Iris pseudacorus - L.
Yellow Iris
Other English Common Names: Pale-yellow Iris
Other Common Names: paleyellow iris
Taxonomic Status: Accepted
Related ITIS Name(s): Iris pseudacorus L. (TSN 43194)
French Common Names: iris faux-acore
Unique Identifier: ELEMENT_GLOBAL.2.149941
Element Code: PMIRI090T0
Informal Taxonomy: Plants, Vascular - Flowering Plants - Iris Family
Kingdom Phylum Class Order Family Genus
Plantae Anthophyta Monocotyledoneae Liliales Iridaceae Iris
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Concept Reference
Concept Reference: Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Concept Reference Code: B94KAR01HQUS
Name Used in Concept Reference: Iris pseudacorus
Conservation Status

NatureServe Status

Global Status: GNR
Global Status Last Changed: 22Mar1994
Rounded Global Status: GNR - Not Yet Ranked
Nation: United States
National Status: NNA
Nation: Canada
National Status: NNA (03Nov2012)

U.S. & Canada State/Province Status
United States Alabama (SNA), Arkansas (SNA), California (SNA), Connecticut (SNA), Delaware (SNA), District of Columbia (SNA), Georgia (SNR), Idaho (SNA), Illinois (SNA), Indiana (SNA), Iowa (SNR), Kansas (SNA), Kentucky (SNA), Louisiana (SNA), Maine (SNA), Maryland (SNA), Massachusetts (SNR), Michigan (SNA), Minnesota (SNA), Missouri (SNA), Montana (SNA), Nebraska (SNA), Nevada (SNA), New Hampshire (SNA), New Jersey (SNA), New York (SNA), North Carolina (SNA), Ohio (SNA), Oregon (SNA), Pennsylvania (SNA), Rhode Island (SNA), South Carolina (SNA), Tennessee (SNA), Texas (SNA), Utah (SNA), Vermont (SNA), Virginia (SNA), Washington (SNA), West Virginia (SNA), Wisconsin (SNA)
Canada British Columbia (SNA), Manitoba (SNA), New Brunswick (SNA), Newfoundland Island (SNA), Nova Scotia (SNA), Ontario (SNA), Prince Edward Island (SNA), Quebec (SNA)

Other Statuses

NatureServe Global Conservation Status Factors

Other NatureServe Conservation Status Information

U.S. States and Canadian Provinces
Color legend for Distribution Map
NOTE: The distribution shown may be incomplete, particularly for some rapidly spreading exotic species.

U.S. & Canada State/Province Distribution
United States ALexotic, ARexotic, CAexotic, CTexotic, DCexotic, DEexotic, GA, IA, IDexotic, ILexotic, INexotic, KSexotic, KYexotic, LAexotic, MA, MDexotic, MEexotic, MIexotic, MNexotic, MOexotic, MTexotic, NCexotic, NEexotic, NHexotic, NJexotic, NVexotic, NYexotic, OHexotic, ORexotic, PAexotic, RIexotic, SCexotic, TNexotic, TXexotic, UTexotic, VAexotic, VTexotic, WAexotic, WIexotic, WVexotic
Canada BCexotic, MBexotic, NBexotic, NFexotic, NSexotic, ONexotic, PEexotic, QCexotic

Range Map
No map available.

Ecology & Life History Not yet assessed
Economic Attributes Not yet assessed
Management Summary Not yet assessed
Population/Occurrence Delineation Not yet assessed
Population/Occurrence Viability
U.S. Invasive Species Impact Rank (I-Rank)
Disclaimer: While I-Rank information is available over NatureServe Explorer, NatureServe is not actively developing or maintaining these data. Species with I-RANKs do not represent a random sample of species exotic in the United States; available assessments may be biased toward those species with higher-than-average impact.

