Eurasian Watermilfoil
Myriophyllum spicatum
Eurasian Watermilfoil (Myriophyllum spicatum) is a submerged, perennial aquatic plant belonging to the family Haloragaceae. Native to Europe, Asia, and North Africa, it is one of the most widely recognized — and in many regions, most problematic — aquatic plant species in the world.
• Characterized by its feathery, finely dissected submerged leaves arranged in whorls along slender stems
• Produces small reddish flowers that emerge above the water surface on erect spikes
• Capable of forming extremely dense underwater canopies that can dominate entire water bodies
• Considered one of the most aggressive invasive aquatic plants in North America, where it was introduced in the 1940s
• Can grow in water depths ranging from 1 to 10 meters, depending on water clarity
• A single plant fragment containing as little as one node can regenerate into an entirely new plant, making it extraordinarily difficult to control
• Native range extends from Western Europe through Central Asia to Japan and parts of the Middle East
• First documented in North America in the 1940s, likely introduced through the aquarium trade or via ballast water of ships
• By the 1970s–1980s, it had spread aggressively across much of the United States and southern Canada
• Now established in at least 45 U.S. states and several Canadian provinces
• The genus Myriophyllum comprises approximately 60 species worldwide, with the highest diversity in Australia
• Myriophyllum spicatum can hybridize with the native Northern Watermilfoil (Myriophyllum sibiricum), producing hybrids that are even more vigorous and difficult to manage
Stems:
• Slender, smooth, and somewhat brittle; typically 1–3 mm in diameter
• Color ranges from green to reddish-brown depending on light conditions and age
• Branch profusely near the water surface, forming dense surface mats
• Easily fragmented — even minor disturbance (boat traffic, wave action) can break stems into viable propagules
Leaves:
• Submerged leaves arranged in whorls of 4 (occasionally 3 or 5) around the stem
• Each leaf is 1.5–3.5 cm long, pinnately divided into 12–21 pairs of fine, thread-like leaflets
• Leaflets are linear, approximately 3–12 mm long, giving the plant its characteristic feathery appearance
• Emergent leaves (on flowering spikes) are smaller, less divided, and may appear scale-like
• Key identification feature: the number of leaflet pairs per leaf (12–21 pairs) distinguishes it from the native Northern Watermilfoil, which typically has only 5–12 pairs
Flowers & Reproduction:
• Small, inconspicuous, reddish to orange flowers borne on erect spikes that project 5–15 cm above the water surface
• Flowers are monoecious — female flowers at the base of the spike, male flowers toward the tip
• Pollination is anemophilous (wind-pollinated)
• Produces small, hard fruits (~2 mm) divided into four nutlets
• However, sexual reproduction is relatively rare in North America; vegetative reproduction via stem fragmentation is the primary mode of spread
Roots:
• Fibrous, adventitious roots arise from nodes along the stem
• Roots anchor the plant in soft substrates (mud, silt, sand) but the plant can also survive as a free-floating fragment
Habitat:
• Lakes, ponds, reservoirs, slow-moving rivers, and canals
• Prefers nutrient-rich (eutrophic to mesotrophic) waters but can tolerate oligotrophic conditions
• Grows in water depths from 0.5 to 10+ meters, limited primarily by light penetration
• Tolerates a wide pH range (5.5–10.0) and moderate salinity
• Can survive winter dormancy under ice cover, regrowing from root crowns and turions in spring
Ecological Impact:
• Forms dense surface canopies that block sunlight, suppressing native submerged vegetation
• Alters dissolved oxygen dynamics — dense beds can cause significant diel oxygen fluctuations
• Provides habitat for some invertebrates and juvenile fish, but generally supports lower biodiversity than native plant communities
• Decomposing biomass contributes to nutrient cycling and can accelerate eutrophication
• Interferes with recreational activities (swimming, boating, fishing) and can clog water intake structures
Growth & Spread:
• Growth rate is among the highest of any submerged aquatic plant — stems can elongate several centimeters per day under optimal conditions
• Primary dispersal mechanism: stem fragments carried by water currents, boats, trailers, and waterfowl
• Also spreads via turions (specialized overwintering buds) and root fragments
• A single stem fragment with one node can regenerate roots and shoots within days
Management & Control:
• Prevention is the most effective strategy — cleaning boats, trailers, and equipment before moving between water bodies is critical
• Mechanical harvesting can provide temporary relief but often accelerates spread by generating fragments
• Biological control using the native milfoil weevil (Euhrychiopsis lecontei) has shown promise in some regions
• Herbicide treatments (e.g., 2,4-D, triclopyr, endothall) are commonly used but require careful application to minimize impacts on non-target species
• Benthic barriers (bottom screens) can suppress growth in small, high-value areas such as swimming beaches
• Drawdown of water levels during winter can expose and kill root crowns through freezing and desiccation
Legal Status:
• Listed as a noxious weed in multiple U.S. states including Washington, Oregon, Idaho, and Minnesota
• Regulated under various state aquatic invasive species laws
• Active eradication and management programs exist in many affected regions
Fun Fact
Eurasian Watermilfoil is a master of vegetative reproduction — a single stem fragment as short as 1 cm containing just one node can regenerate into a fully independent plant. This extraordinary regenerative ability is the primary reason it has become one of the most widespread and difficult-to-control invasive aquatic plants on the continent. The plant's feathery leaves, with their 12–21 pairs of thread-like leaflets per leaf, are so finely divided that they create an enormous surface area relative to their biomass — an adaptation that maximizes light and nutrient absorption in the underwater environment. Hybrid Vigor: • When Myriophyllum spicatum hybridizes with the native Northern Watermilfoil (M. sibiricum), the resulting hybrids can be even more aggressive and resilient than either parent • These hybrids are notoriously difficult to identify morphologically, requiring genetic analysis for accurate detection • Hybrid watermilfoil has been documented in the Great Lakes region, the upper Midwest, and parts of the northeastern United States A Plant That Changed Boating Laws: • The spread of Eurasian Watermilfoil was a major catalyst for the adoption of "Clean, Drain, Dry" protocols now standard across North America • Many states enacted mandatory boat inspection and decontamination programs specifically because of this species • It is estimated that Eurasian Watermilfoil causes hundreds of millions of dollars in economic damage annually across the United States in lost recreation, reduced property values, and management costs
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