Non-native Invasive Freshwater Plants

Parrotfeather (Myriophyllum aquaticum)

 General Information

Parrotfeather milfoil (Myriophyllum aquaticum) is a native of South America that grows well in Washington waters. Because of its interesting growth habit (sticks up above the water surface) and beauty, parrotfeather is a popular plant for aquatic gardens and is sometimes sold for aquarium use. Washington listed parrotfeather on its quarantine list because it rapidly takes over lakes, ponds, and ditches. Aquatic gardeners cannot legally purchase, trade, or sell parrotfeather in Washington. People can easily identify parrotfeather by its bright green, stiff, feather-like foliage that can extend up to one foot above the water's surface. It looks like tiny green fir trees growing on the water. Because all the parrot feather plants in the United States are female, they produce no seeds. However, the plant spreads readily through fragmentation of the stems and rhizomes.

Growth Habit

Parrotfeather forms dense mats of vegetation that can entirely cover the surface of the water in shallow lakes, ponds, ditches, and backwaters in rivers. The plant does not grow out into deep water, but will colonize all shallow waters. The tough stems make it difficult to boat, swim, fish, or water ski. It provides ideal habitat for mosquito larvae and the mass of the plant can cause flooding to occur. We are particularly concerned about parrotfeather in rivers (such as the Chehalis and Yakima Rivers) because it may block passage for salmon and it causes pH and other water quality issues in backwater areas where juvenile salmon rear.

Parrotfeather is an especially problematic plant because it is so difficult to control. Once it gets into a waterbody, it takes tenacity to eliminate it. Parrotfeather's underwater and above water foliage makes herbicides difficult to deliver effectively, and the emergent leaves and stems are covered in a waxy cuticle that inhibits herbicide uptake. Plant-eating sterile grass carp find parrotfeather unpalatable due to the tough, woody stems and high tannin content of the plants. Harvesting and other mechanical controls produce fragments that help spread the plant.

Management

Parrot feather is a serious, costly pest when introduced to Washington rivers, ponds, ditches, and lakes. Unfortunately, because of its attractiveness, lake residents have deliberately planted this species not knowing how invasive and aggressive its growth could be. The Longview/Kelso Drainage District staff, where parrotfeather is established, estimate that they spend $30,000-$40,000 per year for control (mostly mechanical removal). Another ongoing effort is taking place in backwaters along the Yakima River where local, tribal, and state agencies are combining efforts to eradicate a small backwater infestation of parrotfeather.  

Identification

Parrot feather milfoil is easy to identify.

Look for:

Although they are both in the same genus and closely related, Eurasian watermilfoil doesn't have above water leaves

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