Eurasian Water Milfoil (EWM)

By: Chris Hamerla

Eurasian water milfoil (EWM)

What is EWM?

Eurasian water milfoil (EWM) is a submersed plant that is native to Europe, Asia, and northern Africa. It is thought to have gotten to the United States through the aquaculture and aquarium trade and was first documented in 1942 in a Washington D.C. pond. EWM can grow extremely quick and does not stop growing when it reaches the water surface. Upon reaching the surface, the plant continues to grow forming dense mats that can make boating and swimming almost impossible.

The leaves have a feathery appearance and form in a circle (whorl) around the stem. Typically there are four leaves per whorl. Each leaf is generally made of twelve to twenty-one pairs of leaflets. EWM is often confused with native milfoils of which there are eleven species in North America.

EWM starts growing when water temperatures reach 50° F. It is the earliest of the milfoils to start growing. This early growth gives EWM a jump on other native plants in competing for sunlight. Since the plant continues to grow across the water surface it blocks the sunlight needed by other plants.

On a positive note, this early growth makes detection and targeting the plant for treatment easier. New plants are spread from an existing plant breaking apart or fragmenting.  These fragments root themselves and start new colonies. This method of spreading makes it very important that people clean their equipment of all vegetation before going to another water body. Seeds and runners are the other methods for new plant growth.

Comparison of northern milfoil (bottom) and Eurasian (top). The Eurasian has thirteen pairs of leaflets. 

Why is EWM Harmful?

As mentioned earlier, the dense growth of EWM makes for difficult swimming and boating. Thick colonies don’t allow for native plant growth. Some colonies of EWM are so thick that the vegetation isn’t useable as habitat for fish. Effects of EWM have caused the property values on some waters to drop.

These issues have also caused lake associations and various levels of government to spend millions of dollars in managing this invasive. Finally, in fall, as EWM begins to die back it releases a lot of nutrients into the water. Also, increased amounts of carbon dioxide are released into the water as bacteria break down the plants.

Bacteria use oxygen in the water and give off carbon dioxide. This increase in carbon dioxide comes at a time of year when other plants are dying back and not producing the oxygen the fish need to get through the winter.

How to Control EWM?

If EWM is found early enough control methods can be as simple as hand pulling. Often times EWM will get its start near a boat landing. Fragments of the plant were likely attached to a boat or trailer that was launched at the landing.  Since the area around the landing may have little or no vegetation the EWM doesn’t have much competition and easily starts growing.

At this stage simply pulling and removing the entire plant can prevent the invasive from becoming established.  Once EWM has become established, herbicidal treatments are a more effective solution.  Contact and permits from the DNR are required for chemical treatments. Typically a lake management plan, including mapping of the EWM and a lake survey of the aquatic plant life, needs to be completed before the EWM is treated.

Chris Hamerla is the Aquatic Invasive Species Coordinator of Lumberjack Resource, Conservation, & Development Council serving Lincoln, Langlade, and Forest Counties in northern Wisconsin. As the Aquatic Invasive Species Coordinator, Chris focuses on implementing AIS prevention and control efforts through public awareness and education programs. For more information or have any questions contact Chris at 715-362-3690 e-mail

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