Class B Noxious Weed
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.
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.
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.
Back to aquatic weed identification
For questions about lakes in King County, please contact Rachael Gravon, Water Quality Planner or Chris Knutson, Project Manager, Lake Stewardship Program.