L. (Eurasian watermilfoil )
Family: Haloragaceae (Water-Milfoil Family)
Habit: Submerged aquatic
Description: Like many milfoils, Eurasian watermilfoil is a submersed perennial plant with finely dissected feather-like leaves and thin stems. The leaves are arranged in whorls of 4 (rarely 5) around the stem at each node. Each Eurasian watermilfoil leaf generally has 14 or more leaflet pairs and this feature can be used most of the time to distinguish Eurasian watermilfoil from other milfoil species that have whorls of 4 leaves but fewer than 14 leaflet pairs. However, the number of pairs of leaf divisions is highly variable, with younger plants and growing fragments often having fewer than 14. The tips of Eurasian watermilfoil that emerge from the water are often red—especially early in the growing season—
and these reddish spikes stand several inches above the water and submerge when pollination is complete. Flowers are tiny and pink; lower flowers are pistillate and upper flowers are staminate. Seeds are produced, but seedlings are rare in nature.
History: Eurasian watermilfoil may have been introduced to the North American continent at Chesapeake Bay in the early 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, this invasive plant had been found in 33 states, the District of Columbia, and the Canadian provinces of British Columbia, Ontario, and Quebec. Eurasian watermilfoil was first documented in Texas in 1962.
Biology & Spread: Relative to other submersed plants, Eurasian watermilfoil requires high light, has a high photosynthetic rate, and can grow over a broad temperature range. Most regeneration of Eurasian watermilfoil is from rhizomes, fragmented stems, and axillary buds that develop throughout the year. Flower spikes (i.e., emergent tips) often remain above water until pollination is complete, then resubmerge. Although seeds are usually viable, they are not an important means of dispersal. It appears to be primarily spread from waterbody to waterbody through boating activity although anglers have been known to deliberately plant this species in lakes.
Ecological Threat: 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.
US Habitat: Eurasian watermilfoil is an extremely adaptable plant, able to tolerate and even thrive in a variety of 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, 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 can tolerate a range of pH from 5.4-11. Eurasian watermilfoil grows best on fine-textured, inorganic sediments and relatively poorly on highly organic sediments. 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.
Resembles/Alternatives: Yellow nelumbo (Nelumbo lutea), pond weed (Potamogeton nodosus), butterweed (Senecio glabellus) are some alternative plants to consider for the eastern U.S.
Management: Physical removal of Eurasian watermilfoil can be effective in small areas such as boat slips but treatments with herbicides are necessary to control extensive infestations. Care should be taken during physical removal; fragmentation promotes spread. Because Eurasian watermilfoil is prohibited in Texas, a permit may be required for physical removal. A nuisance aquatic vegetation treatment proposal is required for chemical, physical, or biological control of any aquatic plant species on a public water body. Water level control is also effective.
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Last Updated: 2019-08-30 by Monica McGarrity, TPWD