Identifying Eurasian Watermilfoil
Eurasian watermilfoil identification is complicated by two factors: (1) it must be distinguished from several native North American milfoil species that resemble it; and (2) natural hybridization between Eurasian and native milfoils produces intermediate forms that are genuinely difficult to identify. Correct identification is critically important for management — applying milfoil-targeting treatments to native milfoil is ecologically harmful and potentially illegal, while leaving Eurasian watermilfoil untreated allows infestations to expand. This guide covers the primary features used for field identification and the limits of field identification when hybrids may be present.
Key Identification Features
1. The Leaflet Count Test
The most important identification tool for milfoil species is counting leaflet pairs. Remove a mid-stem leaf from the upper portion of the plant (not the very tip or very base), and count the pairs of thread-like leaflets on one side of the leaf central axis (rachis). Double that count for total pairs. Eurasian watermilfoil typically has 14–24 leaflet pairs — higher than any native North American milfoil. The most common native milfoil in northern states, northern watermilfoil (Myriophyllum sibiricum), typically has 5–12 pairs. This count is the most reliable single identification character: if you count 14+ pairs on a well-developed mid-stem leaf, you almost certainly have Eurasian watermilfoil. Counts between 9–13 pairs suggest the possibility of the Eurasian × Northern hybrid.
2. Leaf Texture and Rachis
The central stalk (rachis) of Eurasian watermilfoil leaves tends to be flexible and somewhat flaccid when the leaf is removed from water — the leaflets droop when the leaf is held horizontally out of water. Native northern watermilfoil tends to have stiffer leaves that hold their shape better out of water. This is a useful supplemental feature but not reliable in all conditions and growth stages.
3. Stem Characteristics
Eurasian watermilfoil stems are typically reddish or pinkish near the stem tip, transitioning to whitish or pale toward the base. Stem diameter is 2–5 mm. The stems are not hollow and do not break easily between nodes. Whorls of leaves (4 per whorl in milfoil) are spaced approximately 1–2 cm apart on the stem. The stem tip typically extends above the water surface in shallow water, creating the distinctive emergent "spiked" inflorescence (flower spike) in summer.
4. Flower Spike
In summer (typically June–August), Eurasian watermilfoil produces an emergent flower spike 5–20 cm above the water surface. The spike bears small, reddish, 4-petaled flowers in whorls of 4. The flower spike is the easiest and most reliable identification feature when present — no submerged aquatic plant other than milfoil species produces this emergent flower spike form. However, flower spikes are only present for a portion of the growing season, requiring reliance on vegetative features at other times.
5. Whorled Leaves, Groups of 4
Like all watermilfoil species, Eurasian watermilfoil has leaves in whorls of 4 around the stem. This 4-leaf whorled arrangement immediately identifies the plant as a milfoil species (not hydrilla or coontail). Combined with the leaflet count, it provides a reliable identification.
Native Milfoil Comparison
Several native milfoil species are present in North American water bodies and must not be treated as Eurasian watermilfoil:
- Northern watermilfoil (Myriophyllum sibiricum): 5–12 leaflet pairs; stiffer leaves; slightly more bluish-green color; most common native milfoil in the northern U.S.
- Variable-leaf watermilfoil (Myriophyllum heterophyllum): Has both pinnately divided submerged leaves AND undivided or finely toothed floating leaves — the heterophyllous (different-leaved) character is diagnostic. Native to eastern North America but is invasive in some western states.
- Whorled watermilfoil (Myriophyllum verticillatum): Very fine, thread-like leaflets; typically 8–20 leaflet pairs but typically has 5 whorled leaves per node rather than 4, which distinguishes it from Eurasian watermilfoil.
When to Seek Laboratory Confirmation
When leaflet counts fall in the ambiguous 9–13 range or when the management decisions are high-stakes (large-scale treatment program), molecular or detailed morphological identification should be sought. Several university labs and state agency labs offer DNA-based identification of milfoil species and hybrids from plant tissue samples. Contact your state DNR aquatic invasive species program for referrals to identification services. For complete management guidance once identification is confirmed, see Eurasian watermilfoil control methods.
References
- Aiken, S.G., et al. (1979). Identification of Myriophyllum spicatum. Aquatic Botany 6:379–390.
- Moody, M.L. & Les, D.H. (2002). Evidence of hybridity in invasive watermilfoil. PNAS 99(23):14867–14871.
- Minnesota DNR. Eurasian Watermilfoil Identification. dnr.mn.us