Myriophyllum spicatum is a perennial submerged aquatic herb, from root crowns, inflorescences emergent, stems, to 7 m long (usually no more than 50 cm longer than water depth) but often in fragments floating along water surface that form roots and sink to the bottom, freely branching at water surface, forming a dense canopy, stems thickened below inflorescence, shoot tip tassel-like, internodes 1-3 cm long.
Leaves in whorls, sessile, pinnately divided, 1-5 cm long, feathery, segments linear 12-20 per side, basal segments much longer than those near tip; emergent inflorescence with short bracts.
Flowers minute, often pink, 4-parted, enclosed by a small bract with entire margin, on a terminal spike 4-10 cm, generally held above water surface at a right angle to submerged stems; flowers and fruit in whorls; blooms and fruits June-Sept.
Fruit hard nutlets, 0.2-0.3 cm long. Seedlings rarely observed.
Wetland status: OBL.
Frequency in NYC: Very infrequent.
Habitat: Still water 1-3 feet deep, tolerant of nitrogenous pollution. Tolerates pH 5.4-11, tolerates up to 15 parts per thousand salinity (brackish) and water depth to 10 feet (depends on light levels).
Notes: Grows best at relatively high temperatures and can grow at low levels of carbon dioxide, characteristics of C4 metabolism in a C3 plant. Effected by “northeast disease/Lake Venice disease” that may be bacterial in origin (Aiken et al. 1979). Listed as an invasive alien plants by the NYS DEC Invasive Species Task Force (Sanford 2007). European water-milfoil displaces native aquatic vegetation, covers water surfaces and impedes water flow (Wooten et al. 1996).