Common water milfoil is a submersed aquatic plant. The leaves are similar to a feather in form, with narrow segments (leaflets) arising from both sides of a central rachis (axis). The leaves are arranged in whorls around the stem (see "stem with leaves" above). Common water-milfoil is very similar to European water milfoil (Myriophyllum spicatum). The most reliable way to tell them apart is to count the leaflets on the feather-like compound leaves. Well developed leaves of common milfoil have 11 or fewer leaflets on each side of the rachis and European water milfoil leaves generally have 14 or more leaflets on each side.

There are other species of Myriophyllum with similar form and species cannot be distinguished with certainty unless flowers are present, unless you are familiar with all the species. You cannot safely identify the species of Myriophyllum by comparing a specimen to photographs. The keys and text in Voss, 1984. Michigan Flora Vol. II are recommended.