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

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).

First Great Lakes record: 1886, Lake Ontario (USEPA 2008).

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from plants escaping cultivation in a north Florida swamp exhibited a germination rate of 62% (Jacono and Ramey, unpublished data).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).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Iris pseudacorus has a high environmental impact in the Great Lakes.
Realized:
Iris pseudacorus is tolerant of a range of water conditions (salinity, poor water quality, drought) that enable it to be a good competitor in a variety of wetland ecosystems (Sarver et al. 2008). The clonal nature of I. pseudacorus causes it to form dense stands and thick, submerged rhizome mats (Idaho Invasives 2007, Lui et al. 2010) that can prevent the germination and growth of native species (sedges, rushes, etc.) and eventually displace them entirely (Lui et al. 2010, MNDNR 2012, Noxious Weed Control Program 2009, ODA 2012). This vegetative growth can also trap sediment, raise local elevation of the ecosystem, and alter wetland hydrology (Noxious Weed Control Program 2009, Sarver et al. 2008). Populations of yellow iris create a positive feedback loop: once established, the roots trap sediment, which enables growth of new seedlings, which in turn trap more sediment (Jacobs et al. 2011). This increase in sedimentation also creates new habitat for shrubs and trees, thereby altering it to a drier ecosystem (Lui et al. 2010, Sarver et al. 2008). This alteration reduces the food supply and nesting habitat of many fish and waterfowl that depend on wetlands (Noxious Weed Control Program 2009, ODA 2012). Iris pseudacorus can also out-compete neighboring plants for pollinators (Dieringer 1982).
Yellow iris contains glycosides that can cause skin irritation in wildlife that come in contact with this plant; animals can also experience non-fatal poisoning if plant matter is ingested (Lui et al. 2010).

Potential:
In wetland ecosystems in the eastern United States, I. pseudacorus has reduced the density of native sedges and rushes that serve as habitat for other species, especially waterfowl (Jacobs et al. 2011). Populations of I. pseudacorus in Montana have been reported to reduce a stream’s width by up to 10 inches a year (Tyron 2006 in Noxious Weed Control Program 2009).

Iris pseudacorus may be a competitive threat to native irises, including I. brevicaulis (listed as threatened in Ohio), I. cristata, I. lacustris (endemic to the Great Lakes), I. robusta [versicolor x virginica] (endemic to the Great Lakes), I. setosa, I. verna (listed as threatened in Ohio,) I. versicolor, I. virginica, and I. virginica var. shrives (USDA and NRCS 2012, ODNR 2012). In Connecticut, I. pseudacorus successfully excluded native arrow-arum (Peltandra virginica), which is an important food source for nesting wood ducks (Cox 1999). 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 later expanded along 1300 miles of irrigation canals and laterals near Flathead Lake in northwestern Montana (Lake County Weed District, Pablo, Mont., pers. comm. 2001).
Yellow iris has a high anoxia tolerance. During the growing season, it can survive at least 28 days of dark and anoxia; this period is increased to 60 days for overwintering plants (Schlüter and Crawford 2001). When exposed to prolonged periods of anoxia, I. pseudacorus increased production of superoxide dismutase enzymes that help the plant cope with oxidative stress (drought, nutrient deficiencies, injury, etc.) (Monk et al. 1987). The ability to cope with and recover from stress faster than neighboring plants may enable it to be a better competitor.

Iris pseudacorus has a moderate socio-economic impact in the Great Lakes.

Realized:
All parts of I. pseudacorus are poisonous (Idaho Invasives 2007). Its resins can cause skin irritation and blistering; if ingested, this plant will cause gastric distress in humans (ISCBC 2012, Lui et al. 2010, Sutherland 1990). Yellow iris (live or dried) can cause gastroenteritis in cattle and sicken other livestock if ingested, although grazing animals tend to avoid it (Lui et al. 2010, Sutherland 1990). Because palatable species go relatively untouched when intermingled with I. pseudacorus, the quality of pastureland can be reduced (Bossuyt et al. 2005).

Potential:
Stands of I. pseudacorus can reduce flow and block irrigation systems and flood control ditches (Noxious Weed Control Program 2009, ODA 2012). Its seeds can clog pipes and water control structures (Noxious Weed Control Program 2009). Removal of plant material from these systems may require herbicides or excavation equipment and can be costly (ODA 2012).

Iris pseudacorus has a moderate beneficial effect in the Great Lakes.
Realized:
Iris pseudacorus is a popular ornamental plant. It can be planted along slopes and shores to prevent erosion control (Jacobs et al. 2011).
Muskrats use this species for building their dens (Jacobs et al. 2011).

Potential:
Many studies have investigated the use of I. pseudacorus in wastewater treatment to reduce total nitrogen and total phosphorus concentration, and some treatment wetlands in the Czech Republic already use this species (Benson et al. 2004, Vymazal and Kröpfelová 2008). Wu et al. (2011) claimed that I. pseudacorus is a preferred plant species for treatment wetlands in Northern China because of its ability to uptake nutrients and its pleasant appearance. Barbolani et al. (1986) determined that I. pseudacorus was able to take up cadmium and copper from contaminated waters, with a preference to take up copper. Yellow iris is able to absorb these metals in proportion to what is available in the environment, with stands exposed to higher initial metal concentrations tending to absorb more metal than stands exposed to lower concentrations (Barbolani et al. 1986). Iris pseudacorus was able to remove copper from solutions that also contained surfactants and chlorides (Piccardi and Clauser 1983). Iris pseudacorus may be an economic choice for treatment wetlands designed to treat complex, urban wastewaters that contain heavy metals (Larue et al. 2010, Piccardi and Clauser 1983, Zhang et al. 2007). The rhizomes of I. pseudacorus can also reduce populations of Escherichia coli, Salmonella, and Enterocoli by 50-70% in a 24-hour period (Jacobs et al. 2011).

