Identification

Eurasian watermilfoil is a submersed, aquatic perennial in the Haloragaceae family that roots to the bottom of water bodies. The roots are slender and fragile. Stems emerge from root crowns, are smooth and hairless (glabrous), and grow up to 21 feet (7 meters) to the water surface, where they branch profusely. Stems have layers of specialized, partially lignified cells that enable the stem to self-fragment without mechanical disturbance. Stem fragments are capable of forming new plants. Leaves are whorled in groups of four at the stem nodes (see Figure 2), are 0.5 to 1.5 inches (1.5 to 4 cm) long, and have 14 to 24 pairs of threadlike divisions giving the leaf a feather-like appearance. Lower stem leaves continuously die and fall off. Pollen and seed flowers are separate on spikes that form at the ends of the stem branch. Spikes grow two to eight inches (5 - 20 cm) long and are often pink. They emerge from the water supported by the stem that is twice as wide as the lower portion of the stem. At the time of flowering, the spike is erect but bends at fruit set to be parallel to the water surface. Seed producing flowers lack sepals and petals but have a four-lobed pistil. Pollen producing flowers have four pink petals that drop off early in development, and eight stamens. Flowers are whorled in groups of four. The female flowers (seed producing) are lower on the spike than the male flowers (pollen producing), and the flowers in the middle are often bisexual. The fruits are globelike in shape, about 1/8 inch (2-3 mm) long with four long narrow grooves and four seeds.

Figure 2. A sketch of the flower and leaf of Eurasian watermilfoil.

In Montana there are two native species of watermilfoil that have similar leaf features as Eurasian watermilfoil: shortspike watermilfoil (Myriophyllum sibiricum), and whorl-leaf watermilfoil (Myriophyllum verticillatum). The easiest way to distinguish the invasive from the natives is by lifting a stem out of the water. The leaves of the invasive will relax and fall against the stem whereas the leaves of the native will remain rigid and spread from the stem. The natives can also be distinguished from Eurasian watermilfoil by the sparse stem branching near the water surface compared to the abundant branching of Eurasian watermilfoil, by the lack of conspicuous change in stem width below the inflorescence compared to the almost doubled stem width of Eurasian watermilfoil, and by the lack of the specialized layers of stem cells that facilitate stem fragmentation. Eurasian watermilfoil dies back to propagating root crown buds in the fall, whereas the natives form prominent cylindrical or cup-shaped perennating shoots (turions) attached to, or detached from the parent plant.

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Last Modified: 02/13/2009