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Iris pseudacorus


Paleyellow iris flower.
Photo by Nancy Loewenstein, Auburn University,

Stone, Katharine R. 2009. Iris pseudacorus. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].



paleyellow iris
pale-yellow iris
water flag
yellow iris
yellow flag
yellow flag iris

The scientific name of paleyellow iris is Iris pseudacorus L. (Iridaceae) [18,34].

Several paleyellow iris cultivars are available (review by [73]).




Information on state-level noxious weed status of paleyellow iris in the United States is available at Plants Database.


SPECIES: Iris pseudacorus

Paleyellow iris is native to Europe, northern Africa, and temperate Asia (reviews by [73,90]). A valued horticultural plant, paleyellow iris was brought to North America and escaped cultivation [47,60,91], often spreading down watercourses [47,50,54] or washing downstream in floods [77]. A review of early floras documented paleyellow iris in Virginia as early as 1771 [91]. Paleyellow iris is widely distributed across most of the United States and Canada. It occurs in almost every state, with the exceptions of North Dakota, South Dakota, Iowa, Wyoming, Colorado, Oklahoma, New Mexico, and Arizona. The Plants Database provides a distributional map of paleyellow iris.

Paleyellow iris occurs in plant communities associated with water. In its native European range, paleyellow iris is found in moderately moist meadow communities in northeastern France [86], hardwood floodplain forest in France [64], and alkaline peat (fen) communities in Britain [23]. Lists of associated species in Europe are available in these publications: [23,64,86]. Though it is a widespread species in North America, as of this writing (2009), there were few published descriptions of plant communities where paleyellow iris occurs in North America. The plant community descriptions that follow are divided geographically into the eastern United States and central and western United States.

Eastern United States: In the eastern United States, paleyellow iris is found in forested wetlands, open wetlands, and in riparian and floodplain communities.

Forested wetlands: In Van Cortland Park, Bronx, New York, paleyellow iris occurred in a swamp forest dominated by red maple (Acer rubrum), river birch (Betula nigra) and green ash (Fraxinus pennsylvanica) [55]. At the Ipswich River Wildlife Sanctuary in northeastern Massachusetts, paleyellow iris occurred in both red maple- and shrub-swamp plant communities. Red maple swamps were dominated by red maple, though green ash was an indicator species of this community type. Shrub swamps were comprised of a mixture of wetland herbs and shrubs including sweetgale (Myrica gale), swamp rose (Rosa palustris), hazel alder (Alnus serrulata), and the nonnatives glossy buckthorn (Frangula alnus) and purple loosestrife (Lythrum salicaria) [1].

Open wetlands: Paleyellow iris occurred in both natural and constructed tidal freshwater marshes along the Delaware River in New Jersey. Natural marshes in this area were dominated by the perennial species rice cutgrass (Leersia oryzoides), pickerelweed (Pontederia cordata), and/or cattails (Typha spp.) [41]. On Theodore Roosevelt Island in the Potomac River near Washington, DC, paleyellow iris occurred in a freshwater tidal marsh inundated daily by high tide. The most abundant species in the marsh included green arrow arum (Peltandra virginica), calamus (Acorus calamus), narrow-leaved cattail (Typha angustifolia), and yellow pond-lily (Nuphar lutea) [77]. In Maryland, paleyellow iris was found in marshes dominated by calamus and in swamps with longbeak arrowhead (Sagittaria australis), Gray's sedge (Carex grayi), shallow sedge (C. lurida), golden ragwort (Packera aurea), marsh blue violet (Viola cucullata), sweet woodreed (Cinna arundinacea), goldenclub (Orontium aquaticum), common winterberry (Ilex verticillata), hazel alder (Alnus serrulata), southern arrowwood (Viburnum dentatum), Virginia sweetspire (Itea virginica), sweetbay (Magnolia virginiana), green ash, and buttonbush (Cephalanthus occidentalis) [66]. In southern West Virginia, paleyellow iris occurred in fringed sedge (Carex crinita) table wetlands containing a high diversity of mostly native sedges (Carex spp.and Scirpus spp.) and rushes (Juncus spp). These wetlands were often associated with beaver activity [71].

