Yellow-Flag Iris | Iris pseudacorus L.

California Pest Rating for
Yellow-Flag Iris  |  Iris pseudacorus L.
Family:  Iridaceae
Pest Rating: B |  Proposed Seed Rating: R

Initiating Event:

Iris pseudacorus was introduced in California in the early 1950’s and had no previous pest rating. A pest rating proposal is required to determine a permanent rating for this pest.

History & Status:

Background: Iris pseudacorus, commonly called yellow-flag iris, is a rhizomatous beardless wetland iris that is native to Europe, northern Asia, the Middle East and northern Africa. It has naturalized in much of North America, particularly in the eastern U.S. It is at home in wet soils, typically forming large colonies along streams, ponds and marshes2.

Iris pseudacorus is a perennial, emergent aquatic plant ranging from 0.5–1.5 m in height. It has bright yellow flowers (3-4” across), with a darker yellow zone and brown or violet veining on each fall. It blooms in late spring to early summer on rigid, upright, branched stalks. Each flower stalk bears 4-12 flowers. Flowers give way to large seed pods4. Plant roots have been used in the past for a variety of purposes including medical treatments, dyes, inks, and snuff. Plant seeds have been used as a coffee substitute with no caffeine2.

Worldwide Distribution: Iris pseudacorus is native to all the countries of Europe except Iceland; it is also native to the Caucasus Mountains, Western Asia and North Africa. In North America, it has been reported in Canada from Newfoundland to British Columbia. It is present in the majority of the United States, with the exception of a handful of western and mid-western states4Iris pseudacorus has been reported from Delaware, Maryland,  North Carolina, Oregon, Tennessee, Virginia, Washington, Wisconsin, West Virginia and all New England states3,4.

Official Control: Iris pseudacorus is listed as noxious weed in these states: CN, MA, MN, NH, OR and WA 7.

California Distribution:  Iris pseudacorus is reported from Sacramento, San Diego, Solano, Siskiyou, Merced, San Luis Obispo, Contra Costa, Butte, Los Angeles, Shasta, San Mateo, Monterey, San Bernardino, Ventura, Santa Barbara, Kern, Mendocino, Riverside,  Orange and Madera counties3.

California Interceptions: Two PDR’s (413865 Siskiyou and 1349823 Contra Costa) were reported in the Pest and Damage Record Database by CDFA5.   There were 69 vouchers from all over the California submitted since the 1990’s.

The risk Iris pseudacorus (Yellow-flag iris) would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Iris pseudacorus could grow on wet soil and it is highly adapted to acidic soil. The overall California climate is perfect fit for its ability to spread aggressively in wetlands. Its widespread distribution demonstrates its ability to occupy wetlands. Therefore, Iris pseudacorus receives a Medium (2) in this category.

Evaluate if the pest would have suitable hosts and climate to establish in California:

Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Iris pseudacorus do not require any one  host, but grow wherever ecological conditions are favorable. It receives a High (3) in this category.

Evaluate the host range of the pest.

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

 – High (3) has a wide host range.

3) Pest Dispersal Potential: Iris pseudacorus propagate via seed and rhizomes.  The corky seeds are buoyant, with 95% of them able to float for up to 2 months and germinate along shore edges; they typically do not germinate while immersed in water. The thick, tuberous rhizomes spreads radially to produce large clonal populations of up to several hundred flowering “stems”. Rhizomes can split to produce up to 10 plants per year. These rhizomes are drought tolerant, but during floods, both rhizomes and seeds may be washed downstream4. It receives a High (3) in this category

Evaluate the natural and artificial dispersal potential of the pest.

Low (1) does not have high reproductive or dispersal potential.

Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Iris pseudacorus can impede irrigation and swimming. It may be reduce flow and block irrigation systems and flood control ditches. Its seeds can clog pipes and water control structures. Removal of plant material from these systems may require herbicides or excavation equipment and can be costly4.

It can cause gastroenteritis in cattle if ingested, and it contains glycosides that can cause skin irritation in wildlife that come in contact with this plant5.

It receives a Medium (2) in this category.

Evaluate the economic impact of the pest to California using the criteria below.

Economic Impact:   F, G

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Economic Impact Score: 2

Low (1) causes 0 or 1 of these impacts.

Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

5) Environmental Impact: Iris pseudacorus could invade the water systems of California, disrupt natural lake communities and potentially lower biodiversity by dominating lake margins. This vegetative growth can also trap sediment, raise the local elevation of the ecosystem, and alter wetland hydrology. The clonal nature of Iris pseudacorus causes it to form dense stands that could affect populations of sensitive species such as Mason’s lilaeopsis (Lilaeopsis masonii), CA Clapper rail (Rallus obsoletus), Suisun aster (Symphyotrichum lentum) and Delta tula pea (Lathyrus jepsonii), San Diego ambrosia (Ambrosia pumila) and water howellia (Howelia aquatilis). Iris pseudacorus receives a High (3) in this category.

Evaluate the environmental impact of the pest on California using the criteria below.

Environmental Impact: A B, C

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Environmental Impact: Score: 3

Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Iris pseudacorus (Yellow-flag iris): High (13)

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Iris pseudacorus is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas. It receives a Medium (-2) in this category.

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included:

Score: -2

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: Medium (11)


Iris pseudacorus has been in California a long time, but has spread slowly in wetlands. It has the potential to spread to more acreage and it is likely that this plant will come to dominant many new areas and increase its density and distribution.

Conclusion and Rating Justification:

A bad weed in wetlands in California. Deserves a “B” rating as it has invaded many areas to which it is adapted and undoubtedly has the ability to spread further. Because of this potential future harm, a “B” rating is justified.

  1. Connick, S. and M. Gerel. Partnering to prevent invasions of plants of horticultural origin. California Invasive Plant Council Symposium 2004. Ventura, CA.   Accessed February 10, 2017
  1. Missouri Botanical Garden on line. Accessed February 15, 2017
  1. Jepson Herbarium. Online  UC Berkeley.  Accessed February 15, 2017
  1. Morgan, V.H., L. Berent and A. Fusaro.    Glansis online. Accessed February 15, 2017.
  1. Pest and Damage Record Database, California Department of Food and Agriculture, Plant Health and Pest Prevention Services.
  1. USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD). Accessed February 15, 2017.

USDA data for State Noxious weeds. Accessed March 1, 2017


Javaid Iqbal, California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6695;[@]

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650;[@]

Pest Rating: B |  Proposed Seed Rating: R

Posted by ls