Breaking Eurasian watermilfoil’s stranglehold on Minnesota Lakes

Sometime in the early 1900s, an aquatic plant known as Eurasian watermilfoil (EWM) found its way to the United States, most likely as a decorative plant for aquariums. Once escaped, this aquatic invader spread quickly across the country, establishing itself in lakes, rivers, and streams from Alabama to California, Illinois, and Minnesota. Today, EWM is found in 394 lakes in Minnesota, including 37 in Washington and Chisago Counties.

Invasive Eurasian watermilfoil (left) has 12 to 21 pairs of leaflets and grows into dense thickets in shallow water. Native northern watermilfoil (right) has only 5 to 10 pairs of leaflets and grows in sparse patches. The native milfoil provides good habitat for fish.

Eurasian watermilfoil creates numerous challenges for lake-lovers and water resource managers. To begin, the plant can grow up to 20 feet tall and creates dense mats at the water’s surface. These mats tangle boat propellers and terrify swimmers. EWM also crowds out native aquatic plants that turtles, fish and aquatic invertebrates like to eat. The end result is a dense, underwater forest where few fish live and no one wants to recreate.

Eurasian watermilfoil is most often found in nutrient rich lakes and rivers. It likes heavily used lakes, disturbed lake beds, and lakes that get a lot of nitrogen and phosphorous runoff. In warmer water, EWM can flower and reproduce more than once in the same summer. Worst of all, any fragment of the plant that breaks off can plant itself in a lake bottom and begin to grow again. In this way, boaters unknowingly spread Eurasian watermilfoil across a lake as they travel, and into new lakes when they fail to clean their boats and equipment thoroughly.

Watercraft inspectors talk with boaters as they are entering and leaving to water to make sure they don’t unknowingly transport AIS through plant fragments, mud, or attached zebra mussels.

Managing Eurasian watermilfoil in infested lakes

Over the past fifteen years, researchers and lake associations have struggled to find ways to permanently and effectively control Eurasian water-milfoil. It can be pulled by hand, harvested with machines (picture a giant aquatic lawn mower), or treated with herbicides such as 2,4-D, triclopyr, endothall or diquat. If any miniscule portion of the plant remains after treatment, however, it will sprout again and reestablish itself in the lake.

Most recently, researchers at the Minnesota Aquatic Invasive Species Research Center have studied the potential for using a native aquatic weevil as a biological control method for Eurasian watermilfoil. Unfortunately, the researchers have determined that weevil populations are too low to control EWM in Minnesota and that sunfish eat too many weevils for their populations to get larger.

Last summer, Valley Branch Watershed District (VBWD) worked with Barr Engineering to evaluate the current extent of EWM in six local lakes: Long Lake and Katherine Abbott Pond in Mahtomedi; DeMontreville, Olson, Jane, and Elmo in Lake Elmo; and Silver Lake in North St. Paul. Point intercept surveys showed that herbicide treatments have helped to reduce the abundance of EWM and encourage more native plant diversity in the lakes over the past ten years. However, it is worth noting that Silver Lake suffered a major decline in water quality and native plant diversity twelve years ago, after a full-lake treatment was conducted to control curlyleaf pondweed and EWM. Since then, the lake has gradually recovered. Of the six lakes studied, Lake Elmo is the only one to rely entirely on mechanical control methods. VBWD has observed similar fluctuations in the abundance of EWM on Lake Elmo as in the other lakes where chemical herbicides are used.  

Silver Lake in North St. Paul conducted a full-lake treatment in 2008-2009 to manage curlyleaf pondweed and Eurasian watermilfoil. Since then, healthy populations of native aquatic plants like water lilies have rebounded.

Big Marine Lake Association (BMLA) in northern Washington County uses a targeted approach to treat small patches of EWM before they can spread. In the spring, BMLA volunteers survey the lake to find and map EWM before herbicide treatment. They then conduct several additional surveys during the summer and fall to evaluate the effectiveness of the treatment.

Jeff Dahlberg, a BMLA board member and certified Aquatic Invasive Species (AIS) Detector, has developed a novel technique for mapping the locations of EWM patches. He sets his GPS graph screen to scan for standing plants and maps a trail while slowly driving through areas where EWM can grow. He then trolls a fishing lure as he passes over plants shown on the GPS graph screen. When he finds plants, he enters a waypoint, and then reels in the plant to identify the species. In this way, Dahlberg is able to distinguish between native plants that should be protected, and invasive Eurasian watermilfoil that should be removed.

Big Marine Lake has a swimming beach, fishing pier and three public boat launches. The lake has excellent water quality and the lake association has worked hard to control Eurasian watermilfoil so that it doesn’t spread within the lake.

Though researchers have not yet discovered a way to eradicate Eurasian watermilfoil, we do know that maintaining good water quality and healthy populations of native plants builds resiliency so that EWM is less likely to take over a lake if it is introduced. Similarly, maintaining good water clarity and allowing native plants to quickly reestablish helps to prevent Eurasian watermilfoil from coming back once it is removed.

Watercraft inspections are the first line of defense in preventing the spread of AIS  

Each year, the Minnesota DNR certifies roughly 100 DNR inspectors and 1,000 local inspectors to check boats moving in and out of lakes and rivers across the state. In Washington County, watercraft inspections are conducted by Washington Conservation District (WCD), the Minnesota DNR, and Comfort Lake – Forest Lake Watershed District (CLFLWD).

2020 was a record year for watercraft inspections, in part because people spent more time outside fishing and boating due to COVID, and in part because new funding allowed CLFLWD and WCD to increase their staff hours. Inspectors conducted a total of 10,363 inspections at CLFLWD lakes – Bone, Comfort, and Forest. WCD conducted an additional 15,513 inspections at 18 other public launches in Washington County. Funding for these inspections came from a variety of sources, including Chisago and Washington County AIS programs, watershed districts, city budgets, lake associations, and community groups.

Happily, watercraft inspections data show that the vast majority of people are following state rules that require boaters to clean and drain watercraft to prevent the spread of AIS. In fact, inspectors discovered violations in only 2-4% of watercraft entering lakes last year.

Learn more and get involved

Lake-lovers and shoreline residents play a critical role in helping to prevent the spread of aquatic invasive species. At minimum, Minnesota State Law requires that people clean all visible aquatic plants, zebra mussels, and other AIS from watercraft, trailers, and equipment before leaving a water access; keep drain plugs out while transporting watercraft; and dispose of unwanted bait, including minnows, leeches, and worms, in the trash. Shore fishers should also take care to clean boots and fishing poles. In addition, shoreline landowners should only hire businesses from the DNR’s list of permitted service providers when installing and removing boats, docks, and lifts.

To learn more about aquatic invasive species, go to

If you are interested in learning more about aquatic invasive species and becoming a certified AIS Detector, University of Minnesota will offer fully online training course this May and June. Visit to learn more.

If you are interested in becoming a paid watercraft inspector, Comfort Lake – Forest Lake Watershed District and Washington Conservation District are currently accepting applications for the 2021 season. To apply:

  • CLFLWD: Monitoring public launches on Forest, Comfort and Bone Lakes
  • WCD: Monitoring at 18 locations around Washington County