Riparian and floodplain communities: Paleyellow iris occurred but was rare in the late 1960s on flats along the Potomac River dominated by bottomland forest species such as sycamore (Platanus occidentalis), boxelder (Acer negundo), American elm (Ulmus americana), and silver maple (A. saccharinum) [76]. At Mt Vernon, Virginia, paleyellow iris established outside of cultivation in a "low woods" plant community occurring along the Potomac River, in lower reaches of small streams, and along edges of an infilled marsh. Common species in this community included boxelder, red maple, river birch, green ash, and sycamore [91].

Paleyellow iris was an uncommon species on floodplain and lowland "woodlands" on the edges of swamps in Maryland. These areas were dominated by swamp white oak (Quercus bicolor), willow oak (Q. phellos), pin oak (Q. palustris), Shumard oak (Q. shumardii), sweetgum (Liquidambar styraciflua), and red maple [66].

In southern West Virginia, paleyellow iris occurred in both floodplain and riparian plant communities. On wooded upper beach areas, paleyellow iris occurred in sycamore-river birch forest. This community was often inundated with high water, and substrate varied from sand and mud flats to gravel to large cobble. In this region, paleyellow iris also occurred in American eelgrass-pondweed (Vallisneria americana-Potamogeton spp.) instream wetlands, establishing from shoreline to well within the streambed. It was also found in black willow (Salix nigra)-river birch communities within tributary streambeds and on riverside beach areas. Inundation with high water was common. Substrates included cobblestone, gravel, or sand. Paleyellow iris also occurred in Lizard's tail (Saururus cernuus) silt accumulations in shallow stretches of slow moving water, often associated with backwater channels and beaver activity [71].

Central and western United States: As of 2009, the only published descriptions of plant communities with paleyellow iris in the central and western United States were broadleaf cattail (T. latifolia) communities. Near Duluth, Minnesota, paleyellow iris appeared the third season after the construction of a sandbar. Dominant plants included sandbar willow (Salix interior) and broadleaf cattail [38]. In Montana, paleyellow iris occurred in "extensive" stands by itself and intermixed with broadleaf cattail and other aquatic plants [54]. In Sonoma County, California, paleyellow iris occurred in a marsh with broadleaf cattail, broadleaf arrowhead (Sagittaria latifolia), Cusick's sedge (Carex cusickii), awlfruit sedge (C. stipata), and the nonnative yellow marsh marigold (Caltha palustris) [61].


SPECIES: Iris pseudacorus
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [15,21,22,25,28,29,51,56,62,69,88,92,97]).

Aboveground description: The few to several leaves of paleyellow iris are stiff and erect [21], linear, and 10 to 35 inches (25-90 cm) long [92]. Single or multiple flowering stems are 20 to 39 inches (50-100 cm) tall, usually shorter than or equaling leaves [21]. Plants take 3 years to mature before flowering (review by [35]). Flowers are bright yellow or cream-colored and 3 to 4 inches (7-9 cm) wide [21]. Paleyellow iris fruits are 6-angled capsules, 2 to 4 inches (5-9 cm) long [21]. The dark brown, smooth, disk-like seeds are closely packed into 3 rows within the capsule (review by [73]). Seeds have a hard seed coat beneath which there is a gas space, allowing seeds to float in water [11].

Belowground description: Paleyellow iris has rhizomes that are 0.4 to 2 inches (1-4 cm) in diameter (reviews by [73,90]). Paleyellow iris invests heavily in root development, particularly as a young plant, allowing it to establish in habitats with fluctuating water levels [93]. Roots are usually 4 to 8 inches (10-20 cm) long but can be up to 12 inches (30 cm) long (review by [73]).

Raunkiaer [57] life form:

Paleyellow iris flowers from late May to early July in North America [12,18,56,69,76,77,96]. In the Carolinas, paleyellow iris fruits from August to October [56]. In Poland, there are 3 bursts of seed germination; the majority of germination occurs in spring, followed by limited summer and autumn germination (review by [73]).


Paleyellow iris reproduces both vegetatively and by seed. In Poland, reproduction from vegetative fragmentation was more common than seedling establishment (review by [73]). In Montana, reproduction by seed was thought to be more important than vegetative reproduction [54]. The method of reproduction used may depend on local site conditions. On the fringes of saltmarshes in Ireland, shore level influenced reproductive method. On high saltmarsh sites, rhizomes were long-lived and seedlings were rare, whereas at low saltmarsh sites, rhizomes were short-lived and there were "considerable" numbers of seedlings [72].

