Salvinia minima Baker

Common Name: common salvinia, water spangles

Taxonomy: Family-Salviniaceae; Division-Polypodiophyta (Pteridophyta), true ferns.
The name Salvinia rotundifolia Willdenow has been incorrectly applied to S. minima for many years (Jacono et al 2001). There are no synonyms for Salvinia minima. The name S. rotundifolia is no longer in use.

 Identification: Free floating, rootless aquatic fern. Horizontal, branching rhizomes float just below