Eurasian watermilfoil (Myriophyllum spicatum L.) is an invasive aquatic plant that was introduced to North America in the early 1940s. It grows rapidly and tends to form a dense canopy on the water surface, which often interferes with recreation, inhibits water flow, and impedes navigation. Eurasian watermilfoil is a particularly problematic aquatic weed, due to its ability to reproduce from fragments and spread rapidly, its high growth rate in a range of temperatures and environmental conditions, and its tendency to reach the surface and form extensive mats of plant at the surface, which can allow it to shade and outcompete native vegetation.
What it is
Eurasian watermilfoil is a submersed vascular plant in the family Haloragaceae. It is native to northern Europe and Asia. Several other members of the genus Myriophyllum are native to North America. In Minnesota, the native northern watermilfoil (M. sibiricum) is common, but is often displaced by Eurasian watermilfoil. Eurasian and northern watermilfoil hybridize and hybrids are now found in a number of lakes in North America, including Minnesota. This hybrid watermilfoil is also considered invasive.
As with most invasives, the best approach is to prevent invasion. Once an invasive species is established it is highly unlikely to be eradicated. New tools are being developed to predict new exotics and to develop strategies for preventing their introduction. Minnesota's program of education, laws that prohibit transport and boater inspections appear to have slowed the spread of Eurasian watermilfoil, however, prevention is not 100% effective and control measures will likely be required for established infestations.
Eurasian watermilfoil reproduces from fragments and seeds. Although reproduction from seeds was thought to be uncommon, the presence of hybrids and viable seeds suggests that sexual reproduction can be important. Plants flower once they grow to the surface in June-September; the emergent flower spikes give the plant its scientific name (M. spicatum). The plant will also produce autofragments in the summer; small branches that break off the plant and form roots which can establish new plants. Any fragment of the plant stem that includes a node (whorl of leaves) can produce a new viable plant. Eurasian watermilfoil stores carbohydrates in the lower stems and root crowns which enables the plant to survive over the winter, even with low or no light under the ice. In the spring when water temperatures approach 10-15 ˚C (50-60 ˚F) the plant will begin growing out of the rootcrowns and sometimes overwintering stems and grow toward the surface. The plants often form a canopy throughout the summer that shades out native plants.
Where it's found
Eurasian watermilfoil is found in more than 45 states and three Canadian provinces and it is estimated that millions dollars are spent annually on control. Although Eurasian watermilfoil was been known in Wisconsin since the 1960s, it was not reported in Minnesota until 1987, when it was found in Lake Minnetonka. Within three years, Eurasian watermilfoil had spread to 22 waterbodies and by 1992 was found in 60 waterbodies. By 1999 it had colonized over 100 waterbodies. Today, 2015, it can be found in roughly 300 lakes in Minnesota.
How it spreads
Eurasian watermilfoil is spread most commonly by inadvertent transport by boaters, however some waterbodies appear to have been infested by natural means (downstream transport or waterfowl). Milfoil can get tangled in boat propellers or become lodged in other areas of the boat or trailer. The most important thing you can do to prevent its spread is to always clean, drain, dispose, and dry your boat between lakes.
What it looks like
Eurasian watermilfoil has whorls of 4 feathery leaves, each with 12 – 21 pairs of leaflets (native northern watermilfoil has 5 – 9 pairs). It forms dense underwater stands of stems and mats of vegetation at the surface of the water. Mature plants have narrow flower spikes that emerge 2-5" above the water surface. Learn more about identification of Eurasian watermilfoil from the Minnesota DNR.
As with most weeds, there are three general control strategies that can be employed: mechanical/manual, chemical, and biological. Learn more about each strategy here. Click here for more information specifically about the milfoil weevil.