Source: California Invasive Plant Council

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Invasive Plants of California's Wildland

Myriophyllum spicatum
Scientific name   Myriophyllum spicatum
Additional name information: L.
Common name Eurasian watermilfoil, spike watermilfoil
Synonymous scientific names Myriophyllum exalbescens
Closely related California natives 3
Closely related California non-natives: 1
Listed CalEPPC List A-1,CDFA nl
By: Carla Bossard

Distinctive features:

Eurasian watermilfoil (Myriophyllum spicatum) is an aquatic plant with branching stems near the water surface, creating dense floating mats. The feather-like leaves grow in whorls of four around the stem, with each leaf divided into nine to twenty-one paired leaflets. This wa-termilfoil is distinguished from the dioecious parrot’s feather (M. aquaticum) by the fact that it is monoecious, does not have flattened midribs, has only submersed foliage, and has longer stems at maturity (Hoffman and Kearns 1997).


Haloragaceae. Perennial, submersed aquatic plant. Stems: branching and leafy, 19.5-28 in (50-70 cm) long, generally >6.5 ft(>2 m) long, reddish or olive-green when dry. Stems thicken below inflorescence and double in width further down, often curving to lie parallel with the water surface. Stem fragments root freely. Leaves: submersed, feather-like leaves in whorls of 4; each leaf 0.6-1.6 in (1.5-4 cm) long, most often with 14-24 pairs of filiform divisions.

Inflorescence: terminal spike, 4-20 cm long and emergent, often pink. Spike erect at anthesis, parallel to water surface at fruit set. Flowers: lower flowers pistillate, upper flowers staminate; flowers verticillate in 4s, whorls 2-ranked, adjacent whorls rotated 45 degrees, occasional hermaphrodite flowers in transition zone. Lower 2-4 whorls of floral bracts usually pictinate and often longer than flowers; upper bracts entire, broader than long and shorter than flowers. Female flowers lack perianth; gynoecium 4-lobed with pink, tufted, recurved stigmas. Male flowers with pink, cauduceous petals; 8 stamens. Fruit: subglobose schizocarp, 0.08-0.13 in (2-3mm) long, 4-sulcate with two rather wrinkled ridges adjacent to lines of dehiscence (from Aiken et al. 1979, Hickman 1993).



Eurasian watermilfoil can be found in freshwater lakes, ponds, and canals with slow-moving waters in northern and central California, particularly in the San Francisco Bay and San Joaquin Valley regions (Anderson 1990) and Lake Tahoe (Goldman, pers. comm.). It is found throughout much of the United States from New England to Florida and westward to California and Washington. Typically, it is rooted at water depths of three to ten feet (1-5 m), but can spread to waters to thirty-three feet (10 m) deep.

This watermilfoil can grow on sandy, silty, or rocky substrates but grows best in fertile, fine-textured, inorganic sediments. It is an opportunistic species that prefers disturbed substrates with much nutrient runoff. High temperatures promote multiple periods of flowering and fragmentation (Hoffman and Kearns 1997).



Eurasian watermilfoil is native to Greenland, North Africa, Europe, and Asia. It was introduced from Europe to the eastern United States in ships’ ballast before 1940 and had spread to California by the 1960s (Couch and Nelson 1985, Aiken et al. 1979).

Eurasian watermilfoil produces some long-viable, often dormant seed, but its spread results primarily from vegetative reproduction. The stems are brittle and fragment easily. These fragments can settle in sediments, producing new plants. Fragments are spread by boats or trailers, in bait buckets, or by floating downstream. They can also be spread by waterfowl and other wildlife (Aiken et al. 1979). Dumping surplus plants from aquatic gardens or aquariums into waterways is another means of spread for this species (Hoffman and Kearns 1997).



Eurasian watermilfoil grows and spreads rapidly, creating dense mats on the water surface. These monotypic mats outcompete native aquatic plants, reducing species diversity. Loss of nutrient-rich native plants reduces food sources for waterfowl, impacts fish spawning grounds, and disrupts predator-prey relationships by fencing out larger fish (Hoffman and Kearns 1997). Infestations can alter aquatic ecosystems by shading out algae in the water column that serve as the basis of the aquatic food web. The mats inhibit recreational use of waterways for boating, fishing, and swimming and cause flooding out of the waterway. Mats also can block irrigation pumps and water intakes and provide optimal habitat for mosquitoes (Aiken 1979, Parsons 1992). Millions of dollars annually are spent in the United States and Canada on efforts to control Eurasian watermilfoil.