Pollination and breeding system: Paleyellow iris is a cross-fertilizing species [20]. In its native range, paleyellow iris is pollinated by bees (Bombus spp.) and long-tongued flies (review by [73]). Paleyellow iris was visited by a syrphid fly in Europe (review by [10]). It attracts hummingbirds and butterflies in its nonnative range (review by [44]).

Seed production: In its native range, paleyellow iris produced an average of 5.6 capsules/plant with an average of 120 seeds/capsule. At least 30% of these seeds failed soon after fertilization (review by [73]). In Montana, flowering stalks produced 3 to 4 capsules, each containing 50 to 60 seeds [54].

Seed dispersal: Paleyellow iris seeds are dispersed by water [93] (reviews by [35,90]). Seeds float on the water surface in fall and early spring [93] and germinate along shorelines when water recedes (reviews by [35,78]). In laboratory tests, 100% of paleyellow iris seeds floated during their 1st week, and at least 95% continued to float for 2 months (review by [73]). In other buoyancy tests, 25% of paleyellow iris seeds were still floating 354 days after placement in water, and 10% were still floating after 429 days, the longest time period of any of the species tested [83].

Seed banking: It is not clear how long paleyellow iris seeds persist in the soil seed bank. Paleyellow iris was abundant in a wet meadow in northeastern France but was absent from the soil seed bank [86]. Though present at 84% of the quaking fen locations sampled in the Netherlands, paleyellow iris germinated from only 25% of the seed bank samples. Similarly, though present at 50% of floating forest locations sampled, paleyellow iris germinated from only 14% of the seed bank samples [84]. Paleyellow iris seedlings emerged from soil samples taken from tidal freshwater marshes along the Delaware River in New Jersey [41].

Germination: Paleyellow iris germination is best in moist [11,77] but not waterlogged [42,77] conditions. One review suggests that paleyellow iris germination may be more dependent on temperature than light [78]. In the field, paleyellow iris germination in its native range is said to be poor due to fat-like substances present on the inner seed coat. Seedlings were rare in most habitats (review by [73]). However, one author states that a "large number" of paleyellow iris seeds in Great Britain germinate and put out roots [93]. In Montana, one author suggests that paleyellow iris germination rates were high based on the high number of seedlings observed in the field and from observations of field-collected seed [54].

In the laboratory, only 25% of fresh-collected seeds from the Netherlands germinated in drained soil in 6 weeks [11]. Germination rates for nonscarified seeds in Great Britain varied from 48% after 15 days to 40% after 12 months (review by [73]). Fresh seed collected from established plants in northern Florida exhibited a germination rate of 62% (Jacono and Ramsey unpublished data cited in [52]). Fall-collected seed from the Theodore Roosevelt Island in the Potomac River had a germination rate of 33% after 97 days of inundated conditions and 90 days of moist conditions [77].

Scarification improves paleyellow iris germination ([11,26], review by [73]). In a laboratory study in northern England, scarified paleyellow iris seeds germinated at a significantly higher rate than those that were not scarified (70% versus 48%) (P<0.01) [26]. In laboratory studies in the United States, paleyellow iris seeds that had their "caps" removed and were placed in water experienced much higher germination; in 30 days, 97% of seeds with caps removed had germinated, while no seeds with intact caps germinated in this time period [13].

Seedling establishment and plant growth: Paleyellow iris generally establishes in areas that are moist but not waterlogged. In many cases, paleyellow iris establishes on the edges of water features (see Site characteristics), as water-dispersed seeds are often deposited along the high water mark (reviews by [35,78]). On Theodore Roosevelt Island in the Potomac River, paleyellow iris established on trash, river debris, and tree roots that were above the general water level of the marsh [77]. As a young plant, paleyellow iris invests heavily in developing a root system to adapt to fluctuating water levels [93].

