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Yellow Flag

Iris pseudacorus

Summary 9

Iris pseudacorus (yellow flag, yellow iris, water flag) is a species in the genus Iris, of the family Iridaceae. It is native to Europe, western Asia and northwest Africa. Its specific epithet, meaning "false acorus," refers to the similarity of its leaves to those of Acorus calamus, as they have a prominently veined mid-rib and sword-like shape.

Habitat 10

Common in wet habitats including wet meadows, wet woods, fens, wet dune-slacks, and the edges of watercourses, lakes and ponds. In the north and west of Britain it may also be found alongside coastal streams, on raised beaches, saltmarsh and shingle. It has in many cases been planted in the wild and escaped from gardens (3).

Impacts and control 11

More info for the terms: cover, fire management, invasive species, marsh, natural, nonnative species, prescribed fire, rhizome, succession, swamp, vine

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 [89]) and characteristic California marsh plants such as cattails (Typha spp.) (Fuller personal communication cited by [58]). Pale-yellow iris may also reduce habitat needed by waterfowl and fish ([77], reviews by [35,78]), including several important salmon species (review by [35]). Pale-yellow iris may also reduce available forage for livestock [54]. 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" [77].

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 [52]). Pale-yellow 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 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 [35]).

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 [52].

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 [73]). On Theodore Roosevelt Island in the Potomac, pale-yellow iris occurred in clumps about 1 m² in area [77]. In southern New York, pale-yellow 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]. 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 [49]. In England, herbicide removal of common reed (Phragmites australis) produced open areas in which pale-yellow iris established (review by [73]). 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 [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 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 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 [77].

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 ([53], 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 [73]. A horticultural guide suggests the removal of seed pods to prevent future establishment from seed [75]. Draining wetlands to remove pale-yellow 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 pale-yellow 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 pale-yellow iris. A horticultural guide states that pale-yellow 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 pale-yellow iris [78]. 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 [73]). 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 [73]). Pale-yellow iris hosts and is susceptible to the rust of pale-yellow 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 pale-yellow 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 pale-yellow iris ([52], reviews by [35,78]).

Sources and Credits

  1. (c) Rita Willaert, some rights reserved (CC BY-NC), https://www.flickr.com/photos/rietje/2502271578/
  2. (c) Donald Hobern, some rights reserved (CC BY), http://farm9.staticflickr.com/8256/8751964172_be58ddc892_o.jpg
  3. (c) José María Escolano, some rights reserved (CC BY-NC-SA), http://farm3.static.flickr.com/2746/4176838274_7982d83115.jpg
  4. (c) José María Escolano, some rights reserved (CC BY-NC-SA), http://farm5.static.flickr.com/4126/4996291891_e0867aebb7.jpg
  5. (c) Jörg Hempel, some rights reserved (CC BY-SA), http://farm6.staticflickr.com/5477/11720179964_9c7ec142a7_o.jpg
  6. (c) Jakob Fahr, some rights reserved (CC BY-NC), uploaded by Jakob Fahr
  7. (c) anonymous, some rights reserved (CC BY-NC), http://www.biopix.com/photos/Iris-pseudacorus-00016.JPG
  8. Jan Ševčík, no known copyright restrictions (public domain), https://www.biolib.cz/IMG/GAL/90909.jpg
  9. Adapted by Kate Wagner from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Iris_pseudacorus
  10. (c) Wildscreen, some rights reserved (CC BY-NC-SA), http://eol.org/data_objects/2611513
  11. Public Domain, http://eol.org/data_objects/24629270

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