[www.invasive.org/] Ecological Threat, Invasiveness: Water-milfoil tends to invade disturbed areas and does not typically spread into undisturbed areas where native plants are well established. It can form large, floating mats of vegetation on the surface of lakes, rivers and other water bodies, that impede water traffic and reduce light for native aquatic species. It thrives in areas that have been subjected to natural and man-made disturbance. (link added by Mary Ann Machi)

[Wikipedia] Europe, Asia, north Africa native: Myriophyllum spicatum (Eurasian watermilfoil[1] or spiked water-milfoil) is native to Europe, Asia, and north Africa, but has a wide geographic and climatic distribution among some 57 countries, extending from northern Canada to South Africa.[2] It is a submerged aquatic plant, grows in still or slow-moving water, and is considered to be a highly invasive species. (link added by Mary Ann Machi)