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, and are whorled in 4s around the stem. Male flowers are at the tip of the spike, have 8 yellow stamens and lack petals or have 4 pinkish petals.

Female flowers are below the males and have a 4-parted yellowish to deep pink style and no petals. At the base of a flower is a green bract, as long as or shorter than the flower, toothless or minutely toothed.

Leaves and stems:

Leaves are whorled all along the stem with usually 4 leaves in a whorl, occasionally 3 or 5, mostly broadly egg-shaped in outline, 1 to 5 cm (3/8 to 2 inches) long with 5 to 12 pairs of thread-like, toothless segments. The lowest leaflet pair is longest and spreading to ascending, the leaflets becoming shorter and more ascending as they ascend the central stalk. Leaves tend to retain their shape when removed from the water.

Stems are yellowish to reddish becoming white when dry, smooth, unbranched or with a few forked branches. Colonies are formed from spreading rhizomes. Turions (winter buds) are stiff, dark, cylindric to club-shaped, and formed on side branches in autumn.

Fruit:

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

Notes:

Northern Water-milfoil is the most common Myriophyllum species in Minnesota, found in soft to hard lakes, ponds and slow-moving rivers with sandy or boggy bottoms, most often in less than 5 feet deep water.

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.

Northern Water-milfoil is distinguished by its floral bracts mostly shorter than the flowers and toothless or minutely toothed, yellowish stems, and leaves mostly broadly egg-shaped in outline with 5 to 12 pairs of leaflets, the lowest leaflet pair longest and spreading. The leaves tend to retain their shape when removed from the water. Turions (winter buds) are produced in fall; they are stiff, dark, cylindric to club-shaped.

Most similar is Eurasian Water-milfoil (M. spicatum), which also has floral bracts shorter than the flower, but 12 to 21 pairs of leaflets per leaf that are more similar in size and ascending, the leaf usually more oval-elliptic in outline, and leaves are very limp and collapse when removed from the water; it also produces no turions. Whorled Water-milfoil (M. verticillatum) may have 12 or fewer pair of leaflets like Northern Water-milfoil, but its stems are green to brown, turions are all club-shaped and reddish to brown at maturity, floral bracts are longer than the flowers and distinctly divided with comb-like linear lobes.