Yellow Iris is still uncommon as a naturalized plant in Illinois, but it is probably becoming more common. It is more common in the heavily populated NE section of the state than elsewhere (see Distribution Map). Habitats include low areas along rivers, borders of ponds, marshes, swamps, calcareous seeps, and ditches. Both degraded and higher quality wetlands can be invaded. Yellow Iris was introduced from Europe as an ornamental plant. It is still cultivated in gardens, from which it occasionally escapes.
Impacts: The tendency for pale-yellow 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 ) and characteristic California marsh plants such as cattails (Typha spp.) (Fuller personal communication cited by ). Pale-yellow iris may also reduce habitat needed by waterfowl and fish (, reviews by [35,78]), including several important salmon species (review by ). Pale-yellow iris may also reduce available forage for livestock .
Stand of pale-yellow iris.
On Theodore Roosevelt Island in the Potomac River near Washington, DC, pale-yellow 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 pale-yellow iris were unable to establish and persist. Pale-yellow iris clones eventually replaced the native green arrow arum, an important plant for wood ducks. Mats of pale-yellow iris rhizomes also prevented the germination and seedling development of willows (Salix spp.), particularly black willow. By suppressing willows and providing a raised surface, pale-yellow 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 pale-yellow 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" .
As of 2001, pale-yellow 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 ). Pale-yellow iris plants may clog small streams and irrigation systems, and seeds clog water control structures and pipes (, review by ). One review cites a study from Montana suggesting that pale-yellow 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 pale-yellow iris seedlings (Tyron 2006 unpublished study cited in ).
Rate of spread: Pale-yellow 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 pale-yellow 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 .
The largest intact pale-yellow iris clone in its native range measured 2.17 feet (0.66 m) across. While individual pale-yellow iris clones may be small, populations of pale-yellow iris may cover large areas. Large clumps of pale-yellow iris measured up to 66 feet (20 m) across in Ireland (review by ). On Theodore Roosevelt Island in the Potomac, pale-yellow iris occurred in clumps about 1 m² in area . In southern New York, pale-yellow iris occurred in a 0.75-acre (0.30-hectare) patch along a creek .
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 . Pale-yellow iris occurs with many other nonnative species of concern in wetlands; removal of pale-yellow iris may lead to the expansion of populations of other nonnative species. For example, pale-yellow 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 . In England, herbicide removal of common reed (Phragmites australis) produced open areas in which pale-yellow iris established (review by ). It is possible that pale-yellow 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 pale-yellow iris seedlings from germinating (review by ).
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 ) and by monitoring several times each year . 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 .
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 . See the Guide to noxious weed prevention practices  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 pale-yellow iris using cultural methods. There is some evidence to suggest that dense cover of other plant species may inhibit pale-yellow iris growth; on Theodore Roosevelt Island in the Potomac River near Washington, DC, pale-yellow iris growth was limited by calamus cover in swamp-marsh transition areas .
Physical and mechanical control: Physical and mechanical methods may be effective in controlling small populations of pale-yellow 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 pale-yellow iris (, reviews by [35,78]). One review states that pale-yellow iris leaves are brittle and susceptible to damage by trampling. Consequently, pale-yellow iris is generally absent from areas of pronounced human or animal activity in its native range . A horticultural guide suggests the removal of seed pods to prevent future establishment from seed . Draining wetlands to remove pale-yellow iris was "slow to succeed" in Montana .
Physical and mechanical control methods may be preferable in wetland settings where use of herbicides is problematic (review by ). However, mechanical removal of pale-yellow iris in sensitive areas may cause extensive substrate disturbance, leading to the establishment of other unwanted plants .
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  for background information and important considerations for developing and implementing biological control programs.
As of 2009, there were no biological control agents for pale-yellow iris. A horticultural guide states that pale-yellow iris in New Jersey suffers from borers, rot slugs, and black vine weevils , and one review states that several invertebrates and fungi feed on pale-yellow iris . In its native range, damage to pale-yellow iris by invertebrate grazers was negligible in woodland, grassland, ponds, saltmarsh, fens and reedswamp plant communities (review by ). Pale-yellow iris is susceptible to pale-yellow iris root rot (Pseudomonas iridis) in its native range, which causes premature yellowing of the leaves as the rhizomes rot (review by ). Pale-yellow iris hosts and is susceptible to the rust of pale-yellow iris (Puccinia iridis) but is generally considered resistant .
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 . See The Nature Conservancy's Weed control methods handbook  for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Herbicides are effective at controlling pale-yellow iris (, reviews by [73,90]), though care must be taken when applying herbicides in wetland ecosystems (review by ). Small populations can be spotsprayed by herbicides (review by ), while foliar applications may be needed in large populations (review by ). Herbicide treatments may be most effective during the growing season because the herbicide is transported to the rhizome (review by ).
Integrated management: Cutting or mowing followed by herbicide application to cut stems and leaves may effectively control pale-yellow iris (, reviews by [35,78]).
