Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Torpedograss, Bullettgrass, Quackgrass
(Panicum repens)L.

Torpedograss (Fig. 3) was first described by Linnaeus in Europe in 1762 (Tarver 1979). It is now found in Asia, Africa, Australia, and in South and North America. It was first collected in North America near Mobile, Alabama, in 1876 (Hodges and Jones 1950). The first report of torpedograss in Florida was in the wet prairies of the lower Kissimmee Valley in the early 1920's (Kretchman 1962). By 1926, seeds of this species were also imported and distributed by the U. S. Department of Agriculture for planting in cattle pastures throughout the southern states (Tarver 1979). Torpedograss was planted in every southern Florida county and in several central and northern locations (Hodges and Jones 1950) because it was well suited to wet prairies on which cattle are grazed in those areas. Ironically, it was later determined that torpedograss is not a good cattle feed. Other grasses are equal or better feed and do not have the potential of creating serious weed problems. Torpedograss also frequently spreads into adjoining lands (Hodges and Jones 1950). This invasive grass can grow in water that is as deep as 1.8 m and in habitats from wetlands to dry fields.

Torpedograss was found most often in the southern part of the state where it was the sixth most abundant aquatic plant during surveys of plants in public lakes and navigable rivers in 1990 (Schardt and Schmitz 1990). It was reported in 281 sites and covered nearly 5,062 ha. Reproduction is primarily by vegetative means; seed production is not known in the United States (Wilcut et al. 1988). Rhizomes, which can extend several meters from the parent plant (Holm et al. 1977), are not vulnerable to mechanical or herbicidal control. Cutting rhizomes and stems produces numerous small pieces, each of which can produce a new plant (Chandrasena and Peiris 1989).

Torpedograss is of little value to fisheries and is seldom used by waterfowl or songbirds (Tarver et al. 1986). It quickly invades disturbed areas and may overwhelm and replace native aquatic and terrestrial plants. Plant and animal diversities are therefore reduced. Torpedograss is now considered a serious problem along the lower coastal plain in Alabama, in Mississippi, and in a large part of Florida (Wilcut et al. 1988). The above-ground portions of torpedograss can be controlled with herbicides (Peng and Twu 1979); however, herbicides are not easily translocated to the starch-laden rhizomes. Torpedograss can thus withstand herbicides; it regrows after the lethal effects dissipate. Biological controls of torpedograss are not known at this time. Aquatic growths of torpedograss in flood-control systems are most often managed with the herbicide glyphosate. As many as 4,860 ha may be controlled at an annual cost of more than two million dollars in Florida. During 1990, approximately 40.5 ha of torpedograss were controlled in navigable rivers and in public lakes for a cost of about $20,000 (Schardt and Schmitz 1990).