Myriophyllum aquaticum • Class B

Parrotfeather gets its name from its feather-like-leaves which are arranged around the stem in whorls of four to six. Its leaves are both submersed and emergent. The submersed leaves are easily mistaken for Eurasian watermilfoil, a close relative. The darker green submersed leaves have 20 to 30 divisions per leaf. The feathery emergent foliage is longer and has only 6 to 18 divisions per leaf; these are bright green and can grow up to a foot above the water surface and look almost like small fir trees. Its blooms are small, white and inconspicuous. 

Dense infestations have caused flooding and drainage problems in shallow rivers and streams. One example of this problem in Washington State is in the Longview Diking District, which estimates that it spends about $40,000 a year on parrot feather control in drainage ditches. 

Parrotfeather is found in freshwater ponds, lakes, streams, canals, and appears to be adapted to high nutrient environments. It tends to colonize slowly moving, or still water rather than in areas with higher flow rates. Because the emergent stems have access to atmospheric carbon dioxide, parrotfeather is probably the most productive milfoil species. 

Infestations can alter aquatic ecosystems by shading out the algae in the water that serves as the basis of the aquatic food system. This plant also serves as choice mosquito larvae habitat. 

As with all invasive plants, control is most easily achieved when it is performed before large infestations have been established. 

• Since it spreads by stem and rhizome fragments, mechanical methods such as cutting, harvesting and underwater tilling are not advisable. These methods can increase infestations by dispersing plant fragments that may root in uninvaded areas. 

• Covering parrotfeather with plastic or other material after a fall/winter drawdown (water level drop) may be effective if large areas can be covered. Covers may have to be maintained for extended periods because of persistence of the rhizomes. 

• There are a number of biocontrol potentials that are being tested. One problem with this plant is that it has high tannin content and most grazers, including grass carp, find it unpalatable. 

• Herbicides are the most common and effective means of controlling parrotfeather. However, it requires multiple applications per year for several years to be effective. A number of aquatic herbicides are registered for use on parrotfeather. Please note that aquatic herbicides are restricted for use in Washington State to licensed applicators only 

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels