Perennial aquatic herb Stem: mostly submersed, somewhat stout, to 3 mm thick, internodes usually less than 1.5 cm long. Leaves: in whorls of four to six. Submersed leaves are 2 - 5 cm long, pinnately divided, with seven to ten divisions per side. Amphibious leaves pinnately divided. Emersed leaves are 0.4 - 3 cm long, 1 - 5 mm wide, lance- to spoon-shaped or elliptic, non-toothed or toothed, and firm. Flowers: either male or female, found on the same plant (monoecious), borne solitary in a terminal spike (3 - 37 cm long) above the water surface, with male flowers near the inflorescence tip. Bracts are usually more than twice as long as the male flowers (4 - 18 mm long), lance-shaped to oblong, toothed near the pointed tip, with comb-like divisions at the water surface. Flowers stalkless, green, small, four-parted, with female flower petals 1.5 - 3 mm long with pointed tips. Fruit: a deeply four-lobed nut-like cluster, 1 - 1.5 mm across, nearly spherical, slightly warty, beaked, splitting into four one-seeded segments that are two-ridged and rounded on the sides. Winter buds: at base of stem or on rhizomes, persistent.
Similar species: Myriophyllum pinnatum is easy to distinguish from M. heterophyllum by having both alternate and whorled leaves. Myriophyllum spicatum differs because its bracts are shorter than the flowers, its leaves have twelve or more pairs of divisions, and it lacks winter buds. Myriophyllum sibiricum has eight stamens on its male flowers and bracts that are shorter than the flowers. Myriophyllum verticillatum has eight stamens on its male flowers and well-formed winter buds growing along the stem.
Flowering: late June to mid August
Habitat and ecology: Found in quiet ponds and lakes (usually calcareous), this species is less common than Myriophyllum sibiricum in the Chicago Region.
Occurence in the Chicago region: native
Etymology: Myriophyllum comes from the Greek words, myrios, meaning many, and phyllon, meaning leaf, referring to its highly divided leaves. Heterophyllum means "diversely leaved."
Stem stout, to 3 mm thick; internodes seldom over 1.5 cm; lvs whorled, (1.5-)2-4 cm, pinnatisect into capillary segments; spikes emersed, 5-15(-40) cm; bracts whorled, persistent and eventually reflexed, lanceolate to oblong or even obovate, sharply serrulate, 4-18 mm (but fairly uniform on each plant); stamens 4; fr subglobose, 1-1.5 mm, each mericarp rounded or more commonly with 2 low keels on the back, scabrous to obscurely tuberculate, with an ascending- recurved beak. N.Y. to w. Ont. and N.D., s. to Fla. and Tex., more abundant southward.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.