I-Rank: High/Medium
Rounded I-Rank: High
I-Rank Reasons Summary: Iris pseudacorus is a popular ornamental species in the United States. It is widespread in riparian and other wetland habitats throughout much of the country, with the exclusion of the Rocky Mountain region and the southwest. Although it is likely near the limits of its generalized range, it continues to spread and increase locally. Pale yellow iris is a very strong competitor that forms dense, monotypic stands. Its rhizome mat can collect sediment, elevate topography, and interfere with stream flow, creating a drier habitat that is less suitable for many native species. Significant changes in community composition and successional trajectories have been reported at invaded sites. Management of this species by either mechanical removal of the rhizome mass or by cutting and spraying with glyphosate herbicide can be labor-intensive, as even small residual rhizome fragments can re-sprout vigorously, necessitating considerable follow-up treatment.
Subrank I - Ecological Impact: Medium/Low
Subrank II - Current Distribution/Abundance: High
Subrank III - Trend in Distribution/Abundance: Medium
Subrank IV - Management Difficulty: High/Medium
I-Rank Review Date: 30Nov2005
Evaluator: Gravuer, K.
Native anywhere in the U.S?
Native Range: Native throughout Europe, as well as in some parts of temperate Asia and northern Africa.
Northern Africa: Algeria, Morocco. Temperate Asia: Iran, Palestine, Syria, Turkey, Azerbaijan, Russian Federation [Ciscaucasia, Western Siberia]. Europe: Denmark, Finland, Ireland, Norway, Sweden, United Kingdom, Austria, Belgium, Czechoslovakia, Germany, Hungary, Netherlands, Poland, Switzerland, Belarus, Estonia, Latvia, Lithuania, Moldova, Russian Federation [European part], Ukraine [incl. Krym], Bulgaria, Greece [incl. Crete], Italy [incl. Sardinia, Sicily], Romania, Yugoslavia, France [incl. Corsica], Portugal [incl. Madeira Islands], Spain [incl. Baleares, Canary Islands] (GRIN 2001).

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Screening Questions

S-1. Established outside cultivation as a non-native? YES
Comments: This species is a non-native that is established outside of cultivation (Kartesz 1999).

S-2. Present in conservation areas or other native species habitat? Yes
Comments: Most frequently found in riparian habitats and in open, freshwater wetland ecosystems, and could probably be considered an aquatic species in some cases. Also found in brackish and salt marshes and in forested wetland habitats. Apparently found rarely along rocky coastal shores and may persist in more upland grassland environments when planted (Egler 1983, Jacono 2000, Mehrhoff et al. 2003, Tu 2003, Wisconsin State Herbarium 2005).

Subrank I - Ecological Impact: Medium/Low

1. Impact on Ecosystem Processes and System-wide Parameters:Medium/Low significance
Comments: The rhizome mat that this species creates collects sediment, elevates topography, and interferes with stream flow, creating a drier habitat (Tu 2003, Heckman 2005). This change in hydrology can have significant impacts on other wetland species in the ecosystem. For example, along the lower Potomac River, this species' elevation of the seed bank facilitated the conversion of riparian marshes into mesic forest dominated by Fraxinus species rather than by the willows (Salix spp.) that had formerly occurred there (Thomas 1980).

2. Impact on Ecological Community Structure:Medium/Low significance
Comments: The changes in layer density caused by this species are moderate to substantial. Although it frequently invades areas dominated by plants of somewhat similar life-form (e.g. freshwater marshes), the almost impenetrable thickets (ISSG 2005) that it forms can be significantly denser than the native vegetation. In addition, the rhizome mats that it creates on the soil surface can alter microhabitats and prevent germination of native species (Heckman 2005). These structural changes can alter successional trajectories (Tu 2003).

3. Impact on Ecological Community Composition:High/Moderate significance
Comments: This species is a fast-growing, strong competitor that forms dense, monotypic stands (Ramey and Peichel 2001, Weber 2003). Even native plant species that are thought to be somewhat aggressive themselves (e.g. Typha latifollia) are frequently out-competed (Washington State Noxious Weed Control Board 2003). The density of the stand can also exclude native animals (Tu 2003). In addition, because all parts of the plant are poisonous, I. pseudacorus does not provide food for native animals, and birds are not known to disperse the seeds (Mehrhoff et al. 2003, Tu 2003).

4. Impact on Individual Native Plant or Animal Species:High/Moderate significance
Comments: In Connecticut, I. pseudacorus was able to exclude the native arrow-arum (Peltandra virginica), a plant whose fruits are an important food of wood ducks during the nesting season (Cox 1999, cited in Tu 2003). It also out-competes the native cat-tail (Typha latifolia) (Washington State Noxious Weed Control Board 2003). In addition, it may impact riparian willows (Salix spp.) at some sites by elevating the seed bank (Thomas 1980). No impacts via hybridization are known (Ramey and Peichel 2001).

