Non-native species like zebra mussels make national news, but the dangerous plant variable milfoil is rarely discussed outside of lake communities.
Myriophyllum heterophyllum, commonly referred to as variable milfoil, is an invasive aquatic plant that has been contaminating lakes throughout the Northeastern United States since the 1960s. It looks harmless enough, resembling a green squirrel tail with the occasional small, reddish flower. Yet, variable milfoil can grow up to 15 feet long, forming dense mats of vegetation that choke out native species. These mats block sunlight from reaching other submerged plants, killing them, and can deplete oxygen levels in the water while decaying, which hurts fish and other aquatic animals. The plant not only destroys ecosystems but also inhibits recreational water activities, as the dense mats of milfoil make boating or swimming impossible. Furthermore, these large clusters of plant matter are the perfect breeding grounds for mosquitoes, more bad news for those who visit lakes.
Variable milfoil most seriously affects Maine and New Hampshire due to an absence of natural predators and ideal water conditions for plant growth. The plant is found in over 90 water bodies in these two states alone, including Lake Winnipesaukee, the largest lake in New Hampshire. Variable milfoil was likely brought to the Northeast from the Southern United States, its native habitat, attached to the bottoms of boats as a kind of “aquatic hitchhiker.” Small fragments of milfoil were chopped up by boat propellers and then floated to different parts of the lake, quickly growing to form masses of plant matter. Milfoil spreads most easily by fragmentation, but loose milfoil seeds can also grow into full plants in a short period of time.
With no practical biocontrol available and heavy regulations on herbicides, lake communities affected by variable milfoil mainly resort to pulling the weeds from the lakebed by hand. These hand-pulling programs have proven to be effective in the long run, but manual removal is a slow and expensive process. With a lack of milfoil awareness, lake communities face insufficient funds for these projects, and improperly supervised harvesting can allow fragments of plant matter to break off of the milfoil plants. This can potentially cause new infestations as these fragments float to other parts of the lake.
The best way to prevent milfoil infestations is to stop the spread of the plant in the first place. For information on how to prevent aquatic hitchhikers like variable milfoil from spreading, check out this video.