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)

Copyright Info
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.

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.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
AL198220082Cahaba; South Atlantic-Gulf Region
AK200620173Chichagof Island; Ketchikan; Kuiu-Kupreanof-Mitkof Islands
AZ202120211Upper Verde
AR1985202010Arkansas-White-Red Region; Bayou Bartholomew; Illinois; Little Red; Lower Arkansas-Maumelle; Lower Ouachita-Bayou De Loutre; Lower Ouachita-Smackover; Lower Saline; Upper Ouachita; Upper Saline
CA1948201945Aliso-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 Sacramento; Middle Kern-Upper Tehachapi-Grapevine; Middle San Joaquin-Lower Chowchilla; Mojave; Monterey Bay; Newport Bay; North Fork American; North Fork Feather; Russian; San Diego; San Francisco Bay; 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
CO201120187Arkansas Headwaters; Cache La Poudre; Clear; Colorado Headwaters-Plateau; Middle South Platte-Cherry Creek; Upper Arkansas; Upper South Platte
CT190320133New England Region; Outlet Connecticut River; Shetucket River
DE189520202Brandywine-Christina; Broadkill-Smyrna
DC195120211Middle Potomac-Anacostia-Occoquan
FL194320204Apalachee Bay-St. Marks; Chipola; Lower Ochlockonee; Oklawaha
GA1961202011Conasauga; Coosa-Tallapoosa; Etowah; Middle Chattahoochee-Lake Harding; Middle Savannah; Middle Tennessee-Chickamauga; Oostanaula; Upper Chattahoochee; Upper Coosa; Upper Ocmulgee; Withlacoochee
ID1974201912American Falls; Brownlee Reservoir; Clearwater; Coeur d'Alene Lake; Hells Canyon; Lower Boise; Lower Clark Fork; Lower Salmon; Middle Snake-Succor; Pend Oreille Lake; South Fork Clearwater; Upper Spokane
IL1955201513Chicago; Des Plaines; Kankakee; La Moine; Lower Fox; Lower Illinois; Lower Illinois-Senachwine Lake; Pike-Root; Rock; Upper Illinois; Upper Mississippi Region; Upper Mississippi-Meramec; Upper Sangamon
IN198220185Kankakee; Little Calumet-Galien; Ohio Region; Upper Wabash; Upper White
KS195920195Little Arkansas; Middle Arkansas-Slate; Middle Kansas; South Fork Beaver; Upper Marais Des Cygnes
KY198619904Lower Ohio; Lower Ohio-Salt; Ohio Brush-Whiteoak; Upper Green
LA197220209Bayou D'Arbonne; Calcasieu-Mermentau; Eastern Louisiana Coastal; Lake Maurepas; Lower Mississippi; Lower Mississippi Region; Lower Ouachita; Lower Red-Ouachita; Red-Saline
ME196720235Lower Androscoggin River; Maine Coastal; Piscataqua-Salmon Falls; Presumpscot; Saco River
MD1969202114Chester-Sassafras; Choptank; Gunpowder-Patapsco; Lower Potomac; Middle Potomac-Anacostia-Occoquan; Middle Potomac-Catoctin; Monocacy; Nanticoke; North Branch Potomac; Patuxent; Pokomoke-Western Lower Delmarva; Severn; Tangier; Upper Chesapeake Bay
MA1884201312Ashuelot River-Connecticut River; Blackstone River; Cape Cod; Charles; Chicopee River; Concord River; Deerfield River; Housatonic; Merrimack River; Narragansett; Nashua River; New England Region
MI1932202141Au 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 River; Maple; Michigamme; Muskegon; Ontonagon; Ottawa-Stony; Pere Marquette-White; Pigeon-Wiscoggin; Raisin; Saginaw; Saginaw; Shiawassee; Southeastern Lake Michigan; St. Joseph; Tahquamenon; Thornapple; Thunder Bay; Tittabawassee; Upper Grand
MN1991201822Beaver-Lester; Big Fork; Buffalo; Buffalo-Whitewater; Chippewa; Clearwater; Cloquet; Crow; Elk-Nokasippi; Kettle; Lake Superior; 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
MO193620208Cahokia-Joachim; Lower Missouri-Moreau; Lower Osage; Meramec; Osage; Peruque-Piasa; Spring; Upper Mississippi-Meramec
MT195820139Bitterroot; Blackfoot; Clarks Fork Yellowstone; Flathead Lake; Flint-Rock; Gallatin; Lower Clark Fork; Lower Flathead; Pend Oreille
NE197720204Big Blue; Lower Platte; Niobrara Headwaters; Salt
NV200820212Las Vegas Wash; Truckee
NH1964201111Ammonoosuc River-Connecticut River; Black River-Connecticut River; Contoocook River; Headwaters Connecticut River; Lower Androscoggin River; Merrimack River; Pemigewasset River; Piscataqua-Salmon Falls; Saco River; Waits River-Connecticut River; West River-Connecticut River
NJ1938202010Cohansey-Maurice; Great Egg Harbor; Hackensack-Passaic; Lower Delaware; Middle Delaware-Mongaup-Brodhead; Middle Delaware-Musconetcong; Mullica-Toms; Raritan; Rondout; Sandy Hook-Staten Island