Water plays a key role in paleyellow iris growth. While paleyellow iris needs moisture, laboratory experiments show that inundated conditions reduce seedling growth [11,43]. Seedlings may recover after inundation [11]. On Theodore Roosevelt Island in the Potomac River near Washington, DC, the length of time that paleyellow iris was inundated by water was the factor most limiting to its growth; areas experiencing short inundation exhibited greater growth than areas with long inundation [77]. In contrast, in Montana, paleyellow iris plants growing in 2 to 3 feet (1 m) of water were larger and more "vigorous" than plants that were not inundated [54]. Paleyellow iris occurred in areas that were flooded for as long as 6 months in its native range [53,86]. One author reports that in England, paleyellow iris seeds germinated and seedlings grew well in marshes burned in late summer and flooded over winter (review by [73]).

Studies in Poland show most mortality of paleyellow iris seedlings occurs in the first 2 months after germination, most likely due to desiccation. Heavy seedling mortality also occurs the first winter due to freezing surface water. In Poland, between 28% and 72% of seedlings survive their 1st year; only 3% to 6% grow to a size at which the rhizomes fragment (review by [73]) (see Vegetative regeneration).

Light conditions impact the growth pattern of paleyellow iris. In its native range, paleyellow iris tends to flower more in open habitats, which leads to a more clumped distribution as rhizome branching is associated with flowering. In wooded or shaded habitats, less flowering occurs and plants tend to spread linearly. Shaded plants tend to have fewer and longer leaves than plants in open areas (review by [73]). One study suggests that low light may limit seedling establishment but not growth of mature paleyellow iris plants [77].

On the fringes of saltmarshes in Ireland, shore level influenced paleyellow iris growth. On high saltmarsh sites plants had more and longer leaves and high rhizome terminal bud survival compared to low saltmarsh sites [72].

Vegetative regeneration: Paleyellow iris regenerates vegetatively via rhizomes [15,28,93], (review by [73]). When plants reach about 10 years of age (review by [73]) rhizomes fragment and contribute to new plant establishment (reviews by [35,73,90]).

Rhizomes may break off during floods (review by [73]) and are moved to new locations by water (reviews by [73,90]). Dry rhizomes remain viable for more than 3 months and may establish if they encounter moisture (reviews by [35,73]). Individual rhizomes may persist for 10 years ([72], review by [73]). In its native range, the conditions in wet fens preserve paleyellow iris rhizomes for many years, making it possible to observe 30 to 40 years of flowering extent, branching, and annual incremental increases in plant size [93].

Paleyellow iris roots and cross-section of rhizomes.
Photo by Leslie J. Mehrhoff, University of Connecticut,

Thick rhizomes tend to prevent mixing of adjacent clones, but often 2 or 3 clones may lie on top of each other, with the bottom rhizome occurring at a depth of about 4 inches (10 cm) (review by [73]). Up to several hundred flowering plants may be connected rhizomatously (review by [35]). Rhizomes may grow over the soil (review by [90]), rocks (review by [73]), or as mats floating in water (reviews by [73,90]).

Paleyellow iris needs moisture to establish and survive [36,87,94]. Consequently, it often occurs on the wet edges of lakes [15,18,29,74,88,96], ponds [15,21,28,29,60,65,92,94], rivers [18,19,53,88], and streams [21,22,25,29,32,54,56,71]. Paleyellow iris also occurs in marshes [16,22,25,26,54,56,69,77,88,96], tidal marshes [70,77], wetlands ([8,71], reviews by [35,89,90]), swamps [18,21,56,96], swampy woodlands [22], open woods [25], wood edges [25], and glacial potholes ([54], review by [58]). Paleyellow iris occurs on beach swales [65] and rocky coastal shorelines (review by [89]).

Paleyellow iris is associated with human-made structures such as ditches ([16,28,59,88], review by [35]), irrigation canal banks ([54,92], Lake County Weed District, Pablo, Montana, 2001 personal communication cited in [52]), constructed gravel trails through wetlands [77], man-made pools [31,37,59], meadows [22,25], wet pastures (review by [35]), and other disturbed sites [71,77,97].

Water characteristics: Paleyellow iris is found in fresh (reviews by [78,90]), brackish ([16,70], review by [78]) and salt (reviews by [78,90]) water. In its native range, paleyellow iris persists in the high zones of saltmarshes and may be found surrounded by estuarine water with a salinity of 24% during high tides (review by [73]).

