Eurasian Watermilfoil: What It Is, Why It's a Problem

If you’ve noticed a thick, feathery green canopy spreading just below the surface of your pond or lake, there’s a good chance you’re looking at Eurasian watermilfoil (Myriophyllum spicatum). It’s one of the most aggressive invasive aquatic plants found across Maryland and the Mid-Atlantic region — and once it gains a foothold, it rarely retreats on its own.

We’ve worked in enough degraded waterways to know what this plant can do to a shoreline, a fishery, and a property owner’s peace of mind. The good news: with the right approach, it can be removed, and your water can recover.

What Is Eurasian Watermilfoil?

Eurasian watermilfoil is a submersed, rooted aquatic plant native to Europe, Asia, and northern Africa. It was first documented in the United States in the 1880s and has since spread to waterways across much of North America, carried largely by recreational boating activity. The plant grows from rooted stems anchored in the sediment, sending feathery, finely dissected leaves outward in whorls of four along flexible, reddish-brown stems. When conditions are right — warm, nutrient-rich, shallow water — it grows fast enough to visibly expand a colony in a matter of weeks.

What makes Eurasian watermilfoil particularly difficult to manage is its ability to reproduce through fragmentation. A single stem piece broken off by a boat propeller, a swimming kick, or even a current can drift to a new location, take root in the sediment, and start an entirely new colony. This is why infestations spread so quickly from one part of a water body to another — and from one lake to the next when boats aren’t thoroughly cleaned between launches.

How to Identify It

Distinguishing Eurasian watermilfoil from native milfoil species matters because management approaches and permitting requirements can differ. The most reliable identifying features include leaf structure and leaflet count. Eurasian watermilfoil leaves are feathery and arranged in whorls of four around the stem. Each leaf typically has 12 to 21 pairs of leaflets — more than most native milfoil species, which usually have fewer than 11 pairs per leaf.

The plant generally grows to the water’s surface, where it forms a dense, horizontal canopy. In late summer, it produces small pinkish flower spikes that extend just above the waterline. If you’re uncertain whether what you’re seeing is Eurasian watermilfoil or a lookalike, it’s worth contacting a professional before attempting any removal. Misidentification can lead to ineffective treatment — or worse, spread the problem further.

Why Eurasian Watermilfoil Is So Destructive

The ecological damage Eurasian watermilfoil causes is more wide-reaching than most property owners realize until the situation has already worsened. The plant forms dense surface mats that block sunlight from reaching the water column below, starving native aquatic vegetation of the light it needs to grow. As native plants die back, the habitat structure that fish, waterfowl, and other wildlife depend on begins to unravel.

Dissolved oxygen levels drop in areas where milfoil mats accumulate, particularly overnight when the plants consume oxygen through respiration rather than producing it through photosynthesis. Low oxygen zones are stressful for fish populations and can contribute to fish kills in severe cases. Decomposing plant matter also adds to the nutrient load in the sediment, which can fuel future algae blooms and accelerate the overall degradation of water quality.

From a recreational standpoint, the impact is immediate and frustrating. Thick milfoil beds tangle boat propellers, wrap around fishing lines, and make swimming uncomfortable or unsafe. Property values along heavily infested shorelines suffer, and HOAs and municipalities that manage shared waterways often find themselves facing mounting complaints and increasingly expensive management costs the longer the problem is left unaddressed.

How Eurasian Watermilfoil Spreads — and How to Stop It at the Source

Understanding how this plant moves from one water body to another is the first step in preventing new infestations. The single most common vector is recreational watercraft. Even a small stem fragment clinging to a boat hull, propeller, trailer axle, or wading gear can survive out of water long enough to be transported to the next lake and establish a new colony.

Prevention is straightforward but requires consistency. Every time a boat exits a water body, the hull, trailer, and all equipment should be carefully inspected and cleared of any plant material. Bilge water, live wells, and bait buckets should be drained before leaving the launch site. Gear that has been in contact with infested water should be allowed to dry thoroughly — ideally for at least five days — or treated with high-pressure hot water before being used elsewhere. These habits don’t eliminate the risk entirely, but they significantly reduce it.

