Yellow Flag Iris
(Iris pseudacorus)

Name and Family

Yellow flag iris, Iris pseudacorus, is part of the iris family (Iridaceae). Members of this family produce regular, bisexual flowers with parts in multiples of three. There are over 2,200 species in this family. 


Yellow Flag Iris originated in Europe, Western Asia, and Northwest Africa. It was introduced to North America in the early 1900s as an ornamental garden flower. It has since spread throughout Canada and the U.S. It can be found in several ponds and streams across Prince Edward Island.

Identification Guide

Yellow flag iris is an emergent aquatic plant that is drought tolerant. Positively identifying yellow flag iris can be difficult, especially when it is not in bloom. Here are some distinguishing features to look for:

  • Vigorous and aggressive growth habit, much more so than native iris species.
  • Leaves are sword-shaped, 0.5-1.5m long, 1-3cm wide, and have a raised ridge in the middle of the leaf
  • Large yellow flower with 3 petals facing up and 3 petals hanging down
  • Brown-purple stripes on petals.
  • Blooms June to August.
  • Flowers yield seed pods that resemble small green bananas.
  • Creates dense mats.
  • Extensive root system of connected rhizomes.

What it does in the ecosystem

  • Yellow flag iris is a popular garden ornamental that escapes from gardens into wet areas such as ditches, wetlands, and around streams and ponds.
    • In these environments, it is an aggressive invader that forms dense thickets harming native plants and wildlife. 
    • When established in ditches, they can raise ditch elevation, resulting in flood control issues. 
    • The seeds have also been known to clog up drainage intakes and irrigation pipes. 
  • Its dense underwater root network can have up to 100 flowering plants connected.
    • The roots collect sediment coming downstream and harden the banks of rivers, restricting water flow by adding an average of 10 inches to the river’s edge annually.
      • This can result in a dried-up system, reducing the amount of open water for fish passage and waterfowl feeding.
  • It impacts the availability of food resources for wildlife by replacing native plants such as cattails, sedges, and rushes
    • This iris is unpalatable to many species, toxic to humans and pets, and has sap which can cause contact dermatitis 



Before selecting a control method, consider 

  • The size of the infestation. 
  • Resource availability. 
  • The amount of effort you are willing to expend. 
  • Often, multiple control methods are used simultaneously. 
  • Consider the local ecosystem and what other organisms or ecological processes may be affected by management. 
  • Consider any cultural or socio-economic impacts of your selected control method, such as impacts to landowners, indigenous groups, or other activities in that area
  • In watercourses, we recommend targeting control efforts upstream.
    • Seeds from this plant that float downstream will establish new populations, recolonizing downstream control areas and frustrating efforts. 
  • We recommend targeting small populations that have established away from the larger outbreak first.
    • These have the greatest potential to spread the infestation outward and are easiest to manage when small.  


  • If managing an invasive species near a watercourse or waterbody, you are required to obtain a Watercourse, Wetland and Buffer Zone Activity Permit from the PEI Department of Environment, Energy, and Climate Action. 
  • This permit should be applied for at least six weeks prior to beginning management activities to allow for processing time. 
  • It is unlawful to apply herbicides within 15m of a wetland or watercourse on PEI. 
  • The permit can be applied for at the following link: Apply for a Watercourse, Wetland and Buffer Zone Activity Permit | Government of Prince Edward Island


  • Digging can be an effective control measure for yellow flag iris but should be done before the plant has gone to seed. 
  • Shovels and other garden tools can be used to remove as much of the plant and root system as possible, as roots left in the ground will give rise to new plants. 
  • Sites should be monitored after treatment to ensure any regrowth is removed. 
  • Protective equipment should be worn when removing the plant to avoid the irritating resin. 


  • Repeated mowing may eventually result in complete mortality of yellow flag iris. 
  • Mowing prevents photosynthesis, gradually depleting the energy stored in the rhizomes which is necessary for plant growth. 
  • Mowing also impedes the plant from producing seeds, depleting the seed bank. 
  • It is necessary to repeat cutting practices yearly to inhibit regrowth. 
  • Mowing should occur before plants go to seed. 


