While Eurasian Water Milfoil is not common in NH lakes, it is present and we need to be vigilant to its introduction. Once present in a lake it is very difficult to eradicate.
Eurasian water-milfoil is native to Europe and Asia. It was introduced to North America in the 1940s and has spread rapidly since then. Eurasian water-milfoil is now present in most US states and in Canada. It thrives in a wide variety of environmental conditions. It grows well in still and flowing waters, and can survive under ice. Eurasian water-milfoil grows rooted in water depths from a few to 30 feet, generally reaching the surface where water depths are less than 15 feet.
The plant has branching stems rooted in the bottom of the lake. The leaves are arranged in whorls of 3 to 6 leaves (4 leaves per whorl is common). The whorls are openly spaced along the stem, with 1 to 3 cm between nodes. The leaves are finely feather-divided, typically with 12 to 24 pairs of thread-like leaflets on each leaf. Since the leaves of other milfoil species generally have fewer than 14 leaflet pairs, counting leaflets can provide helpful clues to identifying Eurasian water-milfoil.
Eurasian watermilfioil can be confused with the native northern watermilfoil. See below for for tips to distinguish them.
Source: Wisconsin Aquatic Invasive Species Early Detector Handbook