Paleyellow iris can tolerate water with low levels of oxygen (reviews by [53,73]). One source suggests that it prefers cool water, which may limit its expansion into warm-water areas [52]. Because deep water can prevent seed germination [42,77] and impairs seedling growth [11,43], paleyellow iris generally grows in shallow water, but it may create extensive mats floating over deeper water (review by [35]). In New Zealand, paleyellow iris occurred in water ranging from 0 to 2.6 feet (0 to 0.8 m) in depth [74]. In Montana, paleyellow iris grew in 2 to 3 feet (1 m) of standing water [54].

Soils: Paleyellow iris usually grows on water-deposited substrates such as silt ([26,38,52,71], review by [35]), sand ([38,71], reviews by [35,73]), gravel ([71,77], review by [73]), and cobbles [52,71]. One review notes that paleyellow iris may be found on "rocky" sites [35]. It is associated with calcareous [17,53], sandy loams, clay loams, and other loamy or clayey [17] soils derived from sandstone and schist [53] in its native range. It is present on peat soils in both its native (review by [73]) and nonnative [70] ranges.

Paleyellow iris occurred on soils with pH ranging from 6.65 to 7.55 in Turkey [17] and 3.6 to 7.7 in Britain (review by [73]). In England, paleyellow iris occurred in spring and seepage waters with pH ranging from 6.8 to 7.2 [53]. One review from its nonnative range suggests that paleyellow iris prefers acidic soils ranging from pH 3.6 to 7.7 and averaging 6.0 [78]. Paleyellow iris prefers high nutrient sites in both its native ([36], review by [73]) and nonnative (review by [78]) ranges.

Climate: Paleyellow iris occurs in temperate climates (review by [78]). Few authors report climate data for sites with paleyellow iris in North America. The Ipswich River Wildlife Sanctuary in northeastern Massachusetts has a mean low temperature of 27.0 °F (-2.8 °C) in January and a mean high of 71.6 °F (22.0 °C) in July. Mean annual precipitation is 44 inches (1,120 mm) [1]. In southwestern Louisiana average maximum temperatures range from 60.1 °F (15.6 °C) in January to 90.0 °F (32.2 °C) in July and average precipitation is 56.32 inches (143.05 cm) [16].

Elevation: Paleyellow iris occurs from sea level to 1,080 feet (330 m) in Britain (review by [73]). In North America, it occurs at sea level in Louisiana [16] up to 328 feet (100 m) in California [28]. Paleyellow iris occurs at 4,200 to 4,315 feet (1,280-1,315 m) near Salt Lake City, Utah [92].

Topography: In North America, paleyellow iris generally occurs on flat ground (review by [73]), but in England it may be found on the wet and waterlogged slopes of hills, mountains, and associated wet valleys where groundwater seepage or springs are present. Paleyellow iris is rare on extensive wet upper slopes and crests of hills and mountains with perched water tables [53].

Paleyellow iris grows best in full sun to partial shade ([87], reviews by [48,75]) and is intolerant of deep shade [93]. Low light may limit seedling establishment but not necessarily mature paleyellow iris growth [77]. Along the Upper Rhine in France, paleyellow iris occurred only on sites that had high light levels within a hardwood floodplain forest [64].

Disturbances such as flooding play a key role in paleyellow iris establishment. Rhizomes may break off during floods (review by [73]) and are moved to new locations by water (reviews by [73,90]). Floods may also transport paleyellow iris seeds ([93], reviews by [35,90]).

There is concern that paleyellow iris may alter historical patterns of plant succession ([77], review by [78]) by displacing native vegetation ([52,77], reviews by [9,35,48,58,78,89,90]). See Impacts for more information on this topic.


SPECIES: Iris pseudacorus
Immediate fire effect on plant: As of this writing (2009), no information was available in the published literature regarding the immediate effects of fire on paleyellow iris plants and seeds. Fire would likely kill paleyellow iris seedlings and mature plants growing near the ground surface. However, one review suggests that underground rhizomes of paleyellow iris may survive fire and sprout [9].

