Ecology: Reproduction and spread: The continuous branching and fragmentation of rhizomes turns out large volumes of vegetative daughter plants throughout the growing season. Copious hairy coverings minimize the desiccation of plants spotted on boats, trailers, alligators, turtles, and even dogs leaving the water. Lateral buds deeply imbedded in the rhizome, may lie dormant during periods of reduced moisture and cold temperature. Small rhizome fragments, commonly sheltered in associating vegetation, provide material for reintroduction on the return of favorable growing conditions.
Habitat: Shallow backwaters of bayous, lakes and ponds, oxbows, ditches, slow flowing streams, cypress swamps, and marshes (Lellinger 1985, Nauman 1993, Jacono et al. 2001). Like Salvinia molesta, S. minima is vulnerable to conditions of salinity. Biologists along the coast of southeastern Texas find Salvinia minima in their coastal study sites only during wintertime, when freshwater outflow is high and salinity measurements decline to 4 – 7 ppt. They regularly control Salvinia minima, and improve waterfowl habitat, by opening gates to allow saline water from the Gulf of Mexico into the bayous (Kirk Blood, Texas Parks and Wildlife Department, Port Arthur, Texas, pers. comm.). During August, on the Waterhole Branch of the Fish River, Alabama, Salvinia minima was registered as growing well with surface water salinity levels at 4 –5 ppt. (Scott Phipps, Weeks Bay National Estuarine Research Reserve, AL, pers. comm.).
Impact of Introduction: During earlier stages of colonization Salvinia minima demonstrates exponential growth rates (Gaudet, 1973), which may be just as high as those of Salvinia molesta.
In Texas and Louisiana, S. minima typically occurs in dense, expansive populations and is known as a very troublesome weed. At Lacassine Bayou, southwestern Louisiana, plants completely blanket a waterway measuring 19.3 km long and 110 m wide (Jacono et al 2001). Mats in Louisiana have been measured as thick as 20 - 25 cm (Montz 1989).
Clatworthy and Harper (1962) studied the competition among three species of duckweed, Spirodela polyrrhiza, Lemna gibba, Lemna minor and, the single temperate species of Salvinia, S. natans. In mixed cultures, they found that Lemna gibba and Salvinia natans were able to actually thrust aside Spirodela polyrhiza and Lemna minor. On the other hand, Lemna minor and Spirodela polyrrhiza coexisted without dominating each other. The authors correlated success in competition not with growth rate in pure culture, but rather with morphological characteristics. The presence of aerenchyma in Lemna gibba and the strong connecting rhizome between the fronds in Salvinia, as well as the stiff hairs of Salvinia, enabled these two species to overtop and displace the thinner, flat fronds of the others (reviewed in Landolt 1986). It should be noted that Salvinia natans is smaller and more delicate than S. minima.
An eight-year study at Jean Lafitte National Historic Park, Louisiana, found complete displacement of native Lemna species by Salvinia minima. (T. Doyle, LA, pers. comm.). The Lemnaceae (duckweeds) contain high protein content and are important food sources for waterfowl.
An investigation of competition among Salvinia minima, Spirodela [Landoltia] punctata (G.F.W. Mey.) C.H. Thompson and Azolla caroliniana Willdenow in north Florida found Salvinia minima dominating during the summer months (Dickinson and Miller 1998). Later in the season, S. minima was impacted by flooding and freezing and Spirodela punctata became the most abundant species (Dickinson and Miller 1998). Also introduced to North America, Spirodela punctata shows greater cold tolerance than Salvinia minima by extending to more northern temperate latitudes (Landolt 1986).
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