The Enemy - Eurasian Watermilfoil ( Myriophyllum spicatum) is a native of Europe, Asia, and northern Africa and is the only aquatic plant listed as a noxious weed in Idaho. Reports suggest that it was introduced to the United States in the late 1800’s, but first found in the United States in the 1940s. This plant is a submersed perennial plant with finely dissected feather-like leaves. The leaves are arranged in whorls of four around the stem at each node and are about 1.2 inches long. The leaf generally has 12 or more leaflet pairs (the native plant Nothern Watermilfoil will only have 8 pairs).
The Strategy - This plant can single handedly destroy an entire lake ecosystem. The plant can root in water up to 30 feet deep. Once the plant reaches a certain size the dense mats will segregate themselves off of the bottom and float around the lake spreading seeds. This plant also spreads by fragmentation, that is, each time the plant is broken up by a propeller or boat it will start a new plant. Wildlife cannot swim through the dense mats, and children have even drowned due to getting caught up in them and not able to swim to stay afloat. When the dense mats start to break down by the bacteria it becomes harmful to fish. It spreads by getting caught on boats and trailers, and some suspect that waterfowl have contributed to the spread of the weed.
The Defense - This is a very difficult weed to control because it spreads so fast. Mechanical weed control is somewhat effective in controlling this weed, but is very expensive and requires scuba divers and gear. Many herbicides are available for use under careful and trained professionals and are approved for this use by the Environmental Protection Agency (EPA). The best control is to make sure your boats and gear is clean of debris when you pull out of the lakes. This weed has not been found on the eastern side of Idaho.
For more information, click on the link below to download the Idaho's Noxious Weeds Control Guidelines publication produced by the University of Idaho Extension.
U of I Idaho's Noxious Weeds Control Guidelines (183 KB PDF download)