5. Conservation Significance of the Communities and Native Species Threatened:Moderate significance
Comments: In Texas, Iris pseudacorus has recently become established in the Frio River, one of the few entirely spring fed and still free flowing rivers in the state (Jacono 2000). In addition, the freshwater marsh ecosystems it invades are uncommon and therefore of conservation significance in some areas of its range (e.g. Miller and Golet 2000).

Subrank II. Current Distribution and Abundance: High

6. Current Range Size in Nation:High significance
Comments: Widely established in low-elevation areas throughout much of the contiguous U.S., except in the Rocky Mountain region (Kartesz 1999, Tu 2003, NRCS 2005). More frequent in the northeastern states than in the south and west, as its arrival in the western states appears to be relatively recent (perhaps the late 1940s; Jacono 2000). Nevertheless, it is abundantly established in many western and southern locations, including areas of TX, CA, WA, and MT (Jacono 2000).

7. Proportion of Current Range Where the Species is Negatively Impacting Biodiversity:High/Moderate significance
Comments: Reported as an invasive species or listed on some form of invasive or noxious list in 20 out of the 41 states in which it is established (Kartesz 1999, GRIN 2001, Mehrhoff et al. 2003, Tu 2003, Washington State Noxious Weed Control Board 2003, Czarapata 2005, MNDNR 2005, NRCS 2005, Rice 2005, Swearingen 2005, Wisconsin State Herbarium 2005). Seems to be most problematic in the northeast, upper Midwest, and Pacific northwest states (Mehrhoff et al. 2003, Washington State Noxious Weed Control Board 2003, Czarapata 2005); potentially not as invasive in the south because of the warmer temperatures there (Jacono 2000).

8. Proportion of Nation's Biogeographic Units Invaded:High significance
Comments: More than 35 ecoregions are invaded, based on visual comparison of the generalized range and ecoregions map (The Nature Conservancy 2001).

9. Diversity of Habitats or Ecological Systems Invaded in Nation:Moderate significance
Comments: Has invaded a diverse range of habitats from sea level to 300 meters (Tu 2003). Usually in sites with a continuously high soil-water content across a range of soil types (Weber 2003, Tu 2003). Tolerant of high soil acidity, long periods of anoxia, and at least some salinity, and established rhizomes can withstand moderate droughts (Tu 2003, Ramey and Peichel 2001, Mehrhoff et al. 2003, King County Noxious Weed Control Board 2005). Requires high levels of nitrogen and prefers sites in sun or partial shade (Czarapata 2005, Whitinger 2005). Most frequently found in riparian habitats (e.g. stream banks and lake shores, including their man-made analogues such as ditches) and in open, freshwater wetland ecosystems. Could probably be considered an aquatic species in some cases (e.g. when rhizomes form floating mats, when it occurs on the edges of floating mats of other vegetation, and when in occurs in coarse-textured stream beds next to riffles (Jacono 2000, Weber 2003, Wisconsin State Herbarium 2005)). Also found in brackish and salt marshes, although some sources indicated that it occurs in these environments more often in its native range than in seems to in the U.S. (Mehrhoff et al. 2003). Several sources listed forested wetland habitats, including swamps and floodplain forests (Flora of North America Editorial Committee 2002, Mehrhoff et al. 2003, Tu 2003). Apparently found rarely along rocky coastal shores (Tu 2003) and may persist in more upland grassland environments when planted (Egler 1983).

Subrank III. Trend in Distribution and Abundance: Medium

10. Current Trend in Total Range within Nation:Moderate significance
Comments: In New England, I. pseudacorus has been planted since at least the mid-1800s and was reported as escaped by 1868 (Mehrhoff et al. 2003). However, its spread to the western U.S. appears to be relatively recent, with a 1948 report from Washington and late-1950s reports from Montana and California being among the species' earliest western records (Ramey and Peichel 2001, King County Noxious Weed Control Board 2005). It also appears to be recently spreading southward, with new Texas populations reported in 1998 (Jacono 2000).