Pale-yellow 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]. Pale-yellow 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 , open woods , wood edges , and glacial potholes (, review by ). Pale-yellow iris occurs on beach swales  and rocky coastal shorelines (review by ).
Pale-yellow iris is associated with human-made structures such as ditches ([16,28,59,88], review by ), irrigation canal banks ([54,92], Lake County Weed District, Pablo, Montana, 2001 personal communication cited in ), constructed gravel trails through wetlands , man-made pools [31,37,59], meadows [22,25], wet pastures (review by ), and other disturbed sites [71,77,97].
Water characteristics: Pale-yellow iris is found in fresh (reviews by [78,90]), brackish ([16,70], review by ) and salt (reviews by [78,90]) water. In its native range, pale-yellow 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 ).
Pale-yellow 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 . Because deep water can prevent seed germination [42,77] and impairs seedling growth [11,43], pale-yellow iris generally grows in shallow water, but it may create extensive mats floating over deeper water (review by ). In New Zealand, pale-yellow iris occurred in water ranging from 0 to 2.6 feet (0 to 0.8 m) in depth . In Montana, pale-yellow iris grew in 2 to 3 feet (1 m) of standing water .
Soils: Pale-yellow iris usually grows on water-deposited substrates such as silt ([26,38,52,71], review by ), sand ([38,71], reviews by [35,73]), gravel ([71,77], review by ), and cobbles [52,71]. One review notes that pale-yellow iris may be found on "rocky" sites . It is associated with calcareous [17,53], sandy loams, clay loams, and other loamy or clayey  soils derived from sandstone and schist  in its native range. It is present on peat soils in both its native (review by ) and nonnative  ranges.
Pale-yellow iris occurred on soils with pH ranging from 6.65 to 7.55 in Turkey  and 3.6 to 7.7 in Britain (review by ). In England, pale-yellow iris occurred in spring and seepage waters with pH ranging from 6.8 to 7.2 . One review from its nonnative range suggests that pale-yellow iris prefers acidic soils ranging from pH 3.6 to 7.7 and averaging 6.0 . Pale-yellow iris prefers high nutrient sites in both its native (, review by ) and nonnative (review by ) ranges.
Climate: Pale-yellow iris occurs in temperate climates (review by ). Few authors report climate data for sites with pale-yellow 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) . 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) .
Elevation: Pale-yellow iris occurs from sea level to 1,080 feet (330 m) in Britain (review by ). In North America, it occurs at sea level in Louisiana  up to 328 feet (100 m) in California . Pale-yellow iris occurs at 4,200 to 4,315 feet (1,280-1,315 m) near Salt Lake City, Utah .
Topography: In North America, pale-yellow iris generally occurs on flat ground (review by ), 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. Pale-yellow iris is rare on extensive wet upper slopes and crests of hills and mountains with perched water tables .
Yellow iris flowers from May to July (6); it reproduces by seeds as well as by vegetative reproduction (3). It is thought by some to be the origin of the 'fleur-de-lis' device used on coats of arms (4). In folk medicine, the rhizome of yellow iris was looked upon as something of a cure-all, being used to treat coughs, convulsions, toothache, diarrhoea, cramp, and as an antidote to poisoning (6). Furthermore, the flowers were used to produce a yellow dye, and the rhizome was used to make black dye (6).
Pale-yellow iris occurs in plant communities associated with water. In its native European range, pale-yellow iris is found in moderately moist meadow communities in northeastern France , hardwood floodplain forest in France , and alkaline peat (fen) communities in Britain . 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 pale-yellow 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, pale-yellow iris is found in forested wetlands, open wetlands, and in riparian and floodplain communities.
Forested wetlands: In Van Cortland Park, Bronx, New York, pale-yellow iris occurred in a swamp forest dominated by red maple (Acer rubrum), river birch (Betula nigra) and green ash (Fraxinus pennsylvanica) . At the Ipswich River Wildlife Sanctuary in northeastern Massachusetts, pale-yellow 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) .
Open wetlands: Pale-yellow 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.) . On Theodore Roosevelt Island in the Potomac River near Washington, DC, pale-yellow 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) . In Maryland, pale-yellow 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) . In southern West Virginia, pale-yellow 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 .
Riparian and floodplain communities: Pale-yellow 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) . At Mt Vernon, Virginia, pale-yellow 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 .
Pale-yellow 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 .
In southern West Virginia, pale-yellow iris occurred in both floodplain and riparian plant communities. On wooded upper beach areas, pale-yellow 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, pale-yellow 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. Pale-yellow 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 .
Central and western United States: As of 2009, the only published descriptions of plant communities with pale-yellow iris in the central and western United States were broadleaf cattail (T. latifolia) communities. Near Duluth, Minnesota, pale-yellow iris appeared the third season after the construction of a sandbar. Dominant plants included sandbar willow (Salix interior) and broadleaf cattail . In Montana, pale-yellow iris occurred in "extensive" stands by itself and intermixed with broadleaf cattail and other aquatic plants . In Sonoma County, California, pale-yellow 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) .