Flower:

Loose spike up to 2 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 typically lack petals. Male flowers are arranged alternately at the tip of the spike and have 4 or 8 stamens.

Female flowers have a 4-parted red style and are below the males, the lowest flowers on the spike usually in a whorl of 3 or 4 and the rest alternately arranged. At the base of a flower is a leaf-like bract, shorter than to about as long as the flower, toothless or minutely toothed; bracts on the lowest flowers are larger and more deeply divided into narrow segments.

Leaves and stems:

Leaves are whorled all along the stem with 4 leaves in a whorl, occasionally 3. Leaves are limp, 4 to 12 mm (to ~½ inch) long with 3 to 10 pairs of thread-like, toothless segments. The distance between whorls (internode) is often shorter than the leaf length. Color is green to red.

Stems are slender, pale green to bright red, smooth and much branched. Turions (winter buds) are not formed.

Fruit:

Fruit is brown, about 1.5 mm long, with 4 lobes that eventually split into 4 seeds.

Notes:

Alternate-flowered Water-milfoil is occasional in soft water lakes with sandy, gravelly or mucky bottoms in northeastern Minnesota, in clear or dark tannin-stained water. It has a circumpolar distribution, native to parts of Europe, Asia and North America and reaches the southern edge of its range in Minnesota. Plants are generally all submersed with just the flowering spikes rising out of the 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.

M. alterniflorum is distinguished by its overall slenderness, leaves usually less than 1 cm (3/8 inch) long with fewer than 10 pairs of leaflets, and terminal spike with mostly alternately arranged flowers and bracts. Plants are green to red-tinged to bright or dark red.