Eurasian watermilfoil

Submersed Plants

Myriophyllum spicatum:

Eurasian watermilfoil

Myriophyllum spicatum L.: Eurasian water milfoil, spiked milfoil. Haloragaceae (watermilfoil family). 

Non-native perennial. Submersed except for emergent flower spike; rooted in substrate and growing in still or flowing water. The spongy, stretchy, reddish stems grow to 10 ft (3 m) long and branch frequently, forming dense canopies at the surface. Rather stiff, feather-like leaves, to 2 in (5 cm) long, are arranged 4 to a whorl. The slender leaf divisions are almost paired on either side of the central midrib; these divisions extend out and then up toward the leaf tip where their ends form a blunt or squared-off edge across the top of the leaf, rather than a point. The number of leaflet divisions per side of an individual leaf is usually more than 12 to 14. In comparison, native Myriophyllum species usually have fewer, from 5 to 12, divisions per side. The emergent flower spike has whorls of inconspicuous flowers spaced at intervals. This plant reproduces rapidly within a season from easily-rooted stem fragments. It also forms dense clusters of leaves at some branch tips; these winter buds can detach, fall to the substrate, and give rise to new plants in spring. Eurasian watermilfoil's thick surface canopies cut off light and oxygen to the lower water levels beneath it, and this competition eliminates native submersed plants. It is an extremely noxious weed and has devastated many aquatic environments.