Milfoil is native to Europe and Asia, but was introduced to the United States between the late 1800’s and the 1940’s. It was collected from a lake near Seattle in 1965 and has since been found in many lakes in that area. This plant is primary spread from lake to lake by boats and trailers, with some spread by water birds. It is very difficult to control and unfortunately has recently been found in Montana. There are multiple species of milfoil, which are very difficult to distinguish amongst. Some of these species are native to Montana. Plant experts often rely on DNA to distinguish between species. The noxious species tends to have its leaflets closer together than the native species.  Â
 Concerns Milfoil forms very dense mats of vegetation on the surface of water, which drastically changes the ecology of the body of water. It interferes with recreational water activities such as swimming, water skiing, boating, and fishing. The mass of vegetation created by this plant can cause flooding and it creates good habitat for mosquitoes. Milfoil starts growing earlier in the season than most native aquatic plants and therefore shades the majority of them out. This has negative impacts on fish and wildlife habitat. This aquatic invader can also invade fish spawning habitat, clog irrigation intake pipes, and can hinder power generators. It has the ability to reproduce through broken off plant parts at a very rapid rate and can take over an entire lake in 2 years. Once milfoil is established it is difficult if not impossible to eradicate. Â
Identification Milfoil species are easy to identify as a group because they all have feather-like leaves arranged in whorls around the stem.  Identifying individual species is not quite as simple. It is so difficult that for many years Eurasian water milfoil and northern milfoil were classified as the same species. There are a few characteristics that help in separating the noxious from the natives but they cannot always be relied upon. Eurasian milfoil typically has twelve or more pairs of leaflets on each leaf, the leaves tend to collapse around the stem when removed from the water, and the mature leaves are typically arranged in whorls of four around the stem.
What can you do? Preventing the spread of this weed is the most important thing we can do at this point. Pick every fragment off of your boat or trailer and put them in a garbage can. If you see plant fragments on someone else’s boat or trailer tell them about the threat that milfoil possesses. Spreading the word is so important because an entire lake can be infested by just one tiny fragment. Once a lake or river becomes infested there is no way to completely eradicate the invader. Herbicide treatments, diver harvesting or hand pulling (containing all fragments), underwater rototilling, and instillation of bottom barriers are all somewhat successful in controlling the growth of milfoil. If you are interested in helping stop the spread of Eurasian water milfoil learn how to identify and safely remove it, get other people involved in the prevention, and report any aquatic vegetation that you suspect could be the noxious species. If you have any questions about Eurasian watermilfoil or any other plant call your local county weed district.
Visit the MWCA Weed ID pages for additional information and pictures of Eurasian Water Milfoil.
This series of articles was developed by Ravalli County. If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.