Native to: Temperate and tropical Eurasia and Northern Africa
This species has been introduced to the US multiple times and was likely first brought to North America in ship ballasts or as an ornamental plant for aquariums or water gardens. Accidental spread of Eurasian watermilfoil within the US is due primarily to transportation of contaminated boat trailers, boat parts and bait containers, but it is also spread through the aquarium trade. First reported in the US in the 1940s it spread rapidly into the mid-Atlantic and midwestern states in the 1960s and 1970s and is now the most widespread submersed aquatic weed in the northern half of the US. It prefers the slow-moving water of lakes and streams and can also tolerate brackish environments. There are three native Myriophyllum species (M. heterophyllum, M. laxum, M. pinnatum) that should be ruled out if this species is suspected.
Habit: submersed rooted, attached to the substrate. Stems slender, smooth, 6 to 20 ft. long, reddish-brown to whitish-pink; branching several times near the water surface.
Leaves: olive-green, less than 2 in. long, soft, deeply divided, feather-like; each leaf with a central axis (midrib) and 14 to 24 or so very slender (filiform) segments on each side of the axis. Leaf whorls are arranged along the stems in whorls of 3 to 6 (usually 4) leaves; whorl nodes are about 3/8 in. apart.
Flowers: on an emersed spike, held erect above the water, the spike to 8 inches long; flowers reddish; arranged in 4-flowered whorls along spike; petals 4; petals 1/8 in. long; sepals 4; stamens 8.
Fruit: 4-lobed; splitting into 4 nutlets.
Distribution in Florida: Scattered reports along the gulf coast from central to north Florida as well as limited reports from the central peninsula.
Once introduced to an aquatic system, it spreads prolifically by stem fragments that are produced both naturally (when stem sections detach from the plant at abscission sites) and as a result of mechanical breakage (when plants come into contact with boat motors and intense wave action). It then forms dense canopies at the surface of the water, which interferes with recreational uses such as boating, fishing and swimming. Dense growth of Eurasian watermilfoil may also obstruct commercial navigation, exacerbate flooding or clog hydropower turbines. In addition, excessive growth of the species may alter aquatic ecosystems by decreasing native plant and animal diversity and abundance and by affecting the predator/prey relationships of fish among littoral plants. A healthy lake is damaged because heavy infestations of Eurasian watermilfoil lower dissolved oxygen under the canopy, increase daily pH shifts, reduce water movement and wave action, increase sedimentation rates and reduce turbidity.
Clean all boat equipment and gear before leaving infested aquatic areas.
Hand harvesting and suction harvesting provide longer term control than mechanical harvesting or raking. Drawdowns, dredging and bottom barriers can reduce or prevent growth of Eurasian watermilfoil by altering the environment.
Mechanical harvesting and raking provide temporary but fair control in bodies of water that are small to moderate in size. Mechanical methods alone do not result in long-term control of Eurasian watermilfoil and should be employed as part of an integrated weed control strategy.
Contact herbicides – including diquat and endothall – provide good control, whereas systemic herbicides such as 2,4–D, florpyrauxifen-benzyl, fluridone and triclopyr provide excellent control. Herbicides should be selected based on site size and conditions, water exchange characteristics, potential water use restrictions, federal, state and local regulations and economic considerations. Reach out to your local UF IFAS Extension for further assistance with management recommendations. Additional management recommendations can be found in Biology and Control of Aquatic Plants Chapter 2.3