Once an infestation is established, prevention shifts to containment. Buoy lines and swim barriers can discourage boating activity through dense beds, which limits mechanical fragmentation and slows spread. Early detection monitoring — walking or snorkeling the perimeter of a water body a few times per season — gives property owners and managers the best chance of catching new colonization before it reaches a scale that requires major intervention.

Management and Removal Options

There is no single best method for removing Eurasian watermilfoil. Effective management depends on the size and density of the infestation, the depth and type of water body, the presence of native vegetation, and any applicable state permits. Legacy Waters approaches each project with this complexity in mind, developing a plan that fits the specific conditions of the site rather than applying a one-size-fits-all solution.

Mechanical Harvesting

Mechanical harvesting is one of the most practical and immediately effective methods for managing Eurasian watermilfoil in ponds and lakes. Specialized watercraft equipped with cutting blades and conveyor systems can cut invasive plants below the waterline and collect the biomass directly on board, which is then transported to shore and disposed of off-site. This approach removes the plants along with the nutrients they contain, preventing that organic matter from decomposing back into the water and fueling future growth.

Legacy Waters operates aquatic vegetation harvesting equipment capable of cutting and collecting invasive plants to depths of six feet. Because the biomass is fully removed from the water body — not simply cut and left to fragment or sink — mechanical harvesting delivers visible, lasting results that go beyond surface-level cosmetic improvement. It’s a method we trust for Maryland waterways because it works without introducing any chemical inputs, which matters in water bodies near drinking water sources, sensitive fisheries, or areas with regulatory restrictions.

It’s worth noting that mechanical harvesting alone, particularly simple cutting without collection, is not recommended for Eurasian watermilfoil. Cut fragments that remain in the water can root and start new colonies, potentially making the infestation worse. That’s why our harvesting approach emphasizes full collection and off-site transport at every project.

Hand Pulling and Diver-Assisted Suction Harvesting

For smaller infestations or early-stage colonization, hand pulling and diver-assisted suction harvesting (DASH) can be effective. In DASH, trained divers remove plants by hand or with suction equipment while working underwater, allowing for precise, targeted removal without disturbing surrounding native vegetation. The method is labor-intensive, but its precision makes it well-suited to situations where protecting native plant communities is a priority or where mechanical equipment can’t safely operate.

Hand removal is most effective when the entire root system is extracted rather than just the above-sediment stems. Roots and root crowns left in the sediment will regenerate, and any fragments that separate during removal must be carefully collected to prevent them from spreading. This is work that benefits enormously from experience — knowing how to move through the water without creating unnecessary fragmentation requires practice and a methodical approach.

Benthic Barriers

Bottom barriers — heavy fabric or mats placed directly on the sediment — can smother existing milfoil and prevent regrowth in targeted areas. They work by blocking both light and gas exchange at the sediment surface, effectively eliminating the growing conditions the plant needs. Barriers are most useful for protecting high-priority areas like swimming beaches, boat docks, and boat launches where a clean, plant-free zone is the goal.

They are not typically practical for treating large infestations across an entire water body, but as part of an integrated management strategy, they can help maintain cleared zones after an initial removal effort.

Biological Control

The milfoil weevil (Euhrychiopsis lecontei) is a naturally occurring North American insect that feeds specifically on Eurasian watermilfoil. In some lakes, weevil populations have contributed meaningfully to milfoil suppression, and there is ongoing research into augmenting natural weevil populations in heavily infested water bodies. Biological control is generally considered a long-term, supplemental strategy rather than a primary removal method, and results can vary significantly depending on site conditions.

Chemical Treatment

In larger, heavily infested water bodies, herbicide application may be the most efficient path to significant population reduction. Systemic herbicides like fluridone and selective contact herbicides are used to target milfoil while minimizing impact on non-target native species. Spring and fall treatments tend to produce the best results when using contact herbicides, as milfoil is more actively growing and native plants are less vulnerable to incidental exposure.

Chemical treatment in Maryland waterways requires permits through the Maryland Department of the Environment, and applications must be conducted by licensed professionals. Legacy Waters can help navigate the permitting process and coordinate with regulatory agencies when chemical options are appropriate for a given site.

