Cabomba caroliniana - Fanwort - A Probem Aquatic Plant in the Western USA

The Western Aquatic Plant Management Society

Description and Variation

Fanwort is a submersed, floating (often rooted), perennial, with short, fragile rhizomes. Erect shoots are upturned extensions of horizontal rhizomes. Shoots are grass green to olive green or sometimes reddish brown. Leaves are of two types: submersed and floating. Submersed leaves are finely divided and arranged in pairs on the stem. Floating leaves are linear and inconspicuous, with an alternate arrangement. Floating leaves are long (less than 1/2 inch) and narrow (less than 1/4 inch), and the leaf blade attaches to the center, where there is a slight constriction. Flowers are white and small (less than 1/2 inch in diameter), and they float on the water surface (Gibbons 1993; Radford 1968; Orgaard 1991).

Cabomba is a small genus of aquatic plants originating in the neotropics and adjoining warmer temperate zones. There is a great deal of vegetative similarity among the taxa, making the genus taxonomically difficult (Orgaard 1991).

Economic Importance

Fanwort is an extremely persistent and competitive plant. Under suitable environmental conditions, it can form dense stands, crowding out previously well-established plants (Riemer and Inicki 1968). Once established, this plant can clog drainage canals and freshwater streams, interfering with recreational, agricultural, and aesthetic uses. In Cullaby Lake of northwest Oregon, it has created severe nuisance conditions (Gibbons et al. 1993).

Cabomba caroliniana is commonly used as an aquarium plant because of its delicate appearance. Large numbers of plants are sent from Florida to the rest of the U.S. for commercial use. Fanwort is also grown commercially in Asia for export to Europe and other parts of the world. Small-scale, local cultivation occurs in some areas. (Orgaard 1991). In its native habitat, fanwort is eaten by waterfowl and some fish. In addition, it provides cover for some small fish and plankton (Mitchell 1979 c.f. Orgaard 1991).

Geographic Distribution

Cabomba caroliniana is native to the subtropic-temperate regions of eastern North and South America. C. caroliniana var. caroliniana is common in the southeastern United States, occurring from Texas to Florida, north to Massachusetts, and west to Kansas, although it is reportedly adventive northeast of Virginia (Fernald 1987). This variety also occurs in South America, including southern Brazil, Paraguay, Uruguay, and northeastern Argentina. The variety has been introduced to other areas, including Malaysia, India, Japan, and New Guinea, via discarded aquarium plants (Orgaard 1991).

Habitat

The species grows rooted in the mud of stagnant to slow flowing water, including streams, smaller rivers, lakes, ponds, sloughs, and ditches. In Texas, associated plants include Nymphaea odorata and Brasenia schreberi (Orgaard 1991). There is no direct evidence of how or when fanwort was introduced to Washington and Oregon. The species was introduced to other parts of the world via discarded or deliberately scattered aquarium plants (Orgaard 1991), and this was the likely method of introduction in the Pacific Northwest, as well. Cabomba is readily available in pet stores for use as an aquarium plant.

Growth, Development, and Reproduction

Fanwort is an herbaceous perennial that spreads primarily by stem fragments or rhizomes. Erect shoots are upturned extensions of horizontal rhizomes. The species forms large clones as new rhizomes and floating shoots arise as axillary branches. Rhizomes are fragile and easily broken, facilitating vegetative spread and transport to new water bodies (Orgaard 1991).

Plants flower from May to September (Radford et al. 1968). In the southeastern U.S., fanwort is self-pollinating and seeds readily germinate. Seeds collected from New Jersey failed to germinate, and no seedlings have been observed in the field; therefore, reproduction by seed in New Jersey is of little or no importance (Riemer and Ilnicki 1968). No information is available on seed production and viability in the Pacific Northwest (Gibbons 1993).

Response to Herbicides

According to Westerdahl and Getsinger (1988), Endothall provides excellent control, but it is a contact herbicide only. Fluridone also provides good control. However, some reports indicate that fanwort is less sensitive to herbicides than other aquatic plants (Gibbons 1993).

Response to Cultural Methods

In the southern United States, drawdowns have been used to reduce fanwort growth. Extreme drying is required to prevent regrowth from seed (Gibbons et al. 1993).

Biocontrol Potential

Grass carp will eat fanwort, but it is not a preferred food (Gibbons et al. 1993). No research is currently underway on other biocontrol agents.

References

Buckingham, G.R. and C.A. Bennett. 1989. Laboratory host range of Parapynx diminutalis (Lepidoptera: Pyralidae), an Asian aquatic moth adventive in Florida and Panama on Hydrilla verticillata (Hydrocharitaceae). Environmental Entomology 18:526-530.

Fassett, N.C. 1953. A monograph of Cabomba. Castanea 13:116-128.

Fernald, M.L. 1987. Gray's Manual of Botany. Dioscorides Press, Portland, Oregon.

Gibbons, M.V., H.L. Gibbons, Jr., and M.D. Sytsma. 1994. A Citizen's Manual for Developing Integrated Aquatic Vegetation Management Plans. Washington State Department of Ecology, Olympia.

Gleason, H.A. and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York.

Hotchkiss, N. 1972. Common Marsh, Underwater and Floating-leaved Plants of the United States and Canada. Dover Publications, Inc., New York.

Orgaard, M. 1991. The genus Cabomba (Cabombaceae) - a taxonomic study. Nordic Journal of Botany 11:179-203.

Radford, A.E., H.E. Ahles, and C.R. Bell. 1968. Vascular Flora of the Carolinas. The University of North Carolina Press, Chapel Hill.

Riemer, D.N. and R.D. Ilnicki. 1968. Overwintering of Cabomba in New Jersey. Weed Science 16:101-102.

Westerdahl, H.E. and K.D. Getsinger. 1988. Aquatic Plant Identification and Herbicide Use Guide. Volume II: Aquatic Plants and Susceptibility to Herbicides. Technical report A-88-9. U.S. Army Corps of Engineers, Washington, D.C.