Blog & News

Benthic Mats – Yellow Flag Iris Control

Published on Monday August 1, 2022
Authored by PEIISC

Have you heard of using a benthic mat to control aquatic weeds?

If you are unfamiliar with the term “benthic” it refers to anything associated with or occurring on the bottom of a body of water, so it stands to reason a benthic mat is a mat that sits on the bottom of a body of water. These types of mats are helpful to control aquatic invasive plants that are rooted at the bottom which would be hard to otherwise target. They smother plants, preventing photosynthesis and reducing the plants respiration.

This season the PEIISC had the opportunity to try out this new technique thanks to the PEI Wildlife Conservation Fund. They funded us on a project to help strengthen PEI’s response to Yellow Flag Iris, Iris pseudacorus, a serious wetland invader that is not yet widespread on PEI. Thank you to all residents who supported this fund, by purchasing a Wildlife Conservation Fund license for your hunting/fishing trapping needs or to those who purchased a conservation plate for their vehicle!

About the Plant:

Yellow Flag Iris can invade both fresh and salt water systems and often escapes from gardens where it is planted by those who admire its beauty. It is undeniably beautiful, so what’s the big deal if escapes into natural areas on PEI? Well… EVERYTHING!! It’s bad for birds, fish, plants, human health, livestock health, infrastructure, rivers, and is almost impossible to get rid of once its established in large numbers!

  • One plant can produce over a hundred other plants connected from it’s root system. These plants will grow in thick, dense patches that limit the movement of wildlife and reduce nesting sites for waterfowl.
  • Root system expands each year by an average of 10 inches. It collects silt that travels through the area and forms a false bank that dries up the system it establishes in. This slowly but surely removes open water in rivers and wetlands and reduces fish passage.
  • Takes over valuable habitat that is important for native species of cattails, sedges and rushes. Cattails are an important food source to many species, however Yellow Flag Iris is unpalatable to many native species. It also prevents opportunities for our native Blue Iris’s.
  • The resin found in this sap is an irritant to the skin of many people that come into contact with it.
  • Cattle who ingest this species experience gastroenteritis and acute diarrhea.
  • This plant does well in ditches and has been known to clog them which results in flooding. It’s seed are large and have been known to build up in pipes and cause blockages.

When patches of yellow flag iris are found it is important action is taken as soon as possible. It spreads prolifically, producing an average of 6-10 banana shaped seed pods per plant each of which has up to 100 seeds. These seeds can float downstream for up to 7 months and establish far from the parent plant. Fragments of the rhizomes can also float and start new plants and so it is very important to avoid breaking them off the main system.

Benthic Mat:

Benthic mats are a great option to control Yellow Flag Iris in a pond, lake or wetland. Recent studies show that native cattails recolonize the area well after the removal of the mat. There are other important factors to consider when choosing this technique, here are some questions we asked ourselves:

  • Is it proven to be effective for this species?
  • Is there standing water present? If so, is it seasonal?
  • Will this mat affect fish spawning/hatching?
  • Is the ground mucky/safe to walk on? Will I need a muck mat?
  • Is there much traffic in the area? Boat traffic should be considered, is the mat deep enough to avoid interference?
  •  Is the iris a monoculture, is it in several distinguishable clumps or is it more scattered and mixed in with other species?
  • How large is the patch? Will it be covered all at once or in sections?

If you think a Benthic Mat is right for you, learn how to build your own with the help of our Photo Guide:

We also recommend visiting Dr. Catherine Tarasoff’s (PhD. P.Ag Agrowest Consulting Scientists) series of four videos on Yellow Flag Iris management:

Are you interested in installing a benthic mat? Reach out to us with your questions!

*Important Reminder: Any work undertaken in a watercourse, wetland or bufferzone on PEI must be approved by the provincial government. Projects must have a permit and submit for it at least six weeks ahead of the projects start date: