Flower:

Flower cluster up to 10 inches (25 cm) long at the top of the stem, rising above the water's surface, with separate male and female flowers on the same plant (monoecious). Flowers are tiny, stalkless, and are whorled mostly in 4s around the stem, sometimes 5s or 6s. Whorls of male flowers are at the tip; male flowers have 8 yellow stamens and 4 yellowish to pinkish petals.

Female flowers are below the males and have a 4-parted yellow to pink style, the petals minute or absent. Often, in between the male and female whorls are one or more whorls of perfect flowers, having both styles and stamens. At the base of a flower is a green leaf-like bract, longer than the flower, the edges strongly toothed or comb-like with teeth cut halfway or more to the middle.

Leaves and stems:

Leaves are whorled all along the stem with usually 4 leaves in a whorl, occasionally 3 or 5, broadly elliptic to egg-shaped in outline, 1.2 to 4.5 cm (½ to 1¾ inches) long with 8 to 17 pairs of thread-like, toothless leaflets. The lowest pair of leaflets are usually shorter than the pair above it and those of at least some leaves are very close to the stem, making the compound leaf stalkless or nearly so. The space between leaf nodes is often very short. Plants are usually submersed, the leaves limp; leaves of emersed plants are lance-elliptic in outline, stiffer, smaller and with fewer leaflets.

Stems are green to brown, smooth, unbranched or with a few branches that can reach 8 feet or more in length. Colonies are formed from spreading rhizomes, sometimes forming dense stands. Turions (winter buds) are club-shaped, brown to reddish-brown at maturity, and formed at leaf nodes and on side branches starting in late summer.

Fruit:

Fruit is round, smooth, olive green to brown at maturity, 2 to 2.5 mm long, lobed in 4 parts and eventually splits into 4 seeds.

Notes:

Whorled Water-milfoil is found in soft to hard lakes, ponds and slow-moving rivers with sandy or boggy bottoms, most often in less than 6 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.

Whorled Water-milfoil is distinguished by its floral bracts longer than the flowers, the edges cut with comb-like lobes, the green to brown stems, and leaves broadly elliptic to egg-shaped in outline with 8 to 17 pairs of leaflets, the lowest leaflet pair of at least some leaves very close to the stem making the compound leaf stalkless or nearly so. Turions (winter buds) are produced starting in mid to late August; they are stiff, club-shaped, brown to reddish-brown at maturity.

Most similar is Northern Water-milfoil (M. sibiricum), which has floral bracts shorter than the flower, yellowish stems, only 5 to 12 pairs of leaflets per leaf, the lowest leaflets usually longest, and turions are more cylindric in shape. Eurasion Water-milfoil (M. spicatum) has leaves more narrowly oval-elliptic in outline, 12 to 21 pair of leaflets, floral bracts are shorter than the flower and mostly toothless, and produces no turions.