War on Weeds — Weeds are Everybody’s Problem

Eurasian Watermilfoil

Eurasian watermilfoil (Myriophyllum spicatum) is a perennial aquatic weed native to Eurasia and Northern Africa. It was first found in the U.S. in the 1940’s. It was probably brought to the U.S. in aquariums and then escaped. It invades canals, streams, lakes, reservoirs or rivers. Its presence reduces the flow of water, depletes oxygen, causes issues for boats/recreation and creates habitat for mosquitos.

Eurasian watermilfoil is a submerged aquatic weed that roots in the lake/river/stream bed. Stems are pinkish in color and often grow to the surface of the water, where small pink flowers can be produced. While Eurasian watermilfoil can reproduce from seed, its ability to spread by broken plant parts is what really makes it a dangerous invader. Its leaves have a feather-like appearance, which are arranged in whorls of four. Eurasian watermilfoil can be distinguished from native watermilfoil species by their leaves. Eurasian watermilfoil will have 12 or more leaflet pairs, while native species will have 10 or less.

Integrated Pest Management (IPM) options

• Prevention — Learn to identify this plant. Never transport unknown plant material. Report any sightings to local authorities.
• Mechanical — Hand digging or pulling can be effective on small infestations. Remove as much plant material as possible. Broken plant parts can form new plants.
• Cultural — Water drawdown can be used to expose the rootstock to cold temperatures. While water drawdown can be effective if done properly, it is not always a realistic option.
• Biological— Grass carp feed on milfoil, but they rarely eat enough to control it.
• Chemical — Extreme care should be taken when using herbicides near a body of water. Always read and follow herbicide label directions. If this weed is found contact local authorities who can help.

Idaho is a wonderful place with awesome natural resources, landscapes, agriculture and recreational opportunities. Weeds threaten all the things that we love about Idaho. To detect, contain and control weeds in Idaho we have declared “War on Weeds.” These short articles are designed to give a brief description of each species, an explanation about why it’s a problem, insights into identification and Integrated Pest Management control options. — Justin Hatch

View or Download (pdf)

• Eurasian watermilfoil, 11-23