Perennial aquatic herb Stem: mostly submersed, unbranched or with few long branches to 2.5 m long. Leaves: in whorls of four to five, pinnately divided. Submersed leaves are 0.8 - 4.5 cm long with nine to seventeen divisions on each side. Emersed leaves and bracts are smaller and more coarsely divided. Flowers: either male or female, found on the same plant (monoecious), some bisexual, borne in whorls of four to six on a terminal spike above the water surface, with male flowers near inflorescence tip. Pinnate or incompletely pinnate bracts extend past the flowers. Small, greenish, four-parted, petals are poorly developed in female flowers and yellowish green in male flowers, which have eight stamens. Fruit: a deeply four-lobed nut-like cluster, 2 - 2.5 mm long, nearly spherical, splitting into four segments, each segment brown and deeply four-furrowed with a round back. Winter buds: well-formed, 1 - 2 cm long, club-shaped, falling off in early winter.
Similar species: Myriophyllum sibiricum and Myriophyllum spicatum differ by having non-toothed or toothed bracts that are shorter than the flower. Myriophyllum pinnatum is easy to distinguish by having both alternate and whorled leaves. Myriophyllum heterophyllum has four stamens on its male flowers, toothed or non-toothed bracts, and winter buds formed at the base of stems or on rhizomes.
Flowering: mid July to late August
Habitat and ecology: Rare in lakes and streams.
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. Verticillatum means whorled.
Author: The Morton Arboretum
From Flora of Indiana (1940) by Charles C. Deam
Our only specimens are from Crooked Lake, Steuben County, but doubtless it is more widely distributed. The reports, made by earlier authors, of Myriophyllum pectinatum from Fulton, Kosciusko, Marshall, Steuben, and Vigo Counties and from the dune area should probably be referred to this variety.
Much like no. 9 [Myriophyllum sibiricum Kom.]; lvs with 9-17 segments per side; floral bracts less strongly reduced and modified, those subtending the pistillate fls (1-)2-several times as long as the fl, sometimes to 1 or even 1.5 cm, even those subtending the staminate fls evidently cleft or laciniate- toothed; pistillate fls with persistent sep mostly 0.5-1 mm; staminate fls with yellow-green pet; stamens 8, less often 4; turions abscised from the stem in late fall or early winter, dark yellow-green, their lvs less modified than in no. 9; 2n=28. In acid to circumneutral waters to 3 m deep; circumboreal, s. to Mass., N.Y., Ind., Ill., Nebr., and Utah. (M. hippuroides, misapplied to the phase with 4 stamens)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.