Yellow flag iris

Identification & Biology
Alias' : Yellow iris, water flag, yellow flag, pale yellow iris
Latin Name : Iris pseudacorus
Category : Terrestrial Plants
Description :
  • Perennial aquatic plant that grows up to 1.5 m tall 
  • Showy yellow flowers (7-10 cm across) with 3 sepals that curve backward and 3 petals pointing upward 
  • Sword-shaped leaves stand erect or bent at the top, and are up to 90 cm long and 3 cm wide 
  • Smooth green stem 
  • Seeds housed in green, banana-like pods and can float on water 


Yellow flag iris is an attractive aquatic plant that was first introduced to North America in the 1800s as an ornamental plant. Plants generally do not flower until the third year of growth. Flowering in May and June is followed by seed production with each plant producing several hundred seeds. This iris species can form impenetrable thickets, with hundreds of flowering plants connected under the water; fragments can form new plants when they break off and drift downstream. Unfortunately, yellow flag iris continues to be a popular ornamental plant for water gardens and ponds, and is sold in nurseries, at farmers’ markets and on the internet.  

Yellow flag iris may be confused with the introduced western blue iris (Iris missouriensis) and Siberian iris (Iris sibirica). Yellow flag iris can be distinguished by its bright yellow flowers that have 3 sepals curving backward and 3 petals pointing upwards. 

Look Alikes

Yellow flag iris thrives in wet areas, such as ditches, irrigation canals, marshes, streambanks, lakeshores, floodplains and shallow ponds. It requires high water content in the soil but does not need to be submerged. It can grow in water up to 25 cm deep, and will thrive in full sun to partial shade. Yellow flag iris is currently widely distributed in British Columbia’s southern interior, and is particularly abundant in the Okanagan Valley and Central Kootenays.  

Impact & Risks
  • Yellow flag iris creates dense stands that outcompete native wetland species such as cattails, sedges and rushes, reducing habitat for birds, fish and amphibians.   
  • Dense patches traps sediments and slow water flow, causing waterbodies to dry out.  
  • Can be toxic to livestock when ingested.  
  • Contact with the resins can cause skin irritation and blisters in people.  
Prevention & Mitigation

The most effective way to ensure that your lands do not become infested with yellow flag iris is by prevention. Here are some recommendations to prevent invasion:  

  • Do not plant yellow flag iris; suitable replacement species include native cattail, a semi-aquatic perennial plant, or Siberian iris.  
  • Maintain natural habitats in a healthy, vigorous condition to ensure a competitive plant community. 
  • Regularly patrol your property for yellow flag iris plants and immediately control or remove infestations before seed set.  
  • Take special care when controlling yellow flag iris near streams or ditch lines, to prevent the movement of plant parts and seeds downstream. 
  • If you live alongside an infested shoreline, cooperate with neighbours and encourage them to prevent yellow flag iris spread. 
  • Immediately re-vegetate or replant disturbed riparian areas or wetlands to prevent invasion.  
  • Do not move contaminated soils to a new area.  
Treatment & Disposal
  • Careful digging of the entire plant can be effective for small outbreaks. Manual removal must be repeated annually. Be sure to wear long sleeves and gloves as the sap can cause skin irritation. All plant parts, including the roots, must be removed and disposed of at a landfill. 
  • Cutting aboveground plants, then covering with pond liner or heavy PVC matting can work on land or in the water if there are few obstacles. This technique is expensive and the cover needs to stay in place 4 – 12 months.  
  • Monitoring of control techniques is required for several years, and re-planting of native species may be required. 
  • No aquatic herbicides for yellow flag iris have been approved for use in Canada.  
  • Backyard composting is not recommended as the temperature will not become hot enough to destroy the seeds and roots.  
  • There are currently no biocontrol agents (natural insect enemies) available for use in BC. 
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.