NRCS−Montana−Technical Note−Invasive Species−MT-23 1

United States Department of Agriculture

NATURAL RESOURCES CONSERVATION SERVICE

Invasive Species Technical Note No. MT-23

February 2009 

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

By 

Jim Jacobs, Plant Materials Specialist, NRCS, Bozeman, Montana 

Jane Mangold, Montana State University Extension Invasive Plant Specialist

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

available from Bugwood.org. 

Abstract

Eurasian watermilfoil is a submersed aquatic forb with branching stems growing two to 21 feet 

(0.5 - 7 m) 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 m) 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. 

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 
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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 at 

http://fwp.mt.gov/fishing/fishingmontana/ans/default.html. 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.

Figure 2. A sketch of the flower and leaf of Eurasian watermilfoil.

PLANT BIOLOGY

Identification

Eurasian watermilfoil is a submersed, aquatic perennial in the Haloragaceae family that roots to

the bottom of water bodies. The roots are slender and fragile. Stems emerge from root crowns, 

are smooth and hairless (glabrous), and grow up to 21 feet (7 m) to the water surface, where they 

branch profusely. Stems have layers of specialized, partially lignified cells that enable the stem 

to self-fragment without mechanical disturbance. Stem fragments are capable of forming new 

plants. Leaves are whorled in groups of four at the stem nodes (see Figure 2), are 0.5 to 1.5 

inches (1.5 to 4 cm) long, and have 14 to 24 pairs of threadlike divisions giving the leaf a 

feather-like appearance. Lower stem leaves continuously die and fall off. Pollen and seed 

flowers are separate on spikes that form at the ends of the stem branch. Spikes grow two to eight 

inches (5 - 20 cm) long and are often pink. They emerge from the water supported by the stem 

that is twice as wide as the lower portion of the stem. At the time of flowering, the spike is erect

but bends at fruit set to be parallel to the water surface. Seed producing flowers lack sepals and 

petals but have a four-lobed pistil. Pollen producing flowers have four pink petals that drop off 

early in development, and eight stamens. Flowers are whorled in groups of four. The female 

flowers (seed producing) are lower on the spike than the male flowers (pollen producing), and 

the flowers in the middle are often bisexual. The fruits are globelike in shape, about 1/8 inch (2-

3 mm) long with four long narrow grooves and four seeds. 
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In Montana there are two native species of watermilfoil that have similar leaf features as 

Eurasian watermilfoil: shortspike watermilfoil (Myriophyllum sibiricum), and whorl-leaf 

watermilfoil (Myriophyllum verticillatum). The easiest way to distinguish the invasive from the 

natives is by lifting a stem out of the water. The leaves of the invasive will relax and fall against 

the stem whereas the leaves of the native will remain rigid and spread from the stem. The

natives can also be distinguished from Eurasian watermilfoil by the sparse stem branching near 

the water surface compared to the abundant branching of Eurasian watermilfoil, by the lack of 

conspicuous change in stem width below the inflorescence compared to the almost doubled stem 

width of Eurasian watermilfoil, and by the lack of the specialized layers of stem cells that 

facilitate stem fragmentation. Eurasian watermilfoil dies back to propagating root crown buds in 

the fall, whereas the natives form prominent cylindrical or cup-shaped perennating shoots 

(turions) attached to, or detached from the parent plant. 

Life History

Although Eurasian watermilfoil produces seeds, most reproduction is asexual from stem 

fragments and root crown buds. Propagating root crowns typically break dormancy in the spring 

when water temperature and light intensity increase. Stem growth is limited by water depth, 

with growth occurring in water up to nine feet deep. However, Eurasian watermilfoil can grow 

in water up to 24 feet deep if it is very clear with high-light penetration. When stems reach the 

water surface, they branch abundantly forming a canopy near the water surface. The dense 

tangled mats can support frogs and wading birds. Lower branches and leaves constantly slough, 

adding nutrients to the water column throughout the growing season. The release of nitrogen and 

phosphorus can be rapid, and can be a significant source of internal nutrient loading to a lake. 

Plants flower mostly in late July to September. The female flowers ripen as the inflorescence 

spikes emerge from the water, well ahead of stamen development, favoring cross pollination. 

Fruits have a stony surface that prevents seed germination. Seed dormancy is prolonged (seven 

years), germination is erratic, and seedlings are considered rare in nature. Growth slows in the 

fall and a protective case over buds (hibernacula) may develop. Eurasian watermilfoil can 

maintain a large biomass throughout the winter which aids in rapid and early seasonal growth. It 

produces phenolic compounds that deter herbivores. 

