last updated 8/29/2019

† Populations may not be currently present.

* HUCs are not listed for states where the observation(s) cannot be approximated to a HUC (e.g. state centroids or Canadian provinces).

Ecology: Iris pseudacorus is a hearty, perennial monocot found in nutrient rich (especially nitrogen) environments such as wetlands, swamps, floodplain forests, and wet shores of rivers and lakes (Forest Health Staff 2009, Kim et al. 2009, Vymazal and Kröpfelová 2008). Plants are highly tolerant to anoxic conditions and are able to grow vigorously in water/wet soil with a wide range of pH values (Blokhina et al. 2003, Forest Health Staff 2009). Iris pseudacorus also tolerates salt, but grows taller as soil salinity decreases (Sutherland and Walton 1990). While initial colonization may be favored in silty areas, colonies can also root in pebbly/rocky substrate associated with stream riffles (Jacono 2001).

Plants require three years of growth before they reach maturity and are able to flower (Tyron 2006 in Noxious Weed Control Program 2009). Iris pseudacorus is pollinated by long-tongues flies and bumblebees, including Bombus pagans, B. ferpidus, and B. pennsyhankus (Dieringer 1982, Noxious Weed Control Program 2009). Buoyant seeds spread in flowing water and will germinate along shore edges; they typically do not germinate while immersed in water (Noxious Weed Control Program 2009). Iris pseudacorus also forms thick, tuberous rhizomes that spread radially to produce large clonal populations of up to several hundred flowering “individuals”. These populations form dense, underwater mats of vegetation (ISCBC 2012). Rhizomes can split to produce up to 10 plants per year (Je´han et al. 1994 in Kim et al. 2009). These rhizomes are drought tolerant, but during floods, both rhizomes and seeds may be transplanted downstream (Sutherland 1990).

Germination from seed is moderately successful. Sutherland (1990) reported a germination rate of 48% from freshly collected seed in the British Isles, yet in the field found seedlings to be rare in most habitats (Britain and Europe). In western Montana seedlings of Iris pseudacorus are numerous (Preece 1964). Fresh seed collected from plants escaping cultivation in a north Florida swamp exhibited a germination rate of 62% (Jacono and Ramey, unpublished data).

Yellow iris is poisonous; insects and animals tend not to feed on this plant in its native range (Forest Health Staff 2006).

Means of Introduction: Yellow iris is a horticultural favorite and often escapes cultivation to spread locally along shorelines, stream flats, and into fresh and brackish marshes. It is planted for its showy yellow spring flowers having sepals (falls) faintly etched in brown or purple and for its erect, flat, swordlike leaves.

Status: Established.

Impact of Introduction: By 1970 yellow iris was found growing to the complete exclusion of Typha and other native marsh plants along the Merced River in California (Raven and Thomas 1970). It currently occurs along 1300 miles of irrigation canals and laterals near Flathead Lake in northwestern Montana (Lake County Weed District, Pablo, Mont., pers. comm. 2001).

Remarks: The Frio River's water originates from a deep artesian source and remains cool year round. This factor likely contributes to the unusual aggressiveness of this European, normally colder climate species, in the southern U.S.

Many thanks to Clare Lee, US Fish and Wildlife Service, for contributing photographs and occurrence information.

References: (click for full references)

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Barbolani, E., M. Clauser, F. Pantani, and R. Gellini. 1986. Residual heavy metal (Cu and Cd) removal by Iris pseudacorus. Water, Air, and Soil Pollution 28:277—282.

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Jacobs, J., M. Pokorny, J. Mangold, and M. Graves-Medley. 2011. Biology, Ecology and Management of Yellowflag Iris (Iris pseudacorus L.). Montana State University Extension. Bozeman, MT. 12 pp.

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Author: Morgan, V.H., L. Berent, and A. Fusaro

Revision Date: 8/8/2018

Citation Information:
Morgan, V.H., L. Berent, and A. Fusaro, 2019, Iris pseudacorus L.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=1115, Revision Date: 8/8/2018, Access Date: 9/15/2019

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2019]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [9/15/2019].

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