Parrotfeather
Myriophyllum aquaticum

This aquatic plant is a Class B noxious weed in Washington State. Control is required in King County and we are interested in knowing where this plant is growing in the county. Because of its limited distribution in King County, there is an opportunity to stop its spread and work towards eradication.

Parrotfeather has been popular as an ornamental in ponds and aquariums, but because it rapidly takes over lakes, ponds and ditches, it is now illegal to buy, sell or trade this plant in Washington State (for more information see the Washington State Prohibited Plants List).

If you see parrotfeather in King County, please contact our program by email or phone at 206-296-0290 or notify us through our online infestation form.

Identification (see below for more photos)

Spikes of feathery leaves emerge up to a foot above the water, looking like miniture pine trees growing on the water's surface. Below the water, the leaves are less stiff and closely resemble other types of milfoil, but the plant is easily distinguished by its stiff, bright green upper stems. The feathery leaves grow in whorls of four to six (meaning that there are four to six "feathers" attached at the same point on the stem that radiate out in a circle around the stem). Stems are robust, even under water, and roots form along the stem. Flowers are white, tiny and inconspicuous and are found where the above-water leaves attach to the stems. Because all plants in the United States are female, parrotfeather doesn't form seeds but it does spread readily by fragmentation of stems and rhizomes. In King County, parrotfeather emerges in late May and persists into October.

Where it Grows

Parrotfeather forms dense mats of vegetation and can be found growing along lake and pond edges and slow-moving waterways. Even when attached to the bank, stems can extend several yards out over the water's surface and it has also been found growing as a floating mat in some lakes. It has even been found growing up onto the soil along the edges of ponds. In King County, parrotfeather is mostly found in private ponds. However, in other parts of the state, it is found in drainage canals, lakes, and rivers. For instance, parrotfeather is found throughout the drainage system of the Longview/Kelso area.

Impacts

Although parrotfeather is a native of the Amazon River in South America, it has naturalized throughout the world. In Washington State it has become entrenched in several drainage systems and even small infestations are highly challenging to control. Although it doesn't grow in deep water, parrotfeather can completely cover and fill in shallow water. The dense growth causes problems for boating, swimming, fishing or other water recreation. It encourages mosquito growth and can increase flooding. In rivers, it can impact salmon habitat, especially in side channels where juvenile salmon rear.

Control

Parrotfeather is costly and difficult to remove once it is established in a waterway. We encourage King County property owners to contact our program for assistance controlling this plant.

Small populations can be pulled or raked up, being very careful to remove all fragments from the water. Manual control requires persistence over many years.

Aquatic herbicides can be effective if applied correctly, but can only be applied by a licensed pesticide applicator with an aquatic endorsement and will likely require a permit to use in water. For more information on permits in Washington State, see the Department of Ecology website. If you would like assistance treating your parrotfeather in King County, please contact our program and ask for the aquatic weed specialist.