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Class B Noxious Weed

Parrotfeather milfoil

History

Parrotfeather milfoil, a native of South America, was sold in the United States as an ornamental for aquatic gardens and aquariums. Because of its attractiveness, some lake residents have deliberately planted this species unaware of its invasive and aggressive growth pattern. Since 1996 parrotfeather has not been offered for sale in Washington State.

Method of spread

Parrotfeather forms dense mats of vegetation that can entirely cover the surface of shallow lakes. The tough stems make it difficult to boat, swim, fish, or water ski. Unfortunately, parrotfeather grows well in the Northwest and is widespread throughout Washington and Oregon. The plant spreads readily through fragmentation of the stems and underground rhizomes.

Methods of control

Parrotfeather is difficult to control. Grass carp used for plant control find it unpalatable. Harvesting and other mechanical controls produce fragments that help spread the plant. Its foliage makes herbicides difficult to deliver effectively, and the waxy leaves and stems inhibit herbicide uptake. Mechanical harvest has been used in Washington with limited results.

Identification

  • bright green, stiff, feather-like foliage that can extend up to one foot above the water's surface
  • bright green leaves, oblong, deeply cut and feathery looking, are arranged in whorls (four - six) around the stem and remain submerged until the plant flowers
  • stems can be five feet long, trailing along the ground or water surface, turning erect and leafy at the ends, they appear somewhat "fir-tree-like"
  • female flowers lack petals, male flowers have four small petals
  • dense mat of intertwined brownish stems (rhizomes) in the water
  • blooms May to July

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For questions about lakes in King County, please contact lakes@kingcounty.gov or call the Water and Land Resources Division front desk at 206-477-4800.