Eurasian Watermilfoil Identification & Treatment
Scientific Name: Myriophyllum spicatum Category: Submerged Tier: 1 — High-Priority — Most prevalent and/or most damaging across the US
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Identification
Eurasian watermilfoil has feather-like leaves arranged in whorls of 3–5 around the stem. Each leaf is finely divided into 12–21 pairs of thread-like leaflets. Stems are long, branching near the surface to form dense canopy mats. Small reddish flower spikes emerge above the water surface in summer. Key distinguishing features:
- Feathery, finely divided leaves in whorls of 3–5
- 12–21 pairs of leaflets per leaf (native milfoils typically have fewer)
- Stems often reddish or brownish
- Leaves tend to collapse around the stem when removed from water (limp), while native milfoils remain more rigid
Commonly confused with: Native watermilfoils (fewer leaflet pairs, stiffer leaves), coontail (leaves fork rather than branch from a central axis), parrot feather (emergent leaves are stiff and feathery above water)
Treatment
Recommended Natural Waterscapes Products:
- Harvester (diquat) — Fast-acting contact herbicide. Best for spot treatments and quick knockdowns. Kills visible plant material within 1–2 weeks but does not prevent regrowth from roots. Not effective in turbid or muddy water. Harvester 32oz | Harvester 1gal
- Navigate Granular (2,4-D) — Systemic granular herbicide. Sinks to the bottom and releases into the root zone. Most effective treatment for watermilfoil — provides season-long control. Apply in spring during active growth. Navigate
- Aquathol Super K (endothall) — Contact granular herbicide for spot treatments. Aquathol Super K
- Sonar A.S. (fluridone) — Systemic option for whole-pond treatment in waters with limited flow. Sonar A.S.
Full Profile
- Native Range: Europe, Asia, North Africa
- US Distribution: Found in at least 48 states. Present in virtually all EPA Level III ecoregions across the contiguous US. Most problematic in northern tier states (Northern Lakes and Forests, North Central Hardwood Forests, Northeastern Highlands) where it outcompetes native milfoils. USDA Hardiness Zones 3–10.
- Regulatory Status: Listed as invasive or noxious in most states. Not on the Federal Noxious Weed List but regulated at the state level in many jurisdictions.
- Habitat: Lakes, ponds, slow-moving rivers, and reservoirs. Prefers nutrient-rich water but tolerates a range of conditions. Typically grows in 3–15 feet of water.
- Reproduction: Primarily by fragmentation — small stem pieces carried by boats, currents, and wildlife root and establish new colonies. Also spreads by stolons and, less commonly, by seed. A single stem fragment can start a new infestation.
- Ecological Benefits: Provides some fish habitat structure, particularly for panfish and juvenile bass. Dense beds can stabilize sediments. However, monoculture mats reduce native plant diversity and can create oxygen-depleted zones beneath the canopy. Waterfowl use is limited compared to native pondweeds.
- Seasonal Behavior: Begins growing earlier in spring than most native species (a key competitive advantage). Peak biomass in mid-summer. Dies back in fall in northern areas; semi-evergreen in southern regions. Auto-fragmentation in late summer contributes to spread.
Sources: Texas A&M AquaPlant | MSU Extension – Aquatic Invasive Plants
Related Species
- Hydrilla (Hydrilla verticillata) — Submerged, Tier 1
- Coontail (Hornwort) (Ceratophyllum demersum) — Submerged, Tier 1
- Curly-Leaf Pondweed (Potamogeton crispus) — Submerged, Tier 1
- Elodea (Common Waterweed) (Elodea canadensis) — Submerged, Tier 1
- Egeria (Brazilian Elodea) (Egeria densa) — Submerged, Tier 1
- Sago Pondweed (Stuckenia pectinata (formerly Potamogeton pectinatus)) — Submerged, Tier 2
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