Eurasian watermilfoil (Myriophyllum spicatum) is a submersed aquatic plant native to Europe, Asia, and northern Africa. Like the native milfoils, the Eurasian variety has slender stems whorled by submersed feathery leaves and tiny flowers produced above the water surface. The flowers are located in the axils of the floral bracts, and are either four-petaled or without petals.
The leaves are threadlike, typically uniform in diameter, and aggregated into a submersed terminal spike. The stem thickens below the inflorescence and doubles its width further down, often curving to lie parallel with the water surface. The fruits are four-jointed nut-like bodies. Eurasian watermilfoil has 9-21 pairs of leaflets per leaf, while Northern milfoil typically has 7-11 pairs of leaflets. Coontail is often mistaken for the milfoils, but does not have individual leaflets. Leaf tips are rounded, and stems are flat and alternately branched. Typically found in water less than 3 meters deep, it is tolerant of low light and low water temperatures and can be found up to 18 meters deep. Inconspicuous, unbranched, and erect flower spikes of 1-2 cm long grow above the water surface in spring to early summer.
The early spring growth of Curly-leaf Pondweed shades-out and inhibits native aquatic plants. This growth can result in dense mats that inhibit boating and swimming. Once the plant becomes dormant in mid-summer, the decay depletes oxygen in the water, potentially encouraging algal blooms.