|Eurasian Water Milfoil
( Myriophyllum spicatum )
DESCRIPTION: Plants of Eurasian watermilfoil are rooted and submersed except for a short (3 to 8 cm) emmersed 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: Eurasian water milfloil's stems are thickened below the inflorescence to almost double the width of the lower stems and usually curved to lie parallel with the water surface. Eurasian water milfoil has 2-3 distinct black scales at the inflorescence nodes. Eurasian water milfoil plants never form turions (young shoots budding from underground stems). In contrast, northern water milfoil Stems are not thickened below the inflorescence. They are straight and have 0-2 black or brown, indistinct scales at the inflorescence nodes. Northern water milfoil plants form turions of black green leaves from October to June.
PATHWAYS/HISTORY: Eurasian water milfoil is an aggressive weed that is native to Europe, Asia, and North Africa. Because of morphological similarities and past taxonomic confusion between Eurasian water milfoil and the native, northern water milfoil (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 water milfoil was established in the United States by the 1940’s, while other investigators report that Eurasian water milfoil may have been in the United States since about 1900 or even earlier (Reed 1977).
RISKS/IMPACTS: Eurasian water milfoil 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 water milfoil 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. Spread of Eurasian water milfoil 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 Eurasian water milfoil. Eurasian watermilfoil may "shade out" and out-compete 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 .
MANAGEMENT: 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.
WHAT YOU CAN DO: Always inspect your boat, trailer, and any equipment you use in water. Remove all vegetation before transporting your boat and equipment to new areas.
PROFILE CREDIT: David Webb - IMAGE CREDIT: Linda Hurley, USFWS