After flowering, plant biomass declines as the result of the fragmentation of stems. Where flowering occurs early, plant biomass may increase again later in the growing season and a second flowering may occur. During fall, plants die back to the root crowns, which sprout again in the spring. In areas with mild winters, like western Washington and Oregon Eurasian watermilfoil frequently overwinters in an evergreen form and may maintain considerable winter biomass. Eurasian watermilfoil plants do not form specialized overwintering structures such as turions.
Carbohydrate storage occurs throughout overwintering shoots and roots.
Although Eurasian watermilfoil can potentially spread by both sexual and vegetative means, vegetative spread is considered the major method of reproduction. In Lake George, New York a young population of Eurasian watermilfoil averaged a seed set of 112 seeds per stalk. Eurasian watermilfoil seeds readily germinate in the laboratory and also germinated in situ in a study conducted in Lake George. Despite the high seed production, it is thought that germination of seed is not a significant factor in Eurasian watermilfoil reproduction. Seedlings have never been observed occurring naturally in situ , therefore colonization of new sites is mainly by vegetative fragments.