Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Alligatorweed (Altenanthera philoxeroides (Mart.) Griseb.)

The first known description of alligatorweed from plants obtained in Brazil (Fig. 5) was published in 1926 (Zeiger 1967). This emersed, rooted species seemingly entered the United States before 1987 as an accidental release from the ballast of sailing ships in Mobile, Alabama (Zieger 1967). Schardt and Schmitz (1990) recorded an unsubstantiated report of alligatorweed in Florida in 1894 (Weldon 1960).

Although alligatorweed was recognized for its potential threat to the waterways of the United States as early as 1901, its weedy nature was not realized until the advent of organic herbicides in 1945 (Coulson 1977). When reductions of the competition from the more aggressive floating waterhyacinth was initiated, herbicide-resistant alligatorweed quickly spread into drainage canals and along shorelines of rivers and lakes. More than 1,215 ha of alligatorweed were in public waters by the mid-1960's (Zieger 1967). This invasive plant grows primarily as an emersed aquatic plant but also can thrive in wet or dry soils. The hollow stems grow to 15 m and allow plants to form dense floating mats that extend far into water bodies. These mats reduce or eliminate native plants, are impenetrable to motor boats, and restrict water movement. Alligatorweed was in 243 public waters in 1990, covered 510 ha of water, and ranked 46th in abundance of all aquatic plant species (Schardt and Schmitz 1990).