Eurasian watermilfoil is a non-native aquatic plant found in much of the shallow areas in the Lake Pend Oreille Watershed.  This incredibly invasive aquatic weed, it is suspected, was recently introduced into the lake from other parts of the Columbia River drainage most likely on boat trailers. Because it is widely distributed and difficult to control, watermilfoil is considered to be the most serious aquatic weed problem in the Northwest.
 

Eurasian watermilfoil or commonly referred to as simply watermilfoil (there is also a less invasive native watermilfoil), is actually an attractive plant that was once commonly sold as an aquarium plant.  As it’s name suggests it has it’s origins in Europe and Asia.  It probably arrived in the United States in the 1940s and quickly spread to all but a few states. 

Even a tiny fragment of the plant is enough to establish a colony that can quickly out compete the beneficial native aquatic plants. These massive rooted colonies can become so dense that they not only compromise the water quality, but can suffocate fish and destroy spawning habitat for some species.

Another serious implication is watermilfoil’s impact on an areas economic well being.  Most water based recreational activities are affected to some degree.  Boats can have trouble navigating through the nearly solid floating mats occasionally getting stuck.  Fish populations can decline and swimmers have been known to drown in rare instances.  Dense floating mats can also clog the inlets of power generating plants and increase the cost of maintenance.

One of the leading causes of the proliferation of Eurasian watermilfoil is the excessive nutrient input into the lake and it’s tributaries both from improper land use practices and from individual home owners.  As you read through the Lake*A*Syst materials keep in mind the effect of nutrient runoff, especially nitrogen and phosphorous, and the effect on the quality of Lake Pend Oreille’s water.  What you do or don’t do can make a difference.

Control of Eurasian Watermilfoil

Once watermilfoil becomes well established within a body of water, it is difficult or impossible to remove. There has been some success using aquatic herbicides, however significant control is difficult considering the expense of herbicide application. 

Timing the application is also critical. Keep in mind that the application by a private landowner of any herbicide to a body of public water or a tributary of public water is illegal no matter what the herbicide package may say.  Leave this up to the professionals.

Other Methods of control include: Harvesting, underwater rototilling, diver handpulling, and bottom barriers to smother the plant. There has also been some ongoing research into the use of biological controls such as the watermilfoil weevil who’s larvae attacks and kills the stem of the plant.

Identification

As a group watermilfoil are easy to identify however identifying the exact species is more challenging.  All watermilfoil have feather-like leaves arranged in a whorl of four leaves around the stem. The Eurasian variety usually has twelve or more leaflets per leaf while the native northern variety has fewer than ten. The leaves of the Eurasian variety tend to collapse around the stem while the northern species tend to remain more rigid. Also the stem is generally more reddish with the Eurasian species.

Here are some tips to identify Eurasian watermilfoil from the native milfoils.

Eurasian watermilfoil usually has twelve or more leaflet pairs on each leaf.

Eurasian watermilfoil leaves tend to collapse around the stem when removed from the water. Other milfoil species have thicker stems and are usually more robust.

The mature leaves are typically arranged in whorls of four.

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