Western water-milfoil, myriophylle menu

Burshia humilis Rafinesque, Med. Repos., hexade 2, 5: 357. 1808; Myriophyllum ambiguum Nuttall var. limosum Nuttall; M. procumbens Bigelow

Herbs monoecious, aquatic or semi­aquatic, usually not forming dense stands. Stems often branched, to 1 m. Turions absent. Leaves usually alternate or opposite, rarely in whorls of 3(or 4), heteromorphic; petiole to 4 mm; submersed leaves pectinate, ovate to elliptic in outline, (5.5–)10–27(–30) × (4.4–)6–22(–33) mm, segments (2–)4–13(–14), linear-filiform, longest segment (3–)8–17(–22.5) mm; emersed leaves usually pectinate to pinnatifid proximally, linear to spatulate or lobed distally, 5–9(–12.5) × 0.3–3(–6) mm, segments (0–)4–6(–9). Inflorescences to 35 cm, sometimes submersed with flowers in axils of unmodified, pectinate leaves; flowers proximally pistillate, distally staminate; bracteoles cream, oblong to elliptic to ovate or triangular, 0.3–0.7 × 0.1–0.4 mm, margins entire or irregularly lobed, apex often aristate. Staminate flowers: sepals cream to stramineous, triangular, 0.1–0.2 × 0.1–0.2 mm; petals caducous, purple, elliptic to obo­vate, 0.6–1.5 × 0.3–0.7 mm; stamens 4, filaments to 0.9 mm, anthers 0.3–0.8 × 0.1–0.3 mm. Pistillate flowers: sepals cream to stramineous, triangular, 0.1–0.2 × 0.1 mm; petals caducous, purple, elliptic to obovate, 0.3–0.5 × 0.2–0.3 mm; pistils 0.7–0.9 mm, stigmas red to ± purple, to 0.2 mm. Fruits cylindric, deeply 4-lobed. Mericarps tan to red-brown or purple, cylindric to narrowly ovoid, (0.6–)0.8–1.2 × 0.4–0.6 mm, transversely elliptic to ovate, abaxial surface rounded, sparsely to densely tuberculate, tubercles relatively small, shallow, wings and ridges absent.

Flowering and fruiting Jun–Oct. Oligotrophic waters, lakes, ponds, streams; 0–700 m; N.B., N.S., Que.; Conn., Del., Maine, Md., Mass., N.H., N.J., N.Y., Pa., R.I., Vt., Va.

Myriophyllum humile has a diminutive semiterres­trial growth form, referred to as var. limosum by T. Morong (1891), that has alternate and, typically, spatulate floral bracts that can be confused with the emergent form of M. pinnatum. In vegetative form, the leaves of M. pinnatum tend to be pectinate with a greater number of longer segments than those of M. humile, which are often linear, spatulate, or 4–6-lobed. When fruits are present, the two can be distinguished by the presence of winged ridges on the mericarps in M. pinnatum, which are absent in M. humile. Submersed forms of M. humile can be confused with M. farwellii because of the delicate nature of their leaves. M. humile has leaves mostly alternate and opposite; leaves in M. farwellii tend to be whorled or, sometimes, subverticillate, giving these plants a bushy appearance. Myriophyllum humile can be also confused with M. laxum (see 10. M. laxum discussion).

All specimens examined from Minnesota labeled Myriophyllum humile have been misidentified and are other species of the genus. Only one sterile herbarium specimen labeled as M. humile from Wisconsin has been seen, but its identity could not be confirmed.