Why This Identification Matters
Identifying Eurasian watermilfoil and distinguishing it from native milfoil species is one of the most ecologically important — and genuinely challenging — identification problems in freshwater aquatic plant management. Most state aquatic invasive species programs prioritize management and exclusion of Eurasian watermilfoil specifically because of its documented invasive impacts. Treating native milfoil with targeted herbicides is both ecologically harmful (native milfoils provide important fish and wildlife habitat) and legally problematic in many states. Yet leaving Eurasian watermilfoil untreated allows infestations to expand. The existence of natural hybrids between Eurasian and native milfoils adds further complexity. This guide gives you the tools to make correct field identifications and know when to seek professional confirmation.
Side-by-Side Comparison: Key Characters
Character 1: Leaflet Count (Most Reliable)
Collect a well-developed leaf from the mid-stem (not the growing tip, which has immature leaves, and not the base, which may have senescent leaves). Count the pairs of leaflets on one side of the leaf central axis.
Eurasian watermilfoil (M. spicatum): 14–24 leaflet pairs. This is consistently high.
Northern watermilfoil (M. sibiricum): 5–12 leaflet pairs. This is consistently low.
Eurasian × Northern hybrid: 9–13 leaflet pairs. Intermediate and overlapping with both parents — the ambiguous zone.
Count at least 3 leaves from different parts of the plant and average the counts. If all counts are above 14, you have Eurasian watermilfoil with high confidence. If all counts are below 12, you likely have native milfoil. If counts fall between 9–13, the hybrid is possible and laboratory confirmation is advisable.
Character 2: Leaf Texture and Droop
Hold a leaf horizontally out of water and observe the leaflets. Eurasian watermilfoil leaflets tend to droop and the leaf goes limp relatively quickly. Northern watermilfoil leaflets tend to be stiffer and hold their position longer when held out of water. This is a supplemental character — not reliable in isolation because leaf stiffness varies with growth stage and water chemistry — but it supports a leaflet count identification.
Character 3: Leaflet Width and Appearance
Eurasian watermilfoil leaflets are very fine, 0.2–0.5 mm wide, giving the leaf a delicate, lacy appearance. Northern watermilfoil leaflets are similar in width. This character alone does not distinguish the species reliably but contributes to overall appearance — Eurasian milfoil often has a slightly more lacy, more numerous-leafleted appearance that experienced observers learn to recognize.
Character 4: Stem Color
Eurasian watermilfoil often has reddish or pinkish coloring near the growing tip and upper stem. Northern watermilfoil tends toward greener coloration throughout. This character is variable and influenced by light, nutrients, and water chemistry — do not rely on it alone. However, a reddish stem tip combined with high leaflet counts strongly suggests Eurasian watermilfoil.
The Hybrid Problem
The Eurasian × Northern watermilfoil hybrid (Myriophyllum spicatum × sibiricum) was first described in the 1990s and has since been documented across much of the northern U.S. and southern Canada wherever both parent species co-occur. The hybrid can be more vigorous than either parent ("hybrid vigor") and may show enhanced resistance to some herbicides — particularly triclopyr, to which Eurasian watermilfoil is susceptible but some hybrid populations have shown reduced sensitivity. Management programs that apply triclopyr and see unexpectedly poor results should consider hybrid presence and potentially seek genetic identification.
Hybrid identification in the field is not reliably possible based on morphology alone in the 9–13 leaflet pair zone. DNA-based identification (PCR with species-specific primers) from leaf tissue is available through several university and commercial labs. Your state DNR aquatic invasive species program can provide guidance on sample collection and submission for genetic identification.
Native Milfoil Species Worth Knowing
Beyond northern watermilfoil, several other native milfoil species occur in North American water bodies and should not be confused with or treated as Eurasian watermilfoil:
- Variable-leaf milfoil (M. heterophyllum): Has two leaf forms — finely divided submerged leaves and undivided or coarsely toothed floating leaves. Native to eastern U.S., but invasive in some western states. Leaflet counts on submerged leaves 4–14 pairs.
- Whorled milfoil (M. verticillatum): Has 5 leaves per whorl (not 4 like Eurasian). Leaflets 12–20 pairs. Native and ecologically valuable.
- Alternate watermilfoil (M. alterniflorum): Small, delicate; flowers alternate rather than whorled on the spike. Rare, native, ecologically important in oligotrophic systems.
For complete management guidance once Eurasian watermilfoil is confirmed, see Eurasian watermilfoil control methods.
References
- Moody, M.L. & Les, D.H. (2002). Evidence of hybridity in invasive watermilfoil. PNAS 99(23):14867–14871.
- Aiken, S.G., et al. (1979). Myriophyllum spicatum identification. Aquatic Botany 6:379–390.
- Minnesota DNR. Watermilfoil Identification Guide. dnr.mn.us