11. Proportion of Potential Range Currently Occupied:Low significance
Comments: This USDA hardiness zones for this species are zone 4a to zone 9a (Whitinger 2005). Therefore, it may not be suited to most of AK and HI and would probably have difficulty establishing there. Much of the southwest that it has not already invaded may also be unsuitable due to insufficient precipitation, as this species prefers sites with a continuously high soil-water content (Tu 2003). The Rocky Mountains are probably also unsuitable, as the species seems to require lowland areas (< 300 m) (Tu 2003). Nevertheless, there is potentially for additional spread within some of the regions it has already invaded, such as the Pacific Northwest (King County Noxious Weed Control Board 2005).

12. Long-distance Dispersal Potential within Nation:High significance
Comments: Popular in the U.S. primarily as an ornamental species in wet gardens and pond plantings (Jacono 2000). Also used for erosion control, in dye and fiber production, and in waste water and storm-water treatment ponds for heavy metal fitration (WISC 2004). Widely available for purchase, including online purchase (Ramey and Peichel 2001, King County Noxious Weed Control Board 2005, Whitinger 2005). In addition, both rhizome fragments and seeds can also disperse long distances by water; seeds are buoyant and can remain so for at least 7 months (Tu 2003).

13. Local Range Expansion or Change in Abundance:Moderate significance
Comments: Local spread and increase is occurring most rapidly in recently invaded regions, such as the Pacific Northwest (Heckman 2005). However, some degree of local spread is likely occurring throughout much of the range, as it is still widely available and sold as a garden ornamental (Ramey and Peichel 2001, Tu 2003).

14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats:High/Moderate significance
Comments: This species prefers sun or partially shaded light conditions (Whitinger 2005). Therefore, disturbance events often create more favorable habitat for invasion by increasing light levels. However, a number of sources noted that the species does not appear to be dependent on disturbance for establishment (Heckman 2005), and that it can establish in a variety of existing vegetation types (APRS Implementation Team 2001).

15. Similar Habitats Invaded Elsewhere:Low significance
Comments: Also naturalized in at least Canada, Australia, New Zealand, Argentina, and Japan (Randall 2002). Invaded habitats appear to be largely similar (e.g. riparian areas and freshwater wetlands, Csurhes and Edwards 1998).

16. Reproductive Characteristics:High significance
Comments: Strongly exhibits the following characteristics: reproduces readily both vegetatively and by seed, has quickly spreading rhizomes, resprouts readily when burned, and fragments easily with fragments capable of dispersing and subsequently becoming established (Ramey and Peichel 2001, Tu 2003, Washington State Noxious Weed Control Board 2003, Weber 2003, ISSG 2005).

Subrank IV. General Management Difficulty: High/Medium

17. General Management Difficulty:High/Moderate significance
Comments: This species is difficult to manage once firmly established (Tu 2003). Small, isolated patches can be controlled by physical removal of the entire rhizome system, but because rhizomes resprout vigorously, substantial investment of time and labor is required (Tu 2003). Also, resins in leaves and rhizomes can cause skin irritation in humans (King County Noxious Weed Control Board 2005), which may limit the potential for volunteer pulling efforts. Larger patches can be controlled by cutting followed by application of a glyphosate herbicide suitable for aquatic environments (Tu 2003). However, follow-up treatments are necessary with this method as well (Tu 2003).

18. Minimum Time Commitment:Moderate significance
Comments: Although this species does not have a long-term seed bank (Peat and Fitter 2005), the ability of even very small rhizome fragements to resprout means that follow-up control for a number of years will likely be necessary (Tu 2003).

19. Impacts of Management on Native Species:Medium/Low significance
Comments: Mechanical removal in sensitive areas, such as shallow stream beds, may cause extensive disturbance and permit the establishment of other unwanted plants (Jacono 2000). Therefore, if herbicides are not an option at a site, significant impacts on native species may occur. Cutting followed by herbicide will also likely have some collateral impacts, because the large surface area of this species' foliage may make very precise targeting of the herbicide difficult.

20. Accessibility of Invaded Areas:Moderate significance
Comments: Because this species invades wetlands, some accessibility issues may be present. Also, the very high density of the thickets combined with the skin irritation that the plant causes can present substantial obstacles to individuals attempting to control the species on foot.

Botanical data developed by NatureServe and its network of natural heritage programs (see Local Programs), The North Carolina Botanical Garden, and other contributors and cooperators (see Sources).

  • Alien plants ranking system (APRS) Implementation Team. 2001b. Alien plants ranking system version 7.1. Southwest Exotic Plant Information Clearinghouse, Flagstaff, AZ. Online. Available: (Accessed 2005).

