Flower:

Flowers are all submersed, single in the leaf axils, with separate male and female flowers on the same plant (monoecious). Flowers are tiny, stalkless with 4 reddish to purplish-tinged lance-oblong petals about 1 mm long. Male flowers have 4 stamens that are often aborted.

Female flowers have a short 4-parted style at the tip of a warty ovary. At the base of a flower is a membranous bract shorter than the flower.

Leaves and stems:

Leaves are variously arranged all along the stem, many alternate with some opposite or nearly so, and some whorled in 3s or 4s, rarely 5s. Leaves are broadly egg-shaped to nearly round in outline, 1 to 3 cm (3/8 to 1+ inch) long with 5 to 8 pairs of thread-like, spreading to ascending, toothless segments, these often with a minute, black gland in the leaflet axil. Leaves are very limp and collapse when removed from the water.

Stems are slender, reddish-purple to dark reddish-green, smooth, and few-branched. Colonies are formed from spreading rhizomes. Turions (winter buds) are formed at stem tips in fall.

Fruit:

Fruit is reddish, 2 to 2.5 mm long, with 4 angular lobes distinctly warty along the ridges, and eventually splits into 4 seeds.

Notes:

Farwell's Water-milfoil is occasional to common in north-central and northeastern Minnesota, where it reaches the southwestern edge of its range. It is found primarily in the quiet waters of soft water lakes and ponds with sandy or silty bottoms, often in less than 4 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.

Farwell's Water-milfoil is the exception to the characteristic terminal spike of other Myriophyllum species; its flowers and warty fruits are all in the axils of submersed leaves and fertile plants are easily distinguished by this characteristic. While the leaves are relatively unique in their variable arrangement (mixed alternate, opposite and whorled) and 5 to 8 leaflet pairs per leaf often with a tiny black gland in the leaflet axil (magnification required), without flowers or fruit it can be very difficult to distinguish from sterile shoots of Variable-leaf Water-milfoil (M. heterophyllum), which can also have at least some alternate leaves, but also leaves more often whorled in 5s or 6s, as many as 14 pair of leaflets per leaf, lacks the black glands, and plants stranded on shores can also form variable “terrestrial” leaves that are broader and more toothy. It does have an emergent, terminal spike with floral bracts at least twice as long as the flowers, and it forms turions at the base of the stem or on the rhizome, where Farwell's Water-milfoil turions are at stem tips.