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Yellow Flag Iris (Iris pseudacorus L.)

Also known as yellow flag, yellow iris, paleyellow iris, pale yellow iris, water flag.

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Yellow Flag Iris
Photo © Matthew Gerke, CC BY 4.0.


A long-lived perennial with showy yellow flowers, widely grown in gardens and invasive in wetlands and along shorelines in North America.

Range - Expand

Introduced or Not Present

This tentative map is based on our own research. It may have limited data on Canada and/or Mexico, and there is some subjectivity in our assignment of plants as introduced vs. expanded. Read more in this blog post.


Both in its native range, and in North America where it is widely considered invasive, the Yellow Flag Iris grows primarily on land-water borders and in wetlands. Natural habitats include the edges of both still and moving bodies of water, including ponds, lakes, rivers, and streams. It also occurs in various wetlands, including both tidal and non-tidal marshes, swamps, wetlands in glacial potholes, beach swales, and along rocky coastal shorelines. It can occur on wet sites in open forests and forest edges. It frequently occurs in anthropogenic wetlands and land-water borders, including drainage ditches, irrigation canals, constructed gravel trails through natural wetlands, artificial pools or ponds, and wet areas in pastureland.

It tolerates a range of salinity, growing well in fresh water, and tolerating sites in estuaries where brackish water reaches the base of the plant during high tide. Along oceans and inland lakes with higher-salinity water, it cannot tolerate salt water reaching the base of the plant, but can grow in areas slightly farther back where there is some exposure from salt spray or rare flooding from a high tide, including the highest zones of salt marshes. Its salt tolerance has also allowed it to colonize wetlands in arid regions of the interior West where soils have higher salt levels.

Plants tolerate water with low dissolved oxygen. It is more picky, however, about water temperatures. In the warmer parts of its range, it is absent from shallow wetlands as the water temperatures reach to too high levels, and it is restricted to areas where the water is connected to areas of deeper water that do not heat up as much during the hottest spells.

It can grow on sites with standing water as deep as 3 feet. Because it cannot germinate and establish on deep water, it typically establishes first on shallower sites closer to the edge, and then spreads vegetatively, forming thick mats that spread out over the deeper water.

Underneath the water, soils can be highly variable, and can include the full range of soil textures, including clay, silt, clay loam, sandy loam, sand, gravel, cobble, and larger rocks. Soils can be calcareous, or derived from sandstone or schist. It also can grow on organic-dominated peaty soils such as found in bogs.

It is found on sites with a wide range of pH, spanning from 3.6 to 7.7, but is more common on higher pH sites, preferring about 6.0. It prefers sites with high nutrient availability, which coincides with its ideal pH.

It needs some direct sunlight to thrive, and is intolerant of deep or full shade, but tolerates a wide range of lighting conditions from full sun to partial sun or light shade such as is common in swamps or other forested wetlands. On open sites, it can persist through all successional stages; it is often an early colonizer of disturbed sites, and persists indefinitely. On better-drained sites it will eventually be shaded out by trees.

The yellow flag iris tends to alter the habitats in which it occurs, often forming large monocultures that shut out native vegetation. This can happen even around the borders of its range in North America, where it is uncommon, and is especially true in areas where it is abundant. Because of its persistence through all successional stages in open wetlands and land-water borders, it shuts out many different stages of vegetation.

Life Cycle

This species is a long-lived perennial that forms extensive vegetative colonies by rhizomes, but also reproduces by seed and by breaking off of pieces of rhizomes.

Most seed germinates in spring, with a smaller but still significant amount of seed germinating later in the growing season, through summer and fall.

Seedlings establish best on sites that are moist but not completely waterlogged, and cannot establish in standing water. These sites often coincide with the high water mark following a flood event. Once seedlings establish, they can tolerate temporary flooding, but grow fastest on sites that remain moist but without flooding. Seedlings are also highly susceptible to drought in the first 2 months.

Once established, plants form extensive rhizomes and begin moving into both deeper water, and drier areas. Rhizome growth is initially slow, but accelerates as the extent of the plant increases, and as it is consistently able to access both water and aeration through its root system, thus becoming more resistant to both drought and flooding.

Plants begin flowering around 3 years of age, with flowering usually occurring during late May to July, but starting much earlier in the southernmost portions of its range. Seed capsules contain up to 120 seeds. Seeds contain a gas bubble inside a hard coat, such that they float. At the age of first flowering, plants invest relatively little energy in flowering and seed production, instead spending most of their resources on rhizome production.

At about 10 years of age, rhizomes begin to fragment, which aids in the colonization of new sites. Rhizome fragments are often much faster at establishing, and able to colonize a wider variety of sites, relative to seeds. Rhizomes tend to break off during flooding events, which bring faster-moving water that carries rhizome pieces to new sites. Rhizomes can survive without any water for 3 months and perhaps longer for larger pieces. Plants also can reproduce vegetatively through rhizomes growing over long distances while still connected to the parent plant; this method helps this species to move over rocks along rocky shorelines, and also helps plants reach greater light when growing in shaded habitats, where rhizomes tend to become more elongated. In sunny, wet conditions, the rhizomes branch more extensively and form a dense mat which extends out over the water.

Older plants begin investing more in flowering; a single plant can have several hundred flowers at once, and even more non-flowering stems.

The persistence of this species in the seed bank is not known, but is likely low. It likely forms a short-term seed bank, but with most of the seed either germinating or dying in the first full year following seed distribution.

Individual portions of rhizome can live up to 10 years, and a single vegetative clone can live 30-40 years, possibly longer. However, mortality is very high for seedlings. For seedlings, mortality usually occurs during the first couple months, usually from drought. Flooding is less likely to kill seedlings, only slowing their growth. In the north of its range, seedlings are often killed in the first winter, when surface water freezes. Mortality of established plants is much lower, as drought or flooding tends to kill only a portion of a plant, and plants can regrow quickly as conditions renormalize. Extensive colonies can be killed by heavy shading, or longer-term changes in water level or salinity caused by environmental conditions.

