Herbs monoecious, aquatic or semiaquatic, sometimes form­ing dense stands. Stems often branched, to 1 m. Turions absent. Leaves usually in whorls of (3 or)4, often subverticillate or irregular, sometimes alternate, heteromorphic; petiole 0–5 mm; submersed leaves pectinate, ovate to obovate in outline, (9.5–)12–28(–33) × (7–)10–20(–25) mm, segments 6–12(–13), linear-filiform, longest segment (5–)8–15(–20) mm; emersed leaves linear-lanceolate, margins lobed to serrate, pectinate to pinnatifid, (4–)6–17(–19) × 1–6.5 mm, segments (0–)3–10. Inflores­cences to 20 cm; flowers proximally pistillate, medially bisexual, distally staminate; bracteoles cream, ovate to triangular or deltate, 0.5–0.8 × 0.2–0.5 mm, margins serrate to irregularly lobed. Staminate flowers: sepals cream to stramineous, elliptic to linear-lanceolate or narrowly triangular, 0.1–0.3(–0.4) × 0.1–0.2 mm; petals persistent, cream to purple, elliptic to obovate, 0.7–1.7(–1.9) × (0.4–)0.5–0.8(–1) mm; stamens 4, filaments to 1.4 mm, anthers 0.7–1.5 × 0.2–0.5(–0.7) mm. Pistillate flowers: sepals cream, triangular, 0.1–0.2 × 0.1–0.2 mm; petals usually caducous, rarely persis­tent, cream, elliptic to obovate, 1–2 × 0.5–0.7 mm; pistils 0.8–1.2 mm, stigmas red to ± purple, to 0.5 mm. Fruits ovoid, cruciate. Mericarps tan to brown, cylindric to ovoid, (1–)1.2–1.8 × 0.6–0.8(–1) mm, transversely elliptic, flattened, abaxial surface sharply 2-angled, flattened to concave, papillate, sometimes minutely tuberculate, with 2 distinct longitudinal ridges, ridges with prominent, membranous, undulating wings, wings erect to reflexed, with 6–12 perpendicular ribs.

Phenology: Flowering and fruiting Mar–Oct.
Habitat: Oligotrophic to mesotrophic waters, lakes, ponds, sloughs, mudflats.
Elevation: 0–700 m.

Distribution

• Generated Map
• Legacy Map

B.C., N.B., Sask., Ark., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., La., Md., Mass., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Okla., R.I., S.C., S.Dak., Tenn., Tex., Va., W.Va.

Discussion

Myriophyllum pinnatum is often confused with M. heterophyllum and/or M. hippuroides. The most distinctive characters to separate these species are their fruits and mericarps. In M. pinnatum, mericarps are cylindric with sharply angled faces and a prominent ribbed wing; both M. heterophyllum and M. hippuroides have globose to subglobose mericarps with inconspicuous ridges and prominent tubercles. Myriophyllum pinnatum has leaves with significantly fewer segments, and plants of M. hippuroides tend to be very delicate in appearance. Both M. hippuroides and M. pinnatum produce elongate, strap-shaped to linear-lanceolate to spatulate leaves distally, either when they grow submersed and produce emergent flowering racemes or when they grow as emergent plants stranded along shorelines. The former species typically produces pinnatifid emersed leaves with fewer segments. Although M. heterophyllum also produces emergent leaves in response to flowering, the leaves typically grade from pectinate to lobed to entire to ovate serrate leaves distally. Myriophyllum heterophyllum sometimes also produces a low growing semi-terrestrial form on mudflats having thickened and abbreviated pectinate leaves. The pectinate leaves produced under these conditions also have more segments than those seen in the other species. The emergent leaves in M. pinnatum have very shallow dentate margins compared to the lobed leaves of M. hippuroides. Myriophyllum pinnatum has been misidentified as M. verticillatum based on the presence of distally reduced pectinate leaves; in M. verticillatum the transition is almost always associated with flowering.

Records for Vermont and New Brunswick are range extensions for Myriophyllum pinnatum.

Selected References

None.

Lower Taxa

None.