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| NAS - Nonindigenous Aquatic Species |
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Common Name: yellow iris
Synonyms and Other Names: yellow flag
Taxonomy: available through 
Identification: Rhizomes pink, freely branching, producing extensive clumps, 2–3 cm diam., with fibrous remains of old leaves; roots fleshy. Stems usually 1-branched, solid, 7–15 dm. Leaves: basal deciduous, at first erect, then recurved, blade dark green, with prominent median thickening, 4–10 dm × 2–3 cm, slightly glaucous basally; cauline equaling inflorescence unit. Inflorescence units 4–12-flowered; spathes green with brown margins, outer spathe strongly keeled, inner without keel, 6–9 cm, subequal, margins not scarious. Flowers: perianth bright yellow; floral tube 0.6–0.8 cm, with no constriction into ovary; sepals bright yellow or cream colored, lanceolate to ovate or suborbiculate, 5–7.5 × 3–4 cm, base abruptly attenuate, claw ca. 1/2 length of limb, signal a darker yellow basal patch limited by short, brown lines; petals without veining, lanceolate to spatulate, 2–3 cm; ovary triangular in cross section with concave sides and narrow groove at each angle, 1.5 cm; style keeled, 3–4 cm, crests spreading, 1–1.2 cm, laciniate at apex; stigmas rounded with prominent tongue; pedicel 2.5–7 cm. Capsules prismatic to oblong-ovoid, obscurely 3-angled with obvious groove at each angle, 3.5–6 cm, beak 5 mm. Seeds D-shaped, flattened, 6–7 mm, corky, lustrous.
Native Range:
Eurasia
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 has recently become established in the Frio River, an exceptionally scenic stream in south central Texas. The Frio River is one of the few entirely spring fed and still free flowing rivers in Texas. Plants were first observed in the Frio River in 1998, under a bridge where the stream bed had been silted out. The Iris population continues to root and spread from that site as more silt is deposited. Currently a dense colony, over 100 yards long, exists in the river proper and is creeping down the shallow river habitat.
Texas, and especially the southern Frio River, lie outside of the general range of Iris pseudacorus. Only a few scattered records have previously been made in Texas and none compare to the infestation in the Frio River.
Ecology:
Flowering Apr--Jun. Swamps, wet shores of rivers and lakes.
Plants grow vigorously in water or wet soil. Biologists were surprised to find additional colonies rooted outside of the silty areas, growing next to riffles (pebbly/rocky substrate) in the stream.
Iris pseudacorus is a hearty, perennial monocot that regrows from thick, tuberous rhizomes. The rhizomes spread radially to produce large clonal populations. Rhizomes are drought tolerant while both rhizomes and seeds may be transplanted downstream during floods (Sutherland 1990). In the northeastern United States Iris pseudacorus often grows to such an extent that it is considered invasive and problematic. In its native habitat damage by insects or grazing animals is negligible (Sutherland 1990). In fact, yellow iris is poisonous. Even when dry it causes gastroenteritis in cattle (Sutherland 1990). This plant is listed as an injurious weed in Nevada.
Because of its strong tendency to resprout from rhizomes, burning is not recommended for control (Clark et al. 1998). Similarly, fire in late summer was not found to suppress seedling recruitment the following spring (Sutherland 1990).
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 [Britian and Europe]. In western Montana seedlings of Iris pseudacorus are numerous in the field (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).
Care should be taken when pulling or digging yellow iris because resinous substances in the leaves and rhizomes can cause skin irritation (Cooper and Johnson 1984). Mechanical removal in sensitive areas, such as shallow stream beds, can be expected to cause extensive disturbance to the substrate and permit the establishment of other unwanted plants. Cutting followed by herbicide (glyphosate) treatment with a dripless wick may be the best method for controlling plants in sensitive sites, such as the Frio River.
Means of Introduction: It is still 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
Clark, F.H, C. Mattrick and S. Shonbrun (eds.). 1998. Rogues Gallery: New England's Notable Invasives. New England Wild Flower. New England Wildflower Society. Vol. 2, No. 3. Pp. 19-26.
Cooper, M.R. and A.W. Johnson. 1984. Poisonous plants in Britain and their effects on animals and man. Her Majesty's Stationery Office, London, England.
Preece, S.J. 1964. Iris pseudacorus in Montana. Proceedings of the Montana Academy of Sciences 24:1-4.
Raven, P.H. and J.H. Thomas. 1970. Iris pseudacorus in western North America. Madrono 20:390-391.
Rubtzoff, P. 1959. Iris pseudacorus and Caltha palustris in California. Leaflets of Western Botany 9(2): 31-32.
Sutherland, W.J. 1990. Biological Flora of the British Isles, No. 169. Iris pseudacorus. Journal of Ecology 78: 833-848.
Other Resources:
Author: V. Howard Morgan
Contributing Agencies: 
NOAA - GLERL
Revision Date: 11/26/2008 Citation for this information:
V. Howard Morgan. 2009. Iris pseudacorus. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
<http://nas.er.usgs.gov/queries/FactSheet.asp?speciesID=1115> Revision Date: 11/26/2008
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