Eurasian Watermilfoil

Eurasian watermilfoil

What is Eurasian watermilfoil?

Eurasian watermilfoil (EWM) (Myriophyllum spicatum) is a fast-growing aquatic plant found submerged in still or slow-moving water. Native to Europe, Asia, and North Africa, the species was first discovered in the eastern United States in the early 1900s. The plant was able to travel here by clinging to boats and other water equipment from across the Atlantic. In Wisconsin, it was first recorded in Lake Mendota in 1962 and can now be found in all of Madison’s lakes.

Eurasian watermilfoil
Eurasian watermilfoil (left) and coontail (right)

EWM roots itself to the bottom of lakes or streams where it can grow up to 20 feet tall. It has dark green leaves with white or light green stems and produces stalks with vibrant pink flowers. The flowers emerge from the water surface to be wind pollinated.

EMW can produce up to 100 seeds a year, but its most effective means of dispersion is by clonal expansion. A segment of the vegetation can break off from the main plant and travel a significant distance before rooting itself and growing into a new plant. This method of clonal expansion makes it a great invader because it can hitch a ride on the bottom of a boat and colonize in an entirely different lake in Wisconsin or elsewhere.

Why is Eurasian watermilfoil a problem?

Like most invasive species, EWM outcompetes the native vegetation because of a lack of native organisms that can consume or otherwise keep its growth in check. In Lake Mendota, there are 14 native macrophyte species that risk being suppressed by the unchecked growth of EMW. High levels of nutrients in the lake allow this plant to grow to the surface of the water, creating large mats that block out sunlight that other plants need to survive.

Eurasian watermilfoil with attached filamentous algae
A surface mat of Eurasian watermilfoil with attached filamentous algae

The large vegetative mats and dense colonies create poor habitat conditions and reduce food sources for fish and wildlife in our lakes. Fish require diverse structures from which to feed and nest, but EMW creates uniform environments with relatively few niches. In addition, the loss of native plants reduces food available for waterfowl.

EWM mats can also impair recreation as boats and swimmers can easily get tangled in the dense vegetation. This recreational nuisance has even been tied to decreased property values.

Distinguishing Eurasian watermilfoil

There are several different species of native aquatic plants that resemble EWM such as northern watermilfoil (Myriophyllum sibiricum) and coontail (Ceratophyllum demersum). These plants can even hybridize with each other making them difficult to distinguish. However, an easy way to distinguish EWM is to count the number of leaflet pairs at each leaf whorl. EMW has 12 to 21 leaflet pairs, while native species only have 5 to 9 pairs. The native species also produces a winter bud at the end of the stem unlike EWM.


Prevent the spread of Eurasian watermilfoil

The most common and effective way for EWM to spread between bodies of water is from accidental introduction by humans. A single plant fragment stuck on the bottom of a boat or a trailer could potentially colonize a completely different lake or stream. To prevent the spread of EWM, boats should be cleaned and rinsed after being removed. Drain plugs should also be removed during transportation to allow all water to escape from the boat. Finally, boats should be allowed to dry for 21 days before being placed into a different body of water.

Inspecting and washing boating equipment helps prevent the spread of aquatic invasive hitchhikers like Eurasian watemilfoil
Inspecting and washing boating equipment helps prevent the spread of aquatic invasive hitchhikers

If you are a boater, a good way to prevent the spread of EWM within the lake itself is to avoid or motor slowly through areas of shallow depth. Propeller-induced turbulence disturbs the lake bottom and invites the colonization of aquatic weeds like EWM. Propellers also physically chop exposed plants, further encouraging the fragmentation and spread of this pervasive weed.

We can all make changes in our daily lives to help reduce the abundance of Eurasian watermilfoil. Following the above guidelines is the first step to prevent the spread of this problem weed. At home, consider planting rain gardens or keeping leaves out of the street to reduce the amount of phosphorus pollution entering our lakes. Excess phosphorus and other nutrients help fuel EWM growth to the detriment of other species, reducing the overall health of the ecosystem. In the end, a cleaner and more biodiverse watershed and lake will be more resilient to invasive species such as EWM.

More resources

Read more about the health of the lakes of the Yahara River Watershed.

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