Eurasian watermilfoil (EWM) is an invasive species, native to Europe, Asia and North Africa, that has spread throughout the Midwest. EWM can create dense stands and dominate submergent communities, reducing important natural habitat for fish and other wildlife, and impeding recreational activities such as swimming, fishing, and boating.
EWM is typically at its peak growth stage during the latter part of the summer. Each year, the EWM population would be mapped during the late summer. These data can be seen in the 8-page EWM map book posted here.
Hybrid water milfoil (HWM) is a cross between the native northern water milfoil and the invasive EWM. In general, HWM has thicker stems, is a prolific flowerer, and grows much faster than pure-strain EWM. These conditions may likely contribute to this plant being less susceptible to being controlled by standard use rates of certain herbicides. HWM tends to interfere with recreation, navigation, and aesthetics more than pure-strain EWM populations. EWM and HWM have been confirmed from the Minocqua-Kawaguesaga system, with the MKLPA currently conducting research to learn more about the genetic composition of the invasive watermilfoil of the system.
How Scary are EWM Fragments
People often ask about how Eurasian Watermilfoil (EWM) spreads—and whether fragments from boats or harvesters are as concerning as those the plant produces naturally.
EWM does spread through fragmentation, but not all fragments are equally effective. In late summer, the plant undergoes a process called autofragmentation, where it produces specialized fragments that are biologically prepared to establish in new areas. These fragments typically include rooted nodes, contain energy reserves, and are buoyant—making them well-suited to float, settle, and grow.
Fragments caused by boat propellers or mechanical harvesters, on the other hand, tend to be smaller, randomly broken, and less likely to survive. While they can still contribute to spread, they are much less effective than natural reproductive fragments.
Understanding this difference helps MKLPA refine our management strategy.