Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Hygrophila, Hygro (Hygrophila polysperma; Nees) T. Anderson.

Hygrophila (Fig. 13) of India (Rataj and Horeman 1977) and of the East Indies (Les and Wunderlin 1981) was imported into Ohio near the end of World War II by an aquarium nursery (Reams 1953). Hygrophila was imported into Florida during the early 1950's, and several waterways were deliberately stocked by aquarium plant collectors. It was collected in Lee County in 1979 (Les and Wunderlin 1981). Hygrophila is still a popular and widely distributed aquarium plant, but, because of its potential as a weed, the Department of Natural Resources prohibited its sale in Florida. Hygrophila was first noticed in the wild near Tampa in 1965 but was not correctly identified until 1977 (Les and Wunderlin 1981). Its similar appearance to alligatorweed may have delayed its proper identification. Hygrophila is now in many flood-control canals of southern Florida and was in 12 rivers and public lakes in 1990 where it covered 30 ha and ranked 87th in abundance of aquatic plant species (Schardt and Schmitz 1990).

Hygrophila has terrestrial and aquatic growth forms that survive in shallow acidic waters to wet soils. Reproduction is sexual and asexual, although seed production seems to be rare in Florida (Spencer and Bowes 1985). Because much of its biomass is concentrated at or near the water surface, this species can be a problematic weed. Unlike hydrilla, establishment of this species in deeper-than-2-m water is very slow. Hygrophila seems to grow more robustly in enriched waters (Schmitz and Nall 1984) and in flowing waters (Van Dijk et al. 1986).

Control of hygrophila is difficult. Harvesters fragment plants and increase distribution, and grass carp have a low preference for hygrophila. Registered herbicides including diquat, endothal, and fluridone, provide marginal control. Hygrophila's resistance to herbicides and biological controls provides a competitive advantage over hydrilla that can be controlled by several methods. As hydrilla is brought under statewide control, hygrophila may emerge as a greater pest in Florida waters. More than $250,000 in federal and state funds were spent on the control of hygrophila in flood-control canal systems during the 1980's (Schmitz et al. 1991).