Eurasian Watermilfoil
Myriophyllum spicatum
Washington State Classification
Class B — Control Required (Designated)
It is an ornamental aquatic plant that has escaped cultivation.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Eurasian watermilfoil is a perennial, submersed, aquatic plant with dissected leaves. It forms dense mats with flowering stems extending above the water’s surface. There is a land form of the plant that has smaller, stiffer leaves
Leaves
Leaves are finely dissected and occur in whorls of 4 (rarely 5). Each leaf has 12 or more leaflet pairs (sometimes down to 5 pairs of leaflets). Leaves have a feather-like appearance.
Flowers
Flowers are tiny, pinkish and occur on reddish spikes. Male and female flowers are on the same plant.
Fruit & Seeds
Seeds detach from plant, float and then sink if still in water.
Impact
It is an ornamental aquatic plant that has escaped cultivation. Infestations can alter aquatic ecosystems. It forms dense mats that shade out other aquatic plants, degrades water quality, inhibits water flow and impacts recreational activities.
Ecology & Spread
Habitat
It is highly adaptable and is found in streams, ponds, lakes and ditches. Please click here to see a county level distribution map of Eurasian watermilfoil in Washington.
Spread Mechanisms
Reproduction
Eurasian watermilfoil spreads mainly by stem fragments.
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Control Methods
Mechanical
- Mechanical control is not advised unless the area is entirely invaded by plants. Otherwise, mechanical methods may increase the infestation. Hand pulling may be employed, but the entire plant must be removed or it will re-sprout.
Cultural / Prevention
- Reduce weed infestation by handpulling weeds. Digging and carefully removing all rhizomes will effectively eliminate single plants and small infestations.
Biological Control
- The milfoil weevil, Euhrychiopsis lecontei, while native to the United States, is the most promising insect found to use as a biocontrol on Eurasian watermilfoil. Because the insects known to control Eurasian watermilfoil are naturally occurring, and because rearing them is time consuming and sometimes difficult, and there is no local source to purchase them at this time, their natural dispersion is relied on to aid with control.