Eurasian watermilfoil (Myriophyllum spicatum) is an aggressive invasive plant that poses a serious threat to aquatic ecosystems across North America. Introduced from Europe, Asia, and North Africa between the late 1800s and the 1940s, likely via ship ballast water or the aquarium trade, it is not native to the continent. Milfoil has rapidly become one of the most widely distributed invasive aquatic plants, establishing itself in over 40 states and multiple Canadian provinces. Its ability to grow rapidly and form dense beds severely alters the function and health of freshwater waterways.
Ecological Harm and Habitat Alteration
Milfoil’s rapid growth allows it to quickly outcompete native aquatic vegetation, leading to a significant reduction in natural biodiversity. The plant begins growing earlier in the spring than many native species, quickly elongating its shoots to form dense, canopy-like mats that float on the water’s surface. These thick surface mats intercept sunlight, effectively shading out and suppressing the growth of beneficial submerged plants. The resulting milfoil monocultures offer less value as a food source for waterfowl and are unsuitable habitat for native invertebrates, which are a primary food source for fish.
Dense milfoil beds alter the physical structure of the habitat, negatively impacting fish communities. While young fish may experience high survival rates in the initial cover, the extensive growth obstructs the swimming and foraging movements of larger predatory fish. Furthermore, the dense beds create poor spawning areas for many fish species, leading to reduced fish growth in warm-water fisheries.
When large stands of milfoil die off and decompose, typically in the late summer or fall, they consume substantial amounts of oxygen from the water. This biological process can severely deplete dissolved oxygen levels, potentially causing anoxic conditions that lead to fish die-offs. The decomposition of excessive biomass also contributes to nutrient cycling from the sediments back into the water column, which can exacerbate water quality issues and fuel algae blooms.
Hindrance to Recreational Water Activities
The physical growth pattern of milfoil directly interferes with the human use of lakes and waterways. The plant’s ability to form thick, matted canopies on the water surface makes activities like swimming difficult and potentially hazardous. Swimmers face entanglement risks in the dense, tangled stems, which can reach from the lake bottom to the surface.
Boaters encounter serious mechanical problems when navigating milfoil-infested waters. The long stems easily wrap around boat propellers, fouling the engine and making steering difficult or impossible in shallow areas. Milfoil fragments can also clog engine cooling water intakes, leading to overheating and potential damage.
Navigation becomes virtually impossible in areas with severe infestations. Boat traffic often causes fragmentation, which is a primary mechanism for the plant’s spread to new locations within a water body. Anglers find fishing severely disrupted because the dense weed beds make casting lines and retrieving lures nearly impossible. These thick mats also create stagnant water conditions that can serve as habitat for mosquitoes.
Financial Burdens of Infestation
Infestations of Eurasian watermilfoil place significant financial demands on municipalities, lake associations, and property owners. Managing and controlling the plant requires substantial annual investments in specialized methods, such as mechanical harvesting, which can cost an average of approximately $2,000 per hectare for control efforts. Other common control methods include chemical treatments using herbicides like 2,4-D, biological controls, and specialized permits, all of which contribute to high operational costs.
The presence of milfoil also has a demonstrable negative effect on local economies and the value of waterfront real estate. Severe milfoil invasion can depress the value of shoreline properties, with studies showing a significant negative effect on property sales prices. Property values in some affected areas have been found to decline by an average of 8% to 19% once the plant invades a lake. This reduction in property values translates directly into a loss of property tax revenue for local governments, extending the economic impact beyond individual property owners.