Eurasian watermilfoil (Myriophyllum spicatum) is one of the most problematic aquatic invasive species in Minnesota. First documented in the state's lakes in the late 1980s, it has since spread to hundreds of water bodies across the region and continues to expand. Unlike native aquatic weeds that grow as part of a balanced ecosystem, Eurasian watermilfoil has no natural controls in Minnesota waters. It grows aggressively, outcompetes native plants, and fundamentally alters the character of lakes where it takes hold.

Why Eurasian Watermilfoil Is a Serious Problem

The core issue with Eurasian watermilfoil is its growth rate. In ideal conditions, which unfortunately describe most Minnesota lakes in midsummer, it can grow up to an inch per day. It forms dense surface mats that shade out the native aquatic plants beneath it, collapsing the base of the food chain that fish and waterfowl depend on. Once a dense canopy forms, native species like wild celery, coontail, and native milfoil cannot get enough light to survive.

The impact on wildlife is significant. Native fish rely on specific plant structures for spawning and juvenile habitat. When Eurasian watermilfoil monocultures replace diverse native plant beds, those habitats disappear. Waterfowl that feed selectively on native tubers and seeds find less food available. Over time, a lake dominated by Eurasian watermilfoil supports a fraction of the biodiversity it once did.

For homeowners, the recreational impact is immediate and obvious. Dense surface mats make swimming impossible, foul boat propellers, collapse fishing lines, and create stagnant, oxygen-poor water near the shore that can smell and look unhealthy even when the deeper water is fine.

How to Identify Eurasian Watermilfoil

Positive identification is important because two native plants, northern watermilfoil and coontail, are frequently mistaken for the invasive species. Getting it wrong in either direction creates problems: treating a native plant unnecessarily, or failing to treat the actual invasive.

Eurasian watermilfoil has feathery, underwater leaves arranged in whorls of four around a central stem. Each leaf is finely divided into many narrow leaflets, typically 12 to 21 pairs per leaf. The stem is reddish to brownish in colour, though this can vary depending on water conditions and time of year. The most reliable field test is the limp test: pull a stem from the water and hold it up. Eurasian watermilfoil leaves go limp and droop immediately. Northern watermilfoil, by contrast, holds its shape out of water because it has fewer, stiffer leaflets.

Coontail (Ceratophyllum demersum) is the other common look-alike. It shares the whorled leaf structure but is bushier and more chaotic in appearance, with forked leaflets near the tips that give it a Christmas tree-like silhouette. Eurasian watermilfoil has a flatter, more uniform feathery appearance. Coontail also lacks roots and floats freely in the water column, whereas Eurasian watermilfoil is anchored to the lakebed.

In late summer, Eurasian watermilfoil produces small, reddish flower spikes that protrude above the water surface. This is another useful identification marker, though by the time flowering occurs the plant is already well established.

How It Spreads

Eurasian watermilfoil spreads primarily through fragmentation. A single small fragment of stem carrying even one node can root in a new location and establish a new colony within weeks. This is why watercraft are the primary vector for spreading it between lakes. A fragment caught in a propeller, on a trailer, or on fishing equipment can be transported miles and introduced to a clean lake in a single trip. Even wading gear and kayaks that have been in an infested lake can carry viable fragments.

It also spreads within a lake rapidly once established, as natural wave action, boat wakes, and harvesting equipment can fragment stems and distribute them across the water body. This is why mechanical harvesting of Eurasian watermilfoil requires specific techniques to minimise fragmentation and is best left to professionals with appropriate equipment.

Management and Treatment Options

Because Eurasian watermilfoil is a regulated invasive species, its management involves more considerations than native weed control. The Minnesota DNR requires permits for herbicide treatments and some mechanical removal methods, and there are seasonal windows and approved methods that must be followed. Working with a licensed aquatic weed management company is strongly recommended.

For light to moderate infestations, targeted mechanical removal via scuba diving or careful cutting can manage growth in specific areas. The key is minimising fragmentation during removal, which requires trained divers and proper technique. Mechanical harvesting is effective for larger areas but carries fragmentation risk and is best used as part of a broader management strategy rather than as a standalone solution.

For persistent or widespread infestations, EPA-approved aquatic herbicides are often the most effective long-term management tool. Treatments targeting Eurasian watermilfoil, such as fluridone or 2,4-D-based products, can suppress the invasive species while allowing native plants to recover, particularly when applied in early season before the milfoil canopy fully forms. All herbicide treatments in Minnesota require a DNR permit and must be conducted by a licensed applicator.

Early intervention makes a meaningful difference in both cost and outcome. A new or small Eurasian watermilfoil infestation is manageable. An established, widespread infestation covering the majority of a lake requires sustained, multi-year management and is significantly more difficult to reverse.