Water-soluble polysaccharides extracted from I. pseudacorus appear promising for pharmaceutical uses (Sanavova and Rakhimov 2004). Ethanol extracts of I. pseudacorus show larvicidal and mirscidiacidal/cercarcidal (compounds that kills trematode larvae) properties (Ahmed and Hamshary 2005).

Ripe seeds, if well roasted before consumption, can be a substitute for coffee (Sturtevant and Hedrick 1972).

Historically, the flowers of I. pseudacorus have been used to yellow dye and the rhizomes were used as a powerful herbal laxative and emetic (Jacobs et al. 2011).

The New York Invasive Species Council ranked I. pseudacorus as posing a high ecological risk and recommends that it be prohibited (New York Invasive 2010). The Ohio Department of Natural Resources lists yellow iris as a “well-established invasive” (Ohio Division and Nature Conservancy 2000). Under the Michigan Public Acts 70–74 of 2005, I. pseudacorus is a prohibited aquatic plant species: a person cannot have any purebred or hybrid variant of this species, or fragments or seed unless they are being collected for identification, and/or the person is in the process of legally removing/eradicating the species (Latimore et al. 2011). In Illinois, I. pseudacorus cannot be possessed, propagated, bought, sold, bartered, transported, transferred, or loaned with a permit (GLPANS 2008). Iris pseudacorus is established and considered a “moderate threat” to local ecosystems in Minnesota. This has led it to be classified as a restricted species that cannot be planted/released with a permit (GLPANS 2008, Minnesota Invasive 2009).

In 2001, the Great Lakes Indian Fish and Wildlife Commission (GLIFWC) considered I. pseudacorus capable of severe ecological impacts even though it tends to occur in a few small populations and has a wide array of control options (Falck and Garske 2003). Yellow iris was still listed as a “high priority” invasive species in 2009, 2010, and 2011 (Falck et al. 2010, Falck et al. 2011, Falck et al. 2012).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
For large infestations, it is best to start in the areas with the lowest concentration of yellow irises and progressing towards high density areas (i.e. periphery toward the center, upstream toward downward) (Noxious Weed Control Program 2009).

The glycosides in I. pseudacorus leaves and rhizomes cause skin irritation, so care should be taken to protect skin from contact regardless of the control method (Cooper and Johnson 1984, Forest Health 2006, Nature Conservancy n.d.).

Biological
Insects and animals do not exert grazing pressure on I. pseudacorus in its native range (Forest Health 2006). Although numerous pathogens and insects attack yellow iris, there are currently no known biological control agents (Tu 2003).

Physical
Small infestations of Iris pseudacorus seedlings can be easily pulled or dug up by hand, especially in damp or wet soil (Noxious Weed Control Program 2009). This method is also feasible for small stands of mature plants; however, tools (pickaxes, saws, etc.) may be needed to remove the rhizomes (Noxious Weed Control Program 2009). Care should be taken to remove all parts of the rhizomes to prevent resprouting (Forest Health 2006). For populations of yellow iris growing in standing water, removal of the leaves and stems above water before flowering can result in good control and reduced spread (Simon 2008 in Noxious Weed Control Program 2009). Sites should be monitored for the emergence of new plants from the seed bank or from rhizome sections that were not removed after control measures are completed (Noxious Weed Control Program 2009). This control method may need to be repeated for three or four year to be effective.

Repeat mowing or removal of seed pods can control the spread of larger infestations (Forest Health 2006). Repeated removal of the aboveground portions of yellow iris may also deplete the plant’s energy reserves and may eventually kill it (Tu 2003).

Mechanical removal in sensitive areas, such as shallow streambeds, can be expected to cause extensive disturbance to the substrate and permit the establishment of other unwanted plants (Jacono 2001). Plant material should not be composted on-site because rhizomes can continue to growth for up to three months without water (Sutherland 1990).

Burning is not recommended for control because of this plant’s strong tendency to resprout from rhizomes (Clark et al. 1998, Sutherland 1990).

Chemical
Applications with herbicides such a glyphosate (Rodeo™ or Aquamaster™) or imazapyr (Habitat™) can provide control of larger infestations (Forest Health 2006, MN DNR 2012, Noxious Weed Control Program 2009). If yellow iris is mixed with desirable plants species, targeted control can involve cutting the stems of I. pseudacorus and applying the herbicide directly to the cut area (Idaho Invasives 2007). Areas should not be mowed for several weeks after herbicide application to allow the treatment to be effective (Noxious Weed Control Program 2009).

Other
Cutting followed by herbicide (glyphosate) treatment with a dripless wick may be the best method for controlling plants in sensitive sites.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

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