Faunal Associations

Even relative to other invasive species, yellow flag iris is poor at supporting the food web. Very little in North America eats it.

Yellow flag iris is poisonous to most mammals, and is usually ignored by mammalian herbivores, including both wild animals and livestock. In overgrazed pastures, hungry livestock can consume this plant, leading to poisoning. The muskrat (Ondatra zibethicus) occasionally eats the rhizomes and lower stems.

As of 2009, there were no known biological control agents. In the southern hemisphere, where this species is also invasive, some insects have been studied, but we could find no effective control agents in North America. Although 10 Lepidoptera species eat this plant in its native range, none of these are found in North America, and no lepidoptera species have been recorded feeding on this plant in the wild here. The iris thrip (Iridothrips iridis) feeds on young leaves and leaf sheaths, but does only minimal damage to plants. In its native range, sawflies, seed weevils, and flea beetles feed on this plant, sometimes causing significant damage, but none of these types of species appreciably feed on this plant in North America.

Some gardening sources report that this species is eaten by slugs, weevils, borers, and other invertebrates, but these reports are from landscaping plants, where this species is grown in much drier conditions than it occurs in in the wild, and thus plants are likely to be stressed and produce fewer toxins. The iris borer (Macronoctua onusta) has been reported eating garden plants as well as plants in a nursery production setting. Because that insect eats plants in the production process in nurseries, selection for resistant plants may have been a factor in selecting plants for cultivation, which may have contributed to this plant's complete lack of support for the food web.

The flowers are pollinated by bumblebees.


This species is difficult to control, primarily because the most effective control methods cause other harm to the ecosystems in wetlands and aquatic borders where it is most invasive. These areas are also often messy and labor-intensive to work in.

Mechanical control through digging out rhizomes is effective, but causes disturbance which allows other plants to establish. For this reason, care must be taken to time control, and often to first control other invasives, before removing this plant, to prevent colonization of the site by other invasives. Care must be taken to dispose of the rhizomes in a way that minimizes risk of their spreading to new sites, as rhizomes can survive without water for months and resprout when favorable conditions arise.

Sustained mechanical control typically requires follow-up because small pieces of rhizome can resprout, and it is rarely possible to fully remove all rhizomes on the first pass. However, initial efforts usually greatly reduce this plant's extent, and its resprouting is much slower than its growth prior to the intial control effort. Mechanical control can be carried out at any time of year, so the time can be chosen to favor establishment of desireable vegetation.

Mowing or cutting the top portions of this plant tends to harm it less than digging out its rhizomes, and is usually not effective at killing it, but cutting the top parts of this plant can be a low-effort way to slow its growth, or prevent its reproduction by seed. This approach is most effective if carried out as soon as the foliage reaches its maximum height. For preventing seed production, cutting during flowering is effective. Because this species spreads more by seed in some areas, and more by rhizome fragments in others, it is important to first research whether reproduction by seed occurs in your area, or this approach may have minimal benefit.

It can be controlled by herbicide, but the wetland ecosystems in which it occurs are susceptible to greater damage from herbicides, especially from the surfactants typically used to make them effective, so great care must be exercised if using herbicide to control this species. The tough, waxy leaves of this plant require more surfactant than for plants with delicate leaves, thus posing a greater risk to aquatic life. Because there is a large time window in which herbicides are effective, it is often best to apply herbicide towards the end of a long, dry spell where water levels are lowest.

One of the most important aspects of control of this species is the maintenance of healthy flora in wetlands, and the prioritization of control of this species before the carrying out of any projects that may create new habitat for this and other invasive species. For example, this species often is a fast colonizer of new elevated paths created through wetlands. For this reason, it is important to control this species, and create a healthy wetland community in the years before constructing new paths, so that when the paths are constructed, their margins are colonized by a healthy community of native plants.

Healthy populations of weedy, opportunistic native plants are also important for colonizing the temporarily-disturbed sites created after removing this plant. Weedy native plants, which are often neglected in ecological restoration efforts, can play a key role in preventing the reestablishment of this species or the establishment of other invasives, and can help wetlands achieve a more natural succession trajectory leading to more desireable vegetation.

The removal of this species from gardens and landscaping adjacent to wetlands (whether natural or anthropogenic) is also a key ingredient in its control. This species has become so widespread precisely because it is so widely planted. Suburban gardens tend to be a source for seeds and rhizomes that spread downstream into floodplains and wetlands. The replacing of this plant in gardens with native wetland vegetation can ensure that native plants are instead colonizing wetlands downstream.


The yellow flag iris is widely planted as an ornamental, although it use for this purpose is decreasing as it is increasingly recognized as an invasive plant.

It was originally valued for its showy flowers, deer resistance, tolerance of wet conditions, and being generally easy to grow and adaptable to a wide range of soil types and lighting conditions.

Historically, the rhizome was used medicinally, including to treat toothaches, and to induce vomiting. Its use is limited by its toxicity.

This species has also been used to remove heavy metals from contaminated bodies of water, since it tends to absorb these metals at a higher rate than other plants. However, due to its invasive nature, this use is best avoided outside of its native range.

Numerous other Iris species, both native and introduced, occur in North America. It belongs to the Iris ser. Laevigatae, which includes three species found in North America: the non-native Japanese iris (Iris ensata), and the native harlequin blueflag (Iris versicolor) and Virginia iris (Iris virginica).

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Photo gallery

Photo © Bill Keim, CC BY 4.0.
Photo © Matthew Gerke, CC BY 4.0.