Eurasian watermilfoil is a non-native aquatic plant that was once commonly sold as an aquarium plant. As its name suggests it has its origins in Europe and Asia. It probably arrived in the United States in the 1940s and quickly spread to all but a few states. It is suspected that this incredibly invasive aquatic plant was introduced from boat trailers travelling from other parts of the Columbia River Basin. Even a tiny fragment of the plant is enough to establish a colony that can quickly out-compete the beneficial native aquatic plants. 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 is a perennial, which means unless exposed to air and killed by a hard frost, plants will come back the next growing season. Plants can be found growing in up to 30 feet of water, depending on water clarity. It can grow as deep as light will penetrate. 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.
Most water based recreational activities are affected to some degree by Eurasian watermilfoil. 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.
· Twelve or more leaflet pairs on each leaf.
· Leaves tend to collapse around the stem when removed from the water. Other milfoil species have thicker stems and are usually more robust.
· Mature leaves are typically arranged in whorls of four.