Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Paragrass, Californiagrass (Brachiaria mutica; Forsk.) Stapf.

The genus Brachiaria (Fig. 10) contains about 90 species, most of which are in tropical Africa and six are semi-aquatic to aquatic (Cook et al. 1974). Paragrass is an aquatic grass that is present throughout the world's tropics and subtropics where it inhabits wet soils along shorelines and sometimes floating mats (Handley and Ekern 1981). A native of Africa, it has been distributed into both hemispheres as a highly palatable pasture fodder (Vicente-Chandler et al. 1974) but now causes problems in drainage ditches and agricultural fields. Paragrass may have been introduced into Florida in the late 1870's (Austin 1978) as a forage grass (Godfrey and Wooten 1979) and is now common in the southern through central regions. During the 1990 survey, 174 paragrass populations covered 262 ha in public lakes and rivers and ranked 59th in abundance of all recorded aquatic plant species (Schardt and Schmitz 1990).

Paragrass can quickly reach a height of 1.8 m with roots to about the same depth. The hollow, floating stems can extend as far as 6 m across slowly flowing fresh or brackish water (Sainty and Jacobs 1981). Paragrass is an aggressive competitor that can displace most agricultural crops and other shoreline emergent plants. Its competitive advantage is not only its rapid growth and high productivity but its allelopathic nature (Chang-Hung 1977) of producing substances that inhibit growth and germination of other plants. Reproduction is sexual and asexual, although seed production seems to be of lesser importance (Bown et al. 1966). Floating stolons readily become detached and reestablish themselves from roots that develop at the nodes (Sainty and Jacobs 1981).

Paragrass was used in southeastern Florida for camouflage around military installations in World War II (Austin 1978). Its rapid, dense, and monocultural growth has been used in Hawaii to remove nitrogen from domestic effluent (Handley and Ekern 1981).

In Florida, paragrass only occasionally creates problems at access points or other disturbed areas in public lakes and rivers, but it is a serious weed problem in drainage systems. Biological controls are unknown in Florida, and mechanical controls are mowing and excavation. Herbicides are frequently used, and glyphosate provides the most satisfactory results (Schardt and Schmitz 1990).