  • Planning
  • Cutting the plant materials down and then covering the area can be an effective management strategy. 
  • Covering is not a selective method and all species under the tarp will be killed, so it is important to only cover areas that have iris as the dominant species. 
  • Instead of using one big tarp, it is often best to cut the covering to the size needed and make smaller patches. 
  • Tarping works because the root system would be deprived of light and the plant’s roots will be continuously baked by the sun as temperatures below the tarp increase.
  • This is a simple practice on dry land, but  when this plant is being managed in an aquatic environment the challenge increases. 
  • Tarps can still be placed in aquatic areas whether they are submerged year-round or even seasonally. In most cases, the tarps will need to be modified over time to stay in place. 
  • Tarps placed at the bottom of a waterbody are known as “benthic barriers”.
    • They are built to be weighted down and built to prevent billowing when gases are released by the decaying plants. 
    • Commercially available tarps are available but it is also possible to build your own. 
  • Whether you are tarping in a wet or dry area, the tarp should extend at least three feet past the outbreak on all sides. If they overlap they should do so by at least one foot and be secured.
    • When placing the mat in water, here a few more things to consider:
      • How soft is the water substrate? 
      • How deep is the water? 
      • How much boat traffic passes through?
      • What impacts does placing the tarps have this time of year on wildlife (especially fish)? Fish reproduction often relies on access to the benthic layer.
      • How often you will be able to check on the mat? You must return to the site regularly to ensure that the mat is in good condition and that no new growth has pushed through or around the mat.
  • Installation
    • On installation day, cut the leaves as low as possible (below the water level if in standing water). 
    • Use a shovel to go around the edges of the tarp to disconnect the covered area from any roots that may extend past the tarped section.
      • This step is critical to ensure that the plant does not expand horizontally through its root system.
    • To secure the mat during installation we recommend burying the edges into the soil. Place heavy objects over the mat approximately every 3’ when possible with heavy objects (like cinderblocks). 


  • If you did not get a chance in the season to perform long-term management at the site, it is still highly recommended that you cut any developed seed pods off of the plants. 
  • This will prevent seed dispersal and thus prevent the population from spreading over long distances.
    • The seeds of this iris can float for months and establish downstream of the initial population.
  • Wear gloves while cutting to prevent exposure to the irritating sap.


  • Burning is not a recommended control method for yellow flag iris. 
  • Burning does not damage the underground root system, which supplies the energy for plant regrowth. 
  • In addition, the increased light availability due to the loss of vegetation from burning can promote the germination of yellow flag iris seeds. 


  • As is the case with most invasive plants, return visits to the site after management and in subsequent growing seasons will be required to completely eradicate yellow flag iris.
  • Gloves should still be worn during disposal and follow-ups to prevent exposure to the irritating sap.
  • The leaves of yellow flag iris do not need to be bagged and taken from the site.
    • If there are any roots or seed pods (even unripe) they must be bagged and disposed of safely. 
    • Even a small piece of a root has the potential to regenerate and start a new population.
  • Each time you manage the site, collect and dispose of all seeds and root parts properly.
    • Bag the plant material in clear plastic bags.
    • Mark the bags boldly with a permanent marker. Write “INVASIVE PLANT” or “YELLOW FLAG IRIS” on the bags. 
    • If you have a place away from foot and wildlife traffic, you can dry out the material, making it much easier to move.
      • Leave the bags open and in the sun for a week to dry. 
      • Only do this if you can guarantee that the plant will not be spread from where it is drying
    • If the bags are thin, double bag them to prevent accidental dispersal later on.
    • Tie the bags tightly and place them in your usual residential waste collection (THE BLACK BIN)
      • You can place up to two additional bags beside the bin for collection if your bin is full.
      • For loads larger than this reach out to the PEIISC for specific disposal instructions. 
      • Loads larger than a ½ ton truck will require a Special Waste Disposal permit.
    • Never place invasive plants into the compost. This may allow them to become established once they reach the heap. 
  • Clean and remove all plant parts from any equipment, clothing, or vehicles before leaving the site to prevent spread.
    • Hose off your gear and wash with soapy water. 
    • Double-check that all plant parts have been removed and that your tools are dry and dirt-free before storing them.


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