IV. The Nature of Eurasian Water Milfoil (EWM)

What is Eurasian Water Milfoil (Myriophyllum spicatum L.)? EWM is a perennial aquatic plant that grows submersed in water as deep as adequate light will penetrate (essentially less than 20 feet in depth, and in an area of the lake commonly called the littoral zone) and loves clear waters. The only part of the plant that grows out of the water is the short flower stalk which appears late in the summer. The submersed shoots form a dense canopy at the water's surface. In the winter, green stems may be found under the ice, although with little metabolic activity.

EWM was established in the United States by the early 1940's but the specific date of introduction is debated. Some investigators suggest it may have been present by the early 1900s and one report says that it was used experimentally for potential fish habitat improvement. The current EWM distribution includes 45 of the 50 sates and three Canadian providences. EWM is possibly the most widespread non-native aquatic plant in North America. How does EWM spread? EWM is prolific but does not spread well by seeds. It primarily spreads by fragments commonly transported by boats and trailers that may form roots and develop new plants. These plants grow rapidly, outperforming other aquatic plants. These new plants send out shoots and runners that creep along beds of lakes and rivers to form new colonies. These colonies soon outcompete other aquatic plants. A small plant fragment can multiply into thousands/ tens of thousands of plants in one year!

What are the problems with EWM? Here are some of the detrimental effects of EWM:

• The colonies quickly make an unsightly heavy mat just below the water surface.
• Swimming is dangerous and watercraft have difficulty maneuvering through the thick mat of vegetation.
• Loss of plant diversity.
• Degrade water quality through eutrophication.
• Obstructs swimming space of fishes by fencing out larger predatory fish.
• Shelters too many juvenile fishes.
• Creates food shortages for fishes by reducing the diversity of invertebrates.
• Can cause fish kills by depleting dissolved oxygen when shoots decay in autumn.

• Inspect and remove any visible mud, plants, fish or animals before transporting boat/trailer or any ‘water toys’. Place material well into wooded area.
• Drain water from equipment (boat, motor, trailer, live wells) before transporting.
• Dispose of unwanted live bait in the trash.
• Do not drive boats through or near suspected plant beds.
• Volunteer to help monitor boat landings.
• Remove all aquatic plants from SCUBA, water ski and duck hunting equipment.
• Rinse boat and equipment with high pressure hot water (104 degrees F).
• Dry all equipment in the sun for at least five days.
• Report any suspected plant to WDNR.