Photographer: Alison Fox Affiliation: University of Florida Source: Bugwood.org Copyright: CC BY 3.0 US
Like many milfoils, Eurasian watermilfoil is a submersed perennial plant with finely dissected feather-like leaves. The leaves are arranged in whorls of 4 (rarely 5) around the stem at each node. Each Eurasian watermilfoil leaf generally has 12 or more leaflet pairs and this feature can be used about 70 percent of the time to distinguish Eurasian watermilfoil from other milfoil species. However, the number of pairs of leaf divisions are very variable and can range from 5 to 24. Young plants and free floating plant fragments often develop leaves with fewer than 14 divisions. The growing stem tips of Eurasian watermilfoil (and other milfoil species) are tassel-like and often red; especially early in the growing season. Tiny pinkish flowers occur on reddish spikes that stand several inches above the water and submerge when pollination is complete. The stem width of Eurasian watermilfoil almost doubles below the inflorescence. Lower flowers are pistillate, upper flowers staminate. Seeds are produced, but seedings are rare in nature. In situations where water evaporates slowly and the plants gradually become stranded, Eurasian watermilfoil can develop into a land form. The leaves of the land form are smaller, stiffer, and have fewer divisions. If such plants are submerged, new growth with aquatic leaves develops in 7-10 days, but the first leaves formed have relatively few divisions and only later does the number of divisions increases to more than 12 leaflet pairs.
Eurasian milfoil can form large, floating mats of vegetation on the surface of lakes, rivers, and other water bodies, preventing light penetration for native aquatic plants and impeding water traffic. The plant thrives in areas that have been subjected to various kinds of natural and manmade disturbance.
Most regeneration of Eurasian watermilfoil is from rhizomes, fragmented stems, and axillary buds that develop throughout the year. Flower spikes often remain above water until pollination is complete, then re-submerge. Although seeds are usually viable, they are not an important means of dispersal.
Eurasian watermilfoil may have been introduced to the North American continent at Chesapeake Bay in the 1880s, although Couch and Nelson present evidence that the first collection of Eurasian watermilfoil was made from a pond in the District of Columbia during the fall of 1942. By 1985, Eurasian watermilfoil had been found in 33 states, the District of Columbia, and the Canadian provinces of British Columbia, Ontario, and Quebec. The first known record of Eurasian watermilfoil in Washington is from a 1965 herbarium specimen collected from Lake Meridian, King County. However, state officials first became aware of Eurasian watermilfoil as a problem plant in 1974 when Eurasian watermilfoil moved downstream from the Canadian Okanogan Lake Chain into Lake Osoyoos, despite government efforts to halt its downstream spread. From Osoyoos, Eurasian watermilfoil moved downstream into the Okanogan River and the Columbia River. It was also introduced into the Pend Oreille River and by 1995, Eurasian watermilfoil is found in lakes near these rivers. In western Washington Eurasian watermilfoil was found in Lake Washington in 1974 and from there Eurasian watermilfoil has spread along the Interstate 5 corridor into many western Washington lakes.
Europe, North Asia, North Africa
U.S. Habitat: Eurasian watermilfoil is an extremely adaptable plant, able to tolerate and even thrive in a variety of environmental conditions. It grows in still to flowing waters, can tolerate salinities of up to 15 parts per thousand (half the salinity of Puget Sound), grows rooted in water depths from 1 to 10 meters (regularly reaching the surface while growing in water 3 to 5 meters deep), and can survive under ice. It is able to tolerate pHs from 5.4-11. Relative to other submersed plants, Eurasian watermilfoil requires high light, has a high photosynthetic rate, and can grow over a broad temperature range. Eurasian watermilfoil grows best on fine-textured, inorganic sediments and relatively poorly on highly organic sediments.
U.S. Present: All states except Hawaii, Rhode Island and Wyoming.
Texas Native Alternatives: White water lily (Nymphaea odorata), Floating bladderwort (Utricularia radiata), American eelgrass (Vallisneria americana).
Aquatic herbicides such as diquat, complexed copper, and endothall dipotassium salt are effective, while fluridone is selective. Water level control is also effective. Care should be taken when mechanically harvesting; fragmentation promotes spread.
USE PESTICIDES WISELY: Always read the entire pesticide label carefully, follow all mixing and application instructions and wear all recommended personal protective gear and clothing.
Contributions from Texas Invasives for this species page are greatly appreciated.
B.C. Ministry of Environment 1989. Eurasian Water Milfoil in British Columbia (Pamphlet).
Gleason, H.A., A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. The New York Botanical Garden, 910.
APWG WeedUS Database