Stems fragment by natural wave action and by recreational activities such as boating. Differing 

from native milfoil species, Eurasian watermilfoil stems are adapted to self fragment by the 

development of specialized abscission stem cells, which typically occurs after flowering. One 

study found the growth of naturally produced fragments was better than artificially broken stems, 

and the naturally fragmented stems had higher total non-structural carbohydrate which enabled

improved winter survival. Colonization is greatest during late summer in shallow (two feet) 

water on rich organic sediments. Mortality is greatest in deep water with calcareous, nutrient 

poor sediments during early autumn. In lakes in Wisconsin, South Carolina and Tennessee, 

annual biomass production varied from one to five tons per acre. Eurasian watermilfoil is 

adapted to use bicarbonate as a carbon source for photosynthesis, and although it is a C3 plant, it 

has structural adaptations similar to C4 plants that allow more efficient carbon utilization in the 

low carbon, submersed environment. Total non-structural carbohydrate in Eurasian watermilfoil 

may be highest (20%) in October and in plants under ice in the winter, and declines rapidly 

during spring flush to the lowest level (5%) in early summer.

Stem fragments allow for rapid colonization. For example, in New Jersey, researchers cleared a 

one by four meter area on the bottom of a heavily infested lake in mid-July. By early August, 15 
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rooted stems of Eurasian watermilfoil had established and by early September the number of 

stems had increased to 80. The following April, the cleared area was indistinguishable from the 

rest of the infestation, and there were no other plant species present. 

Habitat 

Eurasian watermilfoil can be found on every continent except Antarctica. It is native to Europe, 

Asia, and northern Africa. The details of its introduction to North America are unclear, but it 

may have been introduced near Maryland around 1940, possibly through the aquarium trade. It 

is problematic across the United States and many states have programs in place to prevent its 

establishment and spread. This exotic watermilfoil colonizes rivers, lakes, and other water 

bodies. It grows under a range of trophic conditions, but it is considered an indicator of 

eutrophic (low levels of dissolved oxygen, high levels of organic matter) conditions. It captures 

nutrients from both ambient water and sediments, but mostly from fine sediments containing 10-

25% organic matter. Root anchoring may be impeded by sand, gravel, or flocculent (loose and 

fluffy) substrate textures. Growth is limited by light, preventing colonization of deep waters or 

water with high suspended particles. Optimum water depth for growth ranges from three to 13 

feet (1 to 4 m), and a maximum depth for growth is 39 feet (12 m). Cold temperatures have little 

influence on growth except under reservoir drawdown conditions when plants are exposed to the 

air. Eurasian watermilfoil can use bicarbonate as a source of dissolved inorganic carbon, and 

high growth rates and dominance in hard, alkaline, high pH waters is common. It grows 

vigorously in salinities up to 10 ‰ (10 parts per thousand) and survives at 20 ‰ (20 parts per 

thousand) salinity (the concentration of brackish water). It can tolerate moving water, and water 

currents and wave action facilitate fragmentation. 

Early detection is a priority for management of Eurasian watermilfoil in Montana, and predicting 

where it will colonize can focus surveys to susceptible water bodies. A Wisconsin study 

developed models to predict the likelihood of finding Eurasian watermilfoil based on its presence 

or absence in Wisconsin lakes. Variables associated with dissolved inorganic carbon were the 

most important factors predicting occurrence. These variables included alkalinity, bedrock, and 

forest cover. Lakes with a one percent increase in forest cover in their drainage were five to 50 

times less likely to become infested than other lakes. Variables affecting Eurasian watermilfoil 

growth were better predictors of presence than variables indicating human activities. The 

presence of a public boat launch was the best human activity predictor, followed by the relative 

abundance of walleye and smallmouth bass. Lakes with a public boat launch were 21 to 28 times 

more likely to become infested than lakes without a boat launch. 

Spread

Eurasian watermilfoil disperses primarily by stem fragments. Wind, waterfowl, water flow 

between water bodies, and human related activities are believed to be the main mechanisms of 

fragment dispersal. The most often cited vectors are motor boats and boat trailers. 