Postfire regeneration strategy [67]:
Surface rhizome and/or a chamaephytic root crown in organic soil or on soil surface
Rhizomatous herb, rhizome in soil
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)

Fire adaptations and plant response to fire:
Fire adaptations: Like many wetland plants, paleyellow iris is not specifically adapted to survive fire. Its tendency to grow at or near the ground surface suggests that fire would likely kill plants and seedlings. Rhizomes located below the soil surface or floating on water may survive fire and sprout (review by [9]). Dispersal of seeds ([93], reviews by [35,90]) and rhizomes (reviews by [73,90]) via water may allow paleyellow iris to establish in burned areas adjacent to wetlands, streams, or rivers. No studies report that paleyellow iris seeds respond to heat scarification, but the high germination rates resulting from scarification ([11,26], review by [73]) suggest that fire may improve germination by scarifying seed coats. Because low light may limit seedling establishment and growth [77], it is possible that the open conditions present after fire may benefit seedlings. One review suggests that fire may encourage seed germination due to increased light and disturbance [78].

Plant response to fire: As of this writing (2009), there were no studies documenting the response of paleyellow iris to fire. One review states that paleyellow iris may sprout after fire [9], though no time frame is specified. Another review suggests that fire may encourage seed germination due to increased light and disturbance [78]. One author reports that in England, paleyellow iris seeds germinated and seedlings grew well in marshes burned in late summer and then flooded over winter. No specific details were presented (review by [73]).

Fuels: No information is available on this topic.

Fire regimes: Fire regimes in wetland and riparian areas vary widely across the United States. For example, fire is unusual in northeastern riparian communities and may only occur in times of severe drought or wetland drainage. Riparian plants may not be fire-adapted in these systems. Long fire-return intervals are typical in wetlands of the Northeast (review by [14]). In contrast, fires are a common occurrence in southeastern wetlands, which support large quantities of flammable, herbaceous vegetation that is well-adapted to frequent fires. Stand-replacement fires may occur in coastal wetlands at 1- to 10-year fire-return intervals (review by [68]). As of 2009, there was insufficient information to predict how paleyellow iris might respond to these fire regimes. It is not clear if or how paleyellow iris may influence fire regimes, though it is likely that the impact of paleyellow iris varies given its occurrence in communities with very different fire regimes. See the Fire Regime Table for further information on fire regimes of vegetation communities in which paleyellow iris may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Potential for postfire establishment and spread: As of 2009, there were no studies documenting the establishment and spread of paleyellow iris after fire. One review from England suggests that the removal of aboveground material and plants after a late summer fire and winter flooding facilitated paleyellow iris seed germination and seedling growth [73].

Preventing postfire establishment and spread: Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly management method. This can be accomplished through early detection and eradication, careful monitoring and follow-up, and limiting dispersal of invasive plant seed or rhizomes into burned areas. Specific recommendations include:

For more detailed information on these topics see the following publications: [2,5,24,81].

Use of prescribed fire as a control agent: Prescribed fire may or may not be an appropriate management tool in the wetland and riparian ecosystems where paleyellow iris occurs. Prescribed fire is not likely to be a useful control measure for invasive species like paleyellow iris in plant communities where fires are typically rare and native species are not fire-adapted. For example, prescribed fire is probably not appropriate in the forested wetlands of the Northeast. Prescribed fire may be an appropriate tool in herbaceous wetlands that commonly support native species adapted to frequent fire (review by [14]), but its effects on paleyellow iris were unknown as of 2009. Reviews caution against this control method because fire may stimulate paleyellow iris sprouting [9,35], seed germination, and/or seedling growth [35].


SPECIES: Iris pseudacorus
Paleyellow iris is generally of little value to wildlife or livestock.

Palatability and/or nutritional value: Paleyellow iris is considered poisonous ([15], review by [78]) due to large amounts of glycosides found in foliage and rhizomes (review by [78]). In grazing experiments in Belgium, paleyellow iris was considered unpalatable to cattle and ponies [4]. In its native range paleyellow iris is usually ignored by domestic ponies, cattle, sheep, goats, and rabbits, though foliage was eaten down to the rhizomes by domestic cattle in autumn when other vegetation was unavailable. Domestic sheep have been observed browsing early-season leaves. Fallow deer browsed paleyellow iris in England. In Britain, gastroenteritis occurred after livestock ate hay containing paleyellow iris, and acute diarrhea occurred in domestic cattle after rhizome consumption (review by [73]).