Colonization of new sites is usually by fragments. Once established in an aquatic habitat, Eurasian watermilfoil grows rapidly in spring (March-April). Stolons, lower stems, root crowns, and roots persist over the winter in California. In waters where temperatures do not drop below 50 degrees F (10 degrees C) there is little seasonal die-back (Aiken et al. 1979). Root crowns store starch that fuels early takeover of the water column (Grace and Wetzal 1978). During the growing season this plant undergoes auto-fragmentation, with fragments often developing roots before separation from the parent plant. Sloughing of plant parts is common after flowering.

Flowering usually occurs in spring, but some plants flower in fall as well (Pullman 1993). Flowers generally are wind-pollinated. Spikes lie parallel to the water as fruits mature. Fruits float for some hours, allowing for some water dispersal. Fruits do not release the seeds. Seeds exhibit prolonged dormancy, and seedlings, although viable, are rare in nature (Cobble and Vance 1987; Aiken et al. 1979).


Vegetative reproduction is far more important in this species, with fragments being released throughout the growing season (Madsen et al. 1988). Small axillary buds detach from the root crown in late winter and may establish new plants during the growing season (Aiken et al. 1979).

(click on photos to view larger image)



Like other milfoils, Eurasian watermilfoil is difficult to remove from an aquatic system once established, so monitoring and prevention are the most important methods of control. Efforts to make the public aware of the need to remove weed fragments at boat landings and a watershed management program that keeps nutrients from unnaturally enriching aquatic systems, which stimulates this species, are critical to its control (Hoffman and Kearns 1997).


Physical control:

Mechanical methods: Eurasian watermilfoil can be removed by mechanical harvesters. However, native vegetation tends to be removed by this method simultaneously, eliminating beneficial competitors. Also, fragments are usually created, which contributes to dispersal. Harvesters should be used only where colonies are widespread (Hoffman and Kearns 1997, Pullman 1993). Hand pulling, while time-consuming, can be used to control colonies smaller than one acre (0.6 hectare). All fragments, including roots, must be carefully removed. Where possible, anchored bottom screens can be used to prevent new sproutings, but these must be cleaned of sediments once a year (Hoffman and Kearns 1997).

Raising or lowering water levels can inhibit this species. Raising the water level can deprive plants of access to light. This technique can be augmented with light-limiting dyes or shade barriers. Lowering water level can dehydrate plants or, in winter, freeze them to death (Bowen 1995).


Biological control:

There are no USDA approved biocontrol agents for Eurasian watermilfoil. A North American weevil, Eurhychiopsis lecontei, feeds on Eurasian watermilfoil in the midwestern United States. It has been shown to cause extensive damage in some populations, with a 50 percent reduction in biomass and no significant effect on biomass of ten native plant species (Sheldon and Creed 1995). The weevil’s effectiveness for controlling watermilfoil on a large scale is under investigation in twelve Wisconsin lakes (Hoffman and Kearns 1997).

The fungus Mycoleptodiscus terrestris is also being researched as a potential biocontrol agent in Michigan and Massachusetts (Hoffman and Kearns 1997). Grass carp (Ctenopharyngodon idella) used as a biocontrol on other invasive aquatic species find watermilfoil unappealing.


Chemical control:

Herbicide use is more highly regulated in aquatic systems than in terrestrial systems and, because of dilution effects, more difficult to use. The most recent version of herbicide labels will give recommended rates and information about whether the compound is registered for use in a specific situation. Two herbicides effective on Eurasian watermilfoil are not legal for use in aquatic systems in California (diquat and 2,4,D butoxyethenol ester, 20 percent attaclay). Since these herbicides are non-selective, the potential disruption to and contamination of aquatic ecosystems is deemed more problematic than the damage caused by watermilfoil infestation.

Under some conditions, the herbicide fluridone has been approved for lake or pond treatments in California, in low concentrations (10-13 ppb), applied early in the season when water temperatures are low and most native vegetation is not actively growing. Fluridone inhibits photosynthesis and synthesis of important protein compounds in plants susceptible to it, as is watermilfoil. The degree of impact is related to the concentration and the amount of time the plant is in contact with the herbicide (Anderson 1981). Fluridone is taken up by roots and shoots, but appears to be translocated only from root to shoot (Marquis et al. 1981).

Glyphosate and triclopyr were found to be ineffective in control of Eurasian watermilfoil in studies in Washington (Thurston County 1995).