The leaves are 1.5 to 4.0 cm long and whorled, usually with four in each whorl. The leaves outline is feather-like. The stem is very rigid and characteristically curved so that this portion lies parallel to the water surface. The flowers radiate from the 4 leaf whorls with adjacent whorls rotated 45 degrees. The lower flowers are female, thus contain pistil, and upper flowers are male, thus containing stamen.
Reproduction can either be asexual or sexual.
In asexual reproduction stems release fragments that float, develop roots, and new stems and leaves, before sinking and adhering to the bottom substrate. Eurasian milfoil sexually reproduces via cross-pollination. Its' fruits are 2 - 3 mm long often occurring in bunches. Humans can spread the plant between water body systems from boats and boat trailers carrying the plant fragments. Origin Tests on the response to growth conditions of Eurasian milfoil plants from Quebec, Ontario, British Columbia, Wisconsin, Tennessee, Alabama, Michigan, and Washington imply a clonal origin for all Eurasian milfoil in North America.