Skip to main content
King County logo

Class B Noxious Weed

Eurasian watermilfoil


Milfoil originates from Europe and Asia, but was introduced to North America through the aquarium industry. Milfoil may have arrived as early as the late 1800s, but was first documented in the Eastern United States in the 1940s.

Method of spread

Milfoil forms very dense mats of vegetation on the water's surface, impairing water recreation. It spreads rapidly, mostly by fragmentation of plant parts. In the late summer and fall, the plants become brittle and naturally break apart. Each fragment is capable of growing roots and developing into a new plant. It is competitive with native species and may completely dominate a plant community within a few years.

Milfoil is widespread throughout western Washington and Oregon. Found in Lake Meridian near Seattle in 1965 and Lake Washington by the mid-1970s. The distribution of milfoil now closely follows Interstate 5. Milfoil has probably been spread from lake to lake on boat trailers.

Methods of control

Once milfoil is well-established, it is difficult to eradicate. In smaller lakes, aquatic herbicides have been partially successful. Other control methods include: underwater rototilling, bottom barriers, hand pulling or dredging, and in limited situations, sterile grass carp. Removing fragments from boat trailers and along shorelines is advised to prevent milfoil's spread into new areas.


  • submersed aquatic milfoil grows in dense mats, with stalks of tiny reddish flowers held above the water
  • usually has twelve or more leaflet pairs on each leaf
  • mature leaves are usually arranged in whorls of four and are about three cm long
  • leaves rarely extend above the water and collapse when removed from the water
  • stems may reach lengths of three m or more, are usually two - four mm thick, and are reddish to olive green
  • blooms June to August

Back to aquatic weed identification

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