Ecology and Management of Eurasian Watermilfoil (Myriophyllum spicatum L.)

Invasive Species Technical Note Number MT-23

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Invasive Species Technical Note Number MT-23 (PDF; 117 KB)

By Jim Jacobs, Plant Materials Specialist, NRCS, Bozeman, Montana
Jane Mangold, Montana State University Extension Invasive Plant Specialist

Abstract

Eurasian watermilfoil is a submersed aquatic forb with branching stems growing two to 21 feet (0.5 - 7 meters) long and supporting whorls of featherlike leaves (see Figure 1). The pink inflorescence is a terminal spike two to eight inches long. Watermilfoil reproduces by seed and vegetative fragments, but colonization of new sites is mainly by vegetative fragments. It flourishes in eutrophic (low levels of dissolved oxygen, high levels of organic matter) lakes and waterways. Plants commonly grow in water three to 12 feet (0.5 - 3 meters) deep and form tangled weed beds that can competitively displace submerged plant communities in two to three years. Dense beds reduce recreational qualities of water bodies, reduce water flow, create favorable habitat for mosquitoes, and clog industrial, agricultural, and drinking water supplies. Eurasian watermilfoil was first reported in Montana in Noxon Reservoir (Sanders County) in 2007. It is a Category 3 noxious weed. Water recreationalists are the predominant vector of long distance spread of Eurasian watermilfoil. However, waterfowl can also carry vegetative fragments from one lake to another.

Figure 1. Eurasian watermilfoil stems and leaves. Photo by Alison Fox, University of Florida, available from Bugwood.org.

Early detection and immediate action to contain and eradicate infestations are the most important management actions for Eurasian watermilfoil in Montana. If you find a new infestation, you should save a specimen and report the infestation as soon as possible to your county Extension Agent or the Montana Department of Fish, Wildlife and Parks. It is highly susceptible to 2, 4-D. Repeated mechanical harvests may reduce stem densities. In reservoirs, water drawdowns during cold weather have reduced infestations. Larvae of the European moth Acentria ephemerella, and the native weevil Euhrychiopsis lecontei, are associated with plant declines.

Plant Biology

• Identification
• Life History
• Habitat, Spread and Impacts

Management Alternatives

References

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Last Modified: 11/04/2011