Impacts

The dense weed beds formed by Eurasian watermilfoil have adverse effects on native aquatic 

vegetation that are important food sources for waterfowl and some mammals, and habitat for 

fish. The dense beds create habitat for disease carrying insects, including mosquitoes, and 
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parasites that cause swimmer’s itch. The richness, diversity, and distribution of invertebrate 

species on lake bottoms are reduced where infestations occur. The function of water ecosystems 

are altered, including biomass turnover and nutrient cycling. Reduced dissolved oxygen and 

changes in water temperature are associated with infestations. Eurasian watermilfoil is a

nuisance species to humans when it reduces open areas along lake shores, washes up on beaches, 

and curtails recreational activities. Populations reduce water flow thus interfering with 

industrial, agricultural, and municipal water systems. Irrigation ditches, canals, farm ponds, and 

irrigation equipment can be clogged by the weed. Management of Eurasian watermilfoil is 

difficult and expensive. 

MANAGEMENT ALTERNATIVES

Prevention is the most important management option for Eurasian watermilfoil in Montana. 

Mapping, monitoring, early detection and eradication are critical to prevention. If a new 

infestation is found, save a specimen and report the infestation to your county Extension Agent 

or the Montana Department of Fish, Wildlife and Parks at 

http://fwp.mt.gov/fishing/fishingmontana/ans/default.html. Inspection and sanitation of 

recreational equipment will prevent spread. Any aquatic plant debris on boats, trailers, livewells, boat bilges, and fishing equipment should be disposed of away from lakes, ponds and 

rivers. Establishing washing stations with sanitation instructions at water-based recreational sites 

is recommended.

Herbicide1/ 

Herbicidal control of Eurasian watermilfoil requires direct application of chemical to water. In 

Montana, applicators need a 308 Permit from the Montana Department of Environmental Quality 

before applying aquatic herbicides to water at (http://deq.mt.gov/wqinfo/mpdes/pesticides.mcpx).

Eurasian watermilfoil is very sensitive to 2, 4-D. Only formulations for submerged aquatic 

weeds should be used (DMA*4IVM®) and all label requirements must be followed. Consult 

your local Extension Agent for more information on herbicidal control of Eurasian watermilfoil. 

The label rate maximum for 2, 4-D is 2.84 gallons/acre-foot of water and rate of application is 

dependent on water depth. Consult the herbicide label for rates at different water depths. 

Control is best when applied in early spring or early summer. Re-applications may be needed 

and should be applied before mid-August. 

Mechanical Harvest

 

Repeated mechanical harvests have been successful in reducing stem densities of Eurasian 

watermilfoil. However, the risk of spread by stem fragments is high. Using fragment barriers 

around harvest operations and herbicidal control can prevent spread. 

Water drawdown followed by exposure to freezing temperatures for 96 hours will kill plants. 

This method has reduced infestations, however re-infestation can be rapid. 

1/ Any mention of products in this publication does not constitute a recommendation by the NRCS. 

It is a violation of Federal law to use herbicides in a manner inconsistent with their labeling.

Biological Control 
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The watermilfoil moth (Acentria ephemerella = Acentia nevea), native to Europe, is associated 

with plant declines. The moth was first introduced into the United States in 1927 and it is 

established in eight eastern and Midwestern states. The larvae feed in and on the stems and 

leaves, and along with case-making, cause the leaves and stems to fall off the plant. Availability 

is currently limited. There is also a weevil native to North America, Euhrychiopsis lecontei that 

is associated with plant declines. 

References

Aiken, S.G., P.R. Newroth, and I. Wile. 1979. The biology of Canadian weeds. 34. 

Myriophyllum spicatum L. Canadian Journal of Plant Science 59: 201-215. 

Buchan, L.A.J., and D.K. Padilla. 2000. Predicting the likelihood of Eurasian watermilfoil 

presence in lakes, a macrophyte monitoring tool. Ecological Applications 10 (5): 1442-1455. 

Eiswerth, M.E., S.G. Donaldson, and W.S. Johnson. 2000. Potential environmental impacts and 

economic damages of Eurasian watermilfoil (Myriophyllum spicatum) in western Nevada and 

Northeastern California. Weed Technology 14: 511-518. 

Nichols, SA.. and B.H. Shaw. 1986. Ecological life histories of the three aquatic nuisance 

plants Myriophyllum spicatum, Potamogenton crispus, and Elodea Canadensis. Hydrobiologia 

131: 3-21. 

Piper, G.L., E.M. Coombs, B. Bloosey, P.B. McEvoy, and S.S. Schooler. 2004. Eurasian 

watermilfoil. In: Coombs, E. M., Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. 

(eds) Biological control of invasive plants in the United States. Oregon State University Press, 

Corvallis. 169-173.