Cover value: No information is available on this topic.

Paleyellow iris has been used as a rehabilitation plant to reduce bacterial loads (review by [73]), absorb heavy metals from contaminated water ([3], reviews by [73,78]), and provide erosion control (review by [78]). One review states that it is "one of the few plants flourishing after a nuclear holocaust" (review by [73]). Paleyellow iris was smoked by people during World War II (review by [73]). In Turkey, paleyellow iris rhizomes are used as a diuretic, to prevent gas, and to treat eczema. Seeds are used as a coffee substitute after drying [17]. One author experienced severe attacks of dermatitis from contact with the syrupy covering of the endosperm of paleyellow iris seeds [13].


Impacts: The tendency for paleyellow iris to grow in large, radially spreading clones allows it to form dense stands that may replace native vegetation ([52,54,77], reviews by [9,35,48,58,78,89,90]), including 2 native irises in Massachusetts (review by [89]) and characteristic California marsh plants such as cattails (Typha spp.) (Fuller personal communication cited by [58]). Paleyellow iris may also reduce habitat needed by waterfowl and fish ([77], reviews by [35,78]), including several important salmon species (review by [35]). Paleyellow iris may also reduce available forage for livestock [54].
Stand of paleyellow iris.
Photo by Joseph M. DiTomaso, University of California-Davis,

On Theodore Roosevelt Island in the Potomac River near Washington, DC, paleyellow iris changed local site conditions to the extent that it facilitated its own spread; rhizome growth compacted the soil, a hardpan developed, and species other than paleyellow iris were unable to establish and persist. Paleyellow iris clones eventually replaced the native green arrow arum, an important plant for wood ducks. Mats of paleyellow iris rhizomes also prevented the germination and seedling development of willows (Salix spp.), particularly black willow. By suppressing willows and providing a raised surface, paleyellow iris promoted the spread of species not needing a mineral surface for establishment (e.g., green ash). In turn, this change in species composition facilitated the succession from marsh to swamp vegetation communities. The author concluded that paleyellow iris "apparently speeds up the destruction of the marsh by promoting expansion of the swamp and apparently preempts space and thus reduces the food supply of the wood duck which occurs on the island" [77].

As of 2001, paleyellow iris occurred along 1,300 miles (2,100 km) of irrigation canals and lateral channels near Flathead Lake in northwestern Montana (Lake County Weed District, Pablo, Montana, 2001 personal communication cited in [52]). Paleyellow iris plants may clog small streams and irrigation systems, and seeds clog water control structures and pipes ([54], review by [35]). One review cites a study from Montana suggesting that paleyellow iris plants may reduce stream width by up to 10 inches (25 cm) annually by trapping sediment. This process creates new streambanks which may be dominated by paleyellow iris seedlings (Tyron 2006 unpublished study cited in [35]).

Rate of spread: Paleyellow iris was first observed along the Frio River in south-central Texas in 1988, establishing under a bridge where silt accumulated in the stream bed. Additional silt deposition in the area encouraged paleyellow iris rooting and spread. A colony over 300 feet (90 m) long developed between 1988 and 2001. Colonies also expanded out of the silt substrate into riffle areas with gravelly or rocky substrates [52].

The largest intact paleyellow iris clone in its native range measured 2.17 feet (0.66 m) across. While individual paleyellow iris clones may be small, populations of paleyellow iris may cover large areas. Large clumps of paleyellow iris measured up to 66 feet (20 m) across in Ireland (review by [73]). On Theodore Roosevelt Island in the Potomac, paleyellow iris occurred in clumps about 1 m² in area [77]. In southern New York, paleyellow iris occurred in a 0.75-acre (0.30-hectare) patch along a creek [50].

