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Invasive Plant of the Month: Yellow Flag Iris, Iris pseudacorus

by Lawrence Rozas



This is the first installment of what I hope will become a regular feature of the monthly newsletter. My purpose for writing a monthly article about the Invasive Plants of Louisiana are to: (1) help readers identify them, (2) convince readers to avoid purchasing those still available in the nursery trade and using them in their landscapes, and (3) encourage readers to eliminate those invasive plants already established on their property.


Invasive plants are non-native species (i.e., did not occur in the U.S. before European settlement) that cause environmental damage and economic harm where they have been introduced. They cause environmental harm by spreading uncontrollably and replacing native species, upon which native wildlife depend. Invasive species also take a great toll on our economy by the damage they cause to agricultural crops, fisheries, forests, and native wildlife as well as the high cost required annually to monitor and manage the problems they cause.


This iris is a native of Europe and was introduced to the U.S. as an ornamental. Yellow Flag Iris has infested a variety of wetland habitats in the state including swamps, and the edges of lakes, ponds, and bayous. Its ability to withstand droughts and extended anoxic conditions gives this species a competitive advantage over native wetland plants. Yellow Flag Iris spreads by rhizomes and seeds; it forms large clonal colonies that replace native species including native irises; the plant contains glycosids, which are toxic to grazing animals. Unfortunately, it continues to be sold in the nursery trade because Yellow Flag Iris is admired for its quick establishment, evergreen foliage, and vigorous growth. This invasive species can be distinguished from native Louisiana Irises by the presence of a central vein in the leaf of the Yellow Flag Iris; native irises lack a central vein.


Responsible gardeners have several wetland-adapted native species from which to select that are a better choice than this weed for landscaping their property. Each of the following native species name is linked to pictures and more information available at the “Plants of Louisiana” website supported by the U.S. Geological Survey. Louisiana is blessed with at least six species of native irises from which to choose as a substitute for Yellow Flag Iris. If you are looking for a vigorous iris with tall evergreen foliage, the large Blue Flag Iris Iris giganticaerulea is for you. Other Louisiana irises include the Zigzag I. brevicaulis, Copper I. fulva, Dixie I. hexagona, and Abbeville Red I. nelsonii. Two other wetland plants Swamp Lily Crinum americanum and Spider Lily Hymenocallis lireosme, would also be fine replacements for this invasive species. Read more about this invasive species here.


Photo credit: Larry Allain

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