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Invasive Species in Amherst State Park

Iris pseudacorus - Yellow Flag Iris, Yellow Iris, Water Flag, Yellow Flag

Close up Photo

photo by  WNY PRISM

June 5, 2015

Infestation Photo

photo by  WNY PRISM

June 5, 2015

Cirsium arvense - Canada Thistle, Creeping Thistle

Close up Photo

photo by  WNY PRISM

June 5, 2015

Infestation Photo

photo by  WNY PRISM

June 5, 2015

Infestation Photo

photo by  WNY PRISM

June 5, 2015

Close up Photo

photo by  WNY PRISM

June 5, 2015

    Yellow Flag Iris (Iris pseudacorus) is a perennial native to Europe, eastern Asia, and northern Africa with bright yellow flowers on a robust stalk, often over-topped by long, stiff, sword-like leaves in clumps. It is in the family Iris and flowers in the months June through August..  Yellow Flag Iris invades wetlands, riparian habitats by producing dense monocultures that outcompete native vegetation. This plant is found growing in several spots along Ellicott Creek.

    Smaller patches of yellow flag iris can be dug out of the soil using shovels, but care must be taken to remove the entire root mass and to properly dispose of material, similar to Japanese knotweed. Repeated mowing for several years may suppress patches of yellow flag iris and deplete the plant’s energy. However, mowing may be difficult due to the fact that yellow flag iris prefers wet habitats. Take caution when removing yellow flag iris, by wearing gloves, long sleeves and pants, as the resins from the plant may be a skin irritant. 

    It would be valuable to educate the public about yellow iris. Yellow flag iris is a prohibited species in New York State, though it is a popular plant in water gardens and many members of the public do not understand that it is an invasive species.

Canada thistle (Cirsium arvense) is a perennial, rhizomatous herb native to Europe and Asia. It grows 2 to 5 feet tall. Leaves have spiny crinkled edges and stems branch at the top with numerous fragrant, pale magenta lavender, or white, rayless flowers. It flowers in the months of June through October. Once established, Canada thistle spreads quickly replacing native plants. It grows in circular patches, spreading through roots which can expand up to 10-12’ in one season. Canada thistle produces an extensive, rhizomatous root system, which may extend 20 feet below ground.

    Repeated mowing and cutting the Canada thistle at the base of the stem for several years may suppress the plant, but their substantial root system allows it to recover from damage. Therefore, cutting or mowing must be done several times a year, in order to stress the plant enough to prevent it from spreading. It is impossible to pull up all of the root of Canada thistle, and broken rhizomes only encourage the plant to produce more sprouts. Because of this, hand pulling is not a viable treatment option. Herbicide treatment combined with mowing is the most effective treatment.