Control: In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill their void must be considered [6]. Paleyellow iris occurs with many other nonnative species of concern in wetlands; removal of paleyellow iris may lead to the expansion of populations of other nonnative species. For example, paleyellow iris occurs in the freshwaters of the Hudson River Basin, though it does not exhibit "significant ecological impacts" like the associated nonnatives curly pondweed (Potamogeton crispus), onerow yellowcress (Rorippa nasturtium), water chestnut (Trapa natans), Eurasian watermilfoil (Myriophyllum spicatum), and purple loosestrife [49]. In England, herbicide removal of common reed (Phragmites australis) produced open areas in which paleyellow iris established (review by [73]). It is possible that paleyellow iris may show a similar response in other areas where other nonnative species are targeted for control. Care should be taken to minimize local site disturbance to prevent paleyellow iris seedlings from germinating (review by [35]).

Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.

Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [45,63] (e.g., avoid road building in wildlands [80]) and by monitoring several times each year [33]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [30].

Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [81]. See the Guide to noxious weed prevention practices [81] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: As of 2009, there were no studies on controlling paleyellow iris using cultural methods. There is some evidence to suggest that dense cover of other plant species may inhibit paleyellow iris growth; on Theodore Roosevelt Island in the Potomac River near Washington, DC, paleyellow iris growth was limited by calamus cover in swamp-marsh transition areas [77].

Physical and mechanical control: Physical and mechanical methods may be effective in controlling small populations of paleyellow iris. Some sources suggest physical removal of the entire plant and rhizome system (reviews by [35,78,90]), though all rhizomes must be removed for this method to be effective (reviews by [35,90]). Repeated mowing or cutting of aboveground foliage may eventually kill paleyellow iris ([53], reviews by [35,78]). One review states that paleyellow iris leaves are brittle and susceptible to damage by trampling. Consequently, paleyellow iris is generally absent from areas of pronounced human or animal activity in its native range [73]. A horticultural guide suggests the removal of seed pods to prevent future establishment from seed [75]. Draining wetlands to remove paleyellow iris was "slow to succeed" in Montana [54].

Physical and mechanical control methods may be preferable in wetland settings where use of herbicides is problematic (review by [89]). However, mechanical removal of paleyellow iris in sensitive areas may cause extensive substrate disturbance, leading to the establishment of other unwanted plants [52].

Biological control: Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [85,95] and the Weed Control Methods Handbook [79] for background information and important considerations for developing and implementing biological control programs.

As of 2009, there were no biological control agents for paleyellow iris. A horticultural guide states that paleyellow iris in New Jersey suffers from borers, rot slugs, and black vine weevils [75], and one review states that several invertebrates and fungi feed on paleyellow iris [78]. In its native range, damage to paleyellow iris by invertebrate grazers was negligible in woodland, grassland, ponds, saltmarsh, fens and reedswamp plant communities (review by [73]). Paleyellow iris is susceptible to paleyellow iris root rot (Pseudomonas iridis) in its native range, which causes premature yellowing of the leaves as the rhizomes rot (review by [73]). Paleyellow iris hosts and is susceptible to the rust of paleyellow iris (Puccinia iridis) but is generally considered resistant [46].

Chemical control: Herbicides are often effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [7]. See The Nature Conservancy's Weed control methods handbook [79] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

Herbicides are effective at controlling paleyellow iris ([53], reviews by [73,90]), though care must be taken when applying herbicides in wetland ecosystems (review by [78]). Small populations can be spotsprayed by herbicides (review by [35]), while foliar applications may be needed in large populations (review by [78]). Herbicide treatments may be most effective during the growing season because the herbicide is transported to the rhizome (review by [78]).

Integrated management: Cutting or mowing followed by herbicide application to cut stems and leaves may effectively control paleyellow iris ([52], reviews by [35,78]).


SPECIES: Iris pseudacorus

This Fire Regime Table summarizes characteristics of fire regimes for vegetation communities in which paleyellow iris may occur based on descriptions in available literature. Follow the links in the table to documents that provide more detailed information on these fire regimes. This table does not include plant communities across the entire range of paleyellow iris. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Fire regime information on vegetation communities in which paleyellow iris may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [40], which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
Minimum interval
Maximum interval
Northeast Grassland

Northern coastal marsh

Replacement 97% 7 2 50
Mixed 3% 265 20  
Southern Appalachians
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
Minimum interval
Maximum interval
Southern Appalachians Forested
Bottomland hardwood forest Replacement 25% 435 200 >1,000
Mixed 24% 455 150 500
Surface or low 51% 210 50 250
*Fire Severities:
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [27,39].

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