  • Csurhes, S. and R. Edwards. 1998. Potential Environmental Weeds in Australia. Appendix C, in Candidate Species for Preventitive Control. National Weeds Program, Queensland Department of Natural Resources. Online. Accessed 2005.

  • Czarapata, E. J. 2005. Invasive Plants of the Upper Midwest. The University of Wisconsin Press. Madison, WI. 215 pp.

  • DiTomaso, J.M. and E.A. Healy. 2003. Aquatic and riparian weeds of the West. Regents of University of California, Division of Agriculture and Natural Resources, Publication 3421.

  • Egler, F. E. 1983. The nature of naturalization II. Studies in naturalization: 1925-1980. The introduced flora of Aton Forest, Connecticut. Publication No. 6, Claude E. Phillips Herbarium, Department of Agriculture and Natural Resources, Delaware State College, Dover, Delaware.

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  • Jacono, C.C. 2000, 30 September 2001 last update. Iris pseudacorus. United States Department of the Interior, Biological Resources Division, Center for Aquatic Resource Studies. Online. Available: (Accessed 2005).

  • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.

  • Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.

  • King County Noxious Weed Control Board. 2005. Yellow flag (Iris pseudacorus L.). Natural Resources and Parks, Water and Land Resources Division, Seattle, Washington. Online. Available: (Accessed 2005).

  • Meades, S.J. & Hay, S.G; Brouillet, L. 2000. Annotated Checklist of Vascular Plants of Newfoundland and Labrador. Memorial University Botanical Gardens, St John's NF. 237pp.

  • Mehrhoff, L.J., J.A. Silander, Jr., S.A. Leicht and E. Mosher. 2003. IPANE: Invasive Plant Atlas of New England. Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT. Online. Available:

  • Miller, N. A. and F. C. Golet. 2000. Development of a statewide freshwater wetland restoration strategy. Phase 1: Site identification and prioritization methods. Department of Natural Resources Science, University of Rhode Island, for RIDEM Office of Water Resources.

  • Minnesota Department of Natural Resources (MNDNR). 2005. Yellow iris (Iris pseudacorus). Minnesota invasive non-native terrestrial plants, an identification guide for resource managers. (Accessed 2005).

  • Newhouser, M. 2004. Part IV. Plant Assessment Form, for use with "Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands" by the California Exotic Pest Plant Council and the Southwest Vegetation Management Association: Iris pseudacorus. Available: (Accessed 2005)

  • Peat, H., and A. Fitter. 2005. The Ecological Flora of the British Isles at the University of York. Available: (Accessed 2005).

  • Ramey, V. and B. Peichel. 2001. Non-Native Invasive Aquatic Plants in the United States: Iris pseudacorus. Center for Aquatic and Invasive Plants, University of Florida and Sea Grant, University of Minnesota. Online. Available: (Accessed 2005).

  • Randall, R.P. 2002. A global compendium of weeds. R.G. and F.J. Richardson, Melbourne. 905 pp.

  • Rice, P.M. 2005. Invaders Database System. Division of Biological Sciences, University of Montana, Missoula. Online. Available: (Accessed 2005).

  • Swearingen, J. 2005. Alien plant invaders of natural areas. Plant Conservation Alliance, Alien Plant Working Group. Available: (Accessed 2005)

  • The Nature Conservancy. 2001. Map: TNC Ecoregions of the United States. Modification of Bailey Ecoregions. Online . Accessed May 2003.

  • Thomas, L. K. 1980a. The impact of three exotic plant species on a Potomac island. National Park Service scientific monograph series; No. 13. U.S. Department of the Interior, Washington, D.C. 179 pp.

  • Tu, M. 2003. Element stewardship abstract: Iris pseudacorus. The Nature Conservancy. Online. Available: (Accessed 2005).

  • USDA NRCS. 2005. The PLANTS Database, Version 3.5 ( National Plant Data Center (, Baton Rouge, LA.

  • USDA, ARS, National Genetic Resources Program. 2001. Germplasm Resources Information Network - (GRIN). [Online Database]. National Germplasm Resources Laboratory, Beltsville, Maryland.URL: (Accessed 2005)

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  • Whitinger, D. 2005. Dave's Garden: PlantFiles. Online. Available: (Accessed 2005)

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