NM200720214Jemez; Mora; Rio Grande-Albuquerque; Rio Grande-Santa Fe
NY1868202241Black; Bronx; Buffalo-Eighteenmile; Cattaraugus; Chemung; Chenango; Conewango; French; Great Lakes Region; Hackensack-Passaic; Headwaters St. Lawrence River; Housatonic; Hudson-Hoosic; Hudson-Wappinger; Irondequoit-Ninemile; Lake Champlain; Lake Ontario; Lower Genesee; Lower Hudson; Lower Hudson; Middle Hudson; Mohawk; Niagara River; Northern Long Island; Oak Orchard-Twelvemile; Oneida; Oswego; Owego-Wappasening; Rondout; Sacandaga; Salmon-Sandy; Sandy Hook-Staten Island; Seneca; Southern Long Island; Southwestern Lake Ontario; St. Lawrence; Upper Allegheny; Upper Delaware; Upper Genesee; Upper Hudson; Upper Susquehanna
NC1967202321Albemarle; Cape Fear; Haw; Lower Catawba; Lower Neuse; Lower Yadkin; Middle Neuse; Neuse-Pamlico; New River; Nolichucky; Pamlico; Seneca; South Atlantic-Gulf Region; Tugaloo; Upper Cape Fear; Upper Catawba; Upper French Broad; Upper Neuse; Upper Tar; Watauga, North Carolina, Tennessee; White Oak River
OH190320229Ashtabula-Chagrin; Black-Rocky; Cuyahoga; Hocking; Lake Erie; Little Scioto-Tygarts; Mahoning; Paint; Tuscarawas
OR1959201950Alsea; Applegate; Brownlee Reservoir; Clackamas; Coast Fork Willamette; Coos; Coquille; Deschutes; Goose Lake; 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 John Day; 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; Wallowa; Willamette; Wilson-Trusk-Nestuccu; Yamhill
PA1923202140Bald Eagle; Chautauqua-Conneaut; Conemaugh; Conewango; Connoquenessing; Conococheague-Opequon; Crosswicks-Neshaminy; French; Lackawaxen; Lake Erie; Lehigh; Lower Allegheny; Lower Delaware; Lower Delaware; Lower Juniata; Lower Monongahela; Lower Susquehanna; Lower Susquehanna; Lower Susquehanna-Penns; Lower Susquehanna-Swatara; Lower West Branch Susquehanna; Mahoning; Middle Allegheny-Redbank; Middle Allegheny-Tionesta; Middle Delaware-Mongaup-Brodhead; Middle Delaware-Musconetcong; North Branch Potomac; Pine; Raystown; Schuylkill; Shenango; Susquehanna; Upper Delaware; Upper Juniata; Upper Ohio; Upper Susquehanna; Upper Susquehanna-Tunkhannock; Upper West Branch Susquehanna; West Branch Susquehanna; Youghiogheny
RI192720064Massachusetts-Rhode Island Coastal; Narragansett; Pawcatuck River; Virginian
SC193120205Coastal Carolina; Cooper; Enoree; Saluda; Seneca
TN1993201914French Broad-Holston; Lower Clinch; Lower Cumberland; Lower French Broad; Lower Little Tennessee; Middle Tennessee-Elk; Nolichucky; Red; South Fork Holston; Upper Clinch, Tennessee, Virginia; Upper Tennessee; Upper Tennessee; Watauga, North Carolina, Tennessee; Watts Bar Lake
TX19725Lower West Fork Trinity; Pine Island Bayou; San Marcos; Upper Frio; West Galveston Bay
UT197420214Jordan; Lower Weber; Provo; Utah Lake
VT190920159Black River-Connecticut River; Hudson-Hoosic; Lake Champlain; Missiquoi River; Otter Creek; St. Francois River; West River-Connecticut River; White River; Winooski River
VA1992202026Appomattox; Blackwater; French Broad-Holston; James; Lower Chesapeake; Lower James; Lower Potomac; Lower Rappahannock; Mid Atlantic Region; Middle James-Willis; Middle New; Middle Potomac-Anacostia-Occoquan; Middle Potomac-Catoctin; North Fork Holston; Pamunkey; Pokomoke-Western Lower Delmarva; Potomac; Rapidan-Upper Rappahannock; Rivanna; South Fork Shenandoah; Upper Clinch, Tennessee, Virginia; Upper James; Upper Levisa; Upper New; Upper Roanoke; Upper Tennessee
WA1971202153Banks 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
WV1978202210Cheat; Gauley; Greenbrier; Kanawha; Lower Kanawha; Monongahela; Tug; Upper Monongahela; Upper Ohio; West Fork
WI1983201934Bad-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 7/12/2024

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

Revision Date: 8/8/2018

Citation Information:
Morgan, V.H., L. Berent, and A. Fusaro, 2024, 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: 7/12/2024

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. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [7/12/2024].

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For general information and questions about the database, contact Wesley Daniel. For problems and technical issues, contact Matthew Neilson.