APIS

Synonyms: NONE

Family: Haloragaceae

Home Range/U.S. Introduction:
Myriophyllum spicatum L. is an aggressive weed that is native to Europe, Asia, and North Africa. Because of morphological similarities and past taxonomic confusion between Eurasian watermilfoil and the native, northern watermilfoil (Myriophyllum sibiricum Komarov), it is difficult to determine the exact time of introduction. A study of herbarium specimens by Couch and Nelson (1985) indicate Eurasian watermilfoil was established in the United States by the 1940’s, while other investigators report that Eurasian watermilfoil may have been in the United States since about 1900 or even earlier (Reed 1977).

U.S. Range Map:

Species Description:
Plants of Eurasian watermilfoil are rooted and submersed except for a short (3 to 8 cm) emersed flowering spike. Primary stems are generally branched and often form a dense canopy on the water’s surface. Leaves are whorled, 4 or rarely 5 leaves per node, each leaf pinnately dissected into narrow, linear segments. The number of pairs of leaf segments is highly variable, ranging from 5 to 24 for each leaf. Leaves cling to the stem above each node when removed from the water. Turions are absent. The flowers are whorled and in spikes with the pistillate flowers at the lower nodes of the spike and staminate flowers at the upper nodes. The stem below the flowering spike is curved to lie parallel with the water surface and is about twice the diameter of the lower stem. Floral bracts subtending the pistillate flowers are equal or slightly longer than the flowers.

The following set of characters are used by Aiken (1981) in distinguishing Eurasian watermilfoil from northern watermilfoil:

Stem thickened below the inflorescence to almost double the width of the lower stem, usually curved to lie parallel with the water surface; scales at the inflorescence nodes 2-3, black, distinct in fresh material; plants never forming turions------Eurasian watermilfoil

Stem not thickened below the inflorescence, straight; scales at the inflorescence nodes 0-2, black or brown, indistinct; plants forming turions of black green leaves from October to June-------Northern watermilfoil

Habitat/Growth Characteristics:
Eurasian watermilfoil is a highly invasive and aggressive species that colonizes reservoirs, lakes, ponds, streams, small rivers and brackish waters of estuaries and bays. As stems of Eurasian watermilfoil near the water surface, they branch profusely and often form a dense canopy that reduces light availability for "understory" species. Myriophyllum spicatum dies back to propagating root crowns during the winter months and does not form turions as does M. sibiricum. Spread of Eurasian watermilfoil is primarily by asexual means. Long range dispersal is primarily by fragmentation that results from mechanical breakage or autofragmentation which occurs after flowering and at the end of the growing season. Fragments produced by either method may be transported over long distances by water currents. Fragments may also be transported from one water body to another when fragments become attached to boat trailers. Once established, individual plants may expand for distances of a few meters by the production of stolons. Although Eurasian watermilfoil produces large quantities of viable seed, very few seedlings have been observed in field situations, and seed are considered to be of minor importance in dispersal of M. spicatum (Smith and Barko 1990).

Problems:
Eurasian watermilfoil may "shade out" and outcompete desirable native species and form monospecific colonies over large areas of some water bodies. Dense mats and colonies of M. spicatum can restrict swimming, boating, bank fishing, and negatively impact aesthetic appeal. Fragments and floating mats may clog water intakes at power generation facilities and potable water intakes. Dense stands of Eurasian watermilfoil provide habitat for mosquitoes and may increase populations of some species of mosquitoes (Aiken et al. 1979, Smith and Barko 1990). Because of the problems caused by Eurasian watermilfoil, large-scale management programs have been implemented by the Tennessee Valley Authority, the U.S. Army Corps of Engineers, and governmental agencies in Canada.

References:
Aiken, S. G. 1981. A conspectus of Myriophyllum (Haloragaceae) in North America. Brittonia 33: 57-69.

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.

Couch, R. and E. Nelson. 1985. Myriophyllum spicatum in North America. In: L. W. J. Anderson (ed.), Proceedings of the First International Symposium on watermilfoil (
Myriophyllum spicatum) and related Haloragaceae species. Aquatic Plant Management Society, Washington, D.C. pp. 8-18.

Reed, C. F. 1977. History and distribution of Eurasian watermilfoil in United States and Canada. Phytologia 36: 417-436.

Smith, C. S. and J. W. Barko. 1990. Ecology of Eurasian watermilfoil. Journal of Aquatic Plant Management 28: 55-64.