Weeds: Parrotfeather and Eurasian watermilfoil – Myriophyllum spp.

• Family: Haloragaceae
• Cycle: Perennial
• Plant type: Broadleaf aquatic

Biology

These rhizomatous perennials exhibit an annual pattern of growth. In the spring, shoots begin to grow rapidly from overwintering rhizomes as water temperatures increase. Rhizomes function as a support structure for adventitious roots and provide buoyancy for emergent growth during the summer. Emergent stems and leaves extend from a few inches to over one foot above the water’s surface. Underwater leaves tend to die back and drop off as the season advances. Bright green emergent leaves are 1/2 inch to 1 inch long and have six to 18 divisions per leaf. Plants usually flower in the spring but some plants may also flower in the fall. The inconspicuous flowers form where the emergent leaves attach to the stem. Parrotfeather typically dies back to the rhizomes in autumn. Unlike Eurasian watermilfoil, parrotfeather does not form autofragments. However, fragments can be formed mechanically and will readily root. A related species, variable-leaf milfoil (M. heterophyllum), is sometimes used as an aquarium plant. All three species may be spread by moving plant fragments on boats, trailers, or water equipment. SPECIAL INFORMATION: Parrotfeather and Eurasian watermilfoil adversely impact aquatic ecosystems by forming dense canopies that shade out native vegetation. Monospecific stands of either species provide poor habitat for waterfowl, fish, and other wildlife. In WASHINGTON and OREGON, parrotfeather and Eurasian watermilfoil are designated as Class ‘B’ noxious weeds. Management of these species may be required by law in your county. In addition, both species are on the Washington and Oregon noxious weed quarantine lists, which prohibit sale, purchase, and transport of plants, seeds, and plant parts. Variable-leaf milfoil is a Class ‘A’ noxious weed in WASHINGTON; control is REQUIRED by law. Consult your county Noxious Weed Control Board for more information.

Habitat

Parrotfeather (Myriophyllum aquaticum) and Eurasian watermilfoil (M. spicatum) are found throughout southwestern Washington in freshwater lakes, ponds, streams, and canals. They appear to be adapted to high nutrient environments, and tend to colonize slowly-moving or still water rather than areas with higher flow rates. While they grow best when rooted in shallow water, they have been known to occur as floating plants in the deep water of nutrient-enriched lakes. The emergent stems can survive on wet banks of rivers and lake shores, so they are well adapted to moderate water level fluctuations. With tough rhizomes, parrotfeather and watermilfoil can be transported long distances on boat trailers.

Management Options

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

• Reduce weed infestation by handpulling weeds.
• Digging and carefully removing all rhizomes will effectively eliminate single plants and small infestations.

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.

• Handpulling and digging is only effective on small infestations.
• Do not use mechanical control methods as they will increase the rate of spread!
• Applications should be made by those holding a current aquatic pest control license.
• In Washington, a special permit is required for use of herbicides in aquatic sites.
• Contact the Washington Department of Ecology or the Washington State Noxious Weed Control Board for more information.

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