Flower:

Spike up to 4 inches long at the top of the stem and branch tips, rising above the water's surface, with separate male and female flowers on the same plant (monoecious). Flowers are tiny, stalkless, typically lack petals, and are whorled in 4s around the stem. Male flowers are at the tip of the spike and have 8 stamens.

Female flowers are below the males and have a 4-parted yellowish to pink style. At the base of a flower is a green bract, mostly shorter than the flower, toothless or minutely toothed. The stem below the lowest whorl is noticeably thickened, up to nearly twice the width as in the spike.

Leaves and stems:

Leaves are whorled all along the stem with 4 leaves in a whorl, occasionally 5, broadly egg-shaped to oval-elliptic in outline, 3 to 3.5 cm (to 1-3/8 inch) long with 12 to 21 pairs of thread-like, toothless segments. Leaves are very limp and collapse when removed from the water.

Stems are light green to yellowish to reddish, smooth, few branched on the lower stem and more heavily branched near the water's surface, often forming a mat near the surface. Larger colonies are formed from spreading rhizomes. Turions (winter buds) are not formed.

Fruit:

Fruit is olive-brown, 2 to 3 mm long, with 4 relatively smooth lobes rounded or somewhat bumpy along the outer edge, and eventually splits into 4 seeds.

Notes:

Eurasian Water-milfoil is a seriously problematic aquatic invasive species. It tends to be few branched on the lower stem and much branched near the water's surface, forming a mat that blocks out the sun, inhibiting the growth of other aquatic plants and degrading or destroying food sources and habitat for native aquatic wildlife. It can also ruin recreational activities, but that really is the lesser evil. Once established, it can be difficult to control, as stem fragments can reroot and re-infest or start new infestations. In Minnesota, it was first discovered on Lake Minnetonka in Hennepin County in 1987 and has since spread to more than half the state, transported primarily on boats, trailers and related equipment.

With the exception of Myriophyllum tenellum, Myriophyllum species are recognized by leaves compound with a central stalk and multiple spreading, thread-like leaflets, usually whorled in 4s; separate male and female flowers on the same plant (monoecious); most with an emersed terminal spike of flowers and fruits. The overall form of Myriophyllum is like some other aquatic species, notably Ceratophyllum (Coontail), which has forked leaves that lack a central stalk and flowers are all in the leaf axils.

Eurasian Water-milfoil is distinguished by its floral bracts mostly shorter than the flowers and toothless or minutely toothed, and leaves with 12 to 21 pairs of leaflets that are frequently all ascending and nearly the same length, the leaf having a more oval-elliptic outline than most other species and plants appearing more feathery overall. The leaves are very limp and tend to collapse when removed from the water. Plants tend to branch more heavily near the water line. No turions (winter buds) are produced.

Most similar is Northern Water-milfoil (M. sibiricum), which also has floral bracts shorter than the flower, but has only 5 to 12 pairs of leaflets per leaf that are more spreading, perpendicular to the central stalk or nearly so, and they tend to retain their shape when removed from the water; it also produces dark, stiff, cylindric to club-shaped turions on side branches and stem tips in autumn that break bud the following spring, the stem is not noticeably thickened below the terminal spike, and is few branched especially near the water line. Whorled Water-milfoil (M. verticillatum) may have 12 or more pairs of leaflets like Eurasian Water-milfoil, but leaflets are longest at the base of the leaf becoming shorter as they ascend the central stalk, and its floral bracts are longer than the flowers and distinctly divided with comb-like linear lobes.