What to Expect After Removal

Successful milfoil removal doesn’t mean the work is over. Eurasian watermilfoil is remarkably persistent, and even a thorough removal effort can leave behind root fragments or dormant seed banks capable of producing new growth. Post-removal monitoring is an essential part of any serious management plan — returning to the site at regular intervals through the growing season to assess regrowth and address it before it regains momentum.

In most cases, the aquatic ecosystem responds positively and visibly after a significant milfoil removal. Water clarity often improves as the surface canopy is eliminated and light penetration increases. Native aquatic vegetation, given the chance to compete without milfoil’s aggressive shade, begins to return over time. Fish populations benefit from restored oxygen levels and improved habitat structure, and the water becomes more inviting for swimming, boating, and fishing.

The pace of recovery depends on the severity of the original infestation, the health of the native plant community, and the ongoing management strategy in place. Legacy Waters provides follow-up monitoring as part of our project work because we understand that removing invasive plants is a process, not a single event. Our goal is water that stays restored — not just water that looks better for a season.

Working With Legacy Waters on Eurasian Watermilfoil Removal

Legacy Waters Environmental Services works with homeowners, HOAs, golf courses, municipalities, and conservation organizations across Maryland and the Mid-Atlantic to restore waterways affected by invasive aquatic vegetation. We handle all aspects of the process, from initial on-site assessment through permitting, removal, and follow-up monitoring — so property owners and managers can focus on enjoying the water rather than managing a problem that keeps coming back.

Every project begins with a free consultation. We visit the site, assess the extent and density of the infestation, evaluate the surrounding ecosystem, and develop a management plan tailored to what we find. There are no generic treatment packages here — the right approach for a small residential pond is different from the right approach for a multi-acre HOA lake or a municipal reservoir. Reach out to our team at (443) 927-4337 to schedule your assessment and take the first step toward cleaner, healthier water.

What Property Owners Ask Us About Eurasian Watermilfoil

What does Eurasian watermilfoil look like, and how can I tell it apart from native plants?

Eurasian watermilfoil has feathery, submersed leaves arranged in whorls of four along a flexible stem, with each leaf containing 12 to 21 pairs of leaflets — more than most native milfoil species. The stems are typically reddish-brown, and the plant grows to form a dense mat at or near the water’s surface in summer. In late summer, it sends up small pinkish flower spikes just above the waterline. If you’re unsure whether what you’re seeing is Eurasian watermilfoil, a professional site assessment is the safest way to confirm the identification before attempting any removal.

Is Eurasian watermilfoil harmful to fish and wildlife?

Yes, dense Eurasian watermilfoil infestations can significantly harm aquatic ecosystems. The thick surface canopy blocks sunlight from reaching native plants, which degrades fish habitat and reduces overall biodiversity. Overnight oxygen depletion beneath dense mats can stress fish populations, and in severe cases, contribute to fish kills. Native waterfowl and invertebrates that depend on a balanced aquatic plant community also suffer when milfoil displaces the native vegetation they rely on for food and shelter.

Can Eurasian watermilfoil spread from one lake to another?

Eurasian watermilfoil spreads primarily through fragmentation, and boats are the most common vehicle. A stem fragment as small as a few inches can survive out of water for hours and take root in a new water body when a contaminated boat is launched. This is why thoroughly inspecting and cleaning boats, trailers, and gear after every use is so important. Draining all water from live wells and bilges before leaving the launch site is also critical to preventing the spread of plant fragments between water bodies.

What is the most effective method for removing Eurasian watermilfoil?

The most effective method depends on the size and location of the infestation. For larger areas, mechanical harvesting with full biomass collection is one of the most practical and environmentally responsible approaches, removing both plants and their nutrient content without chemicals. For smaller, early-stage colonization, diver-assisted suction harvesting or targeted hand removal can be highly effective. In some cases, an integrated management plan combining mechanical removal with biological control and ongoing monitoring produces the best long-term results.

Do I need a permit to remove invasive aquatic plants in Maryland?

In Maryland, any work involving aquatic vegetation management in regulated waterways typically requires authorization from the Maryland Department of the Environment. The specific permits required depend on the scope of work, the method being used, and the characteristics of the water body involved. Chemical treatments always require permits and must be performed by licensed applicators. Legacy Waters handles all permitting coordination on behalf of our clients, ensuring that every project is conducted in full compliance with state regulations from start to finish.