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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Nonindigenous Occurrences: Yellow iris is widespread in the northeastern United States, where it has been found in the wild for close to 140 years. Although recorded from over 40 states, yellow iris is not equally distributed or problematic throughout. It is relatively new to the western United States, where notable early records from California and western Montana date to the 1950s (Rubtzoff 1959; Preece 1964).

Iris pseudacorus was first observed in the Frio River, Texas in 1998, under a bridge where the streambed had been silted out. The established iris population continues to root and spread from that site as more silt is deposited. As of 2000, a dense colony, over 100 yards long, existed in the river proper and was creeping down the shallow river habitat. Texas, and especially the southern Frio River (i.e. outside of its general range). Only a few scattered records have previously been made in Texas and none compare to the infestation in the Frio River (Jacono 2001).

Iris pseudacorus is reported as invasive in Connecticut, Delaware, Maryland, New Hampshire, North Carolina, Oregon, Tennessee, Virginia, Vermont, Washington, Wisconsin, and West Virginia (Forest Health Staff 2006).

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

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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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: 9/23/2012

Citation Information:
Morgan, V.H., L. Berent and A. Fusaro. 2017. Iris pseudacorus. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. Revision Date: 9/23/2012

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.

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Citation information: U.S. Geological Survey. [2017]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/24/2017].

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