Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Eurasian Watermilfoil (Myriophyllum spicatum)L.

Eurasian watermilfoil (Fig. 7) was first described by Linnaeus in 1753 from specimens collected in Europe. Its origin is Europe, Asia, and North Africa. The estimated date of its arrival in North America considerably varies from as early as 1814 (Grace and Wetzel 1978) to the late 1800's in the Chesapeake Bay (Holm et al. 1977) to as recently as the early 1940's (Couch and Nelson 1986) in the District of Columbia. These discrepancies may be in part due to the similarity between M. spicatum and M. sibiricum (= M. exalbescens), a native of North America (Smith and Barko 1990). Shortly after Bertholdt (1958) published an article promoting this species for aquarium hobbyists, Eurasian watermilfoil spread quickly across the country. In temperate North America, Eurasian watermilfoil is a serious problem weed in 34 of the United States and in three Canadian provinces. The first infestations in Florida began with deliberate planting by aquarium plant collectors in 1964 in the Crystal and Homosassa rivers (Blackburn and Weldon 1967).

Reproduction of this submersed species is sexual and asexual. It is competitive, particularly in disturbed freshwater. Rapid colonization through fragment dispersal, accelerated growth in spring, and profuse, light-limiting branching and matting at the water surface allow milfoil to dominate or replace native plant species (Smith and Barko 1990). In Florida, however, hydrilla seems to be better adapted to the environment and displaces milfoil, except in brackish waters where Eurasian watermilfoil has a greater salt tolerance (Tiller 1982). Eleven Eurasian watermilfoil populations covered 499 ha in 1990 and ranked 48th in abundance of aquatic plant species. All but one population were in the brackish waters of northern Gulf Coast rivers (Schardt and Schmitz 1990).

Management of this species in Florida is uncommon because Eurasian watermilfoil only occasionally presents problems (Schardt and Schmitz 1990). Grass carp have been used for control of this species in Deer Point Lake, Florida, a potable water reservoir, at great expense to native vegetation because milfoil is not a preferred food by waterfowl. Florschutz (1972) discussed the importance of Eurasian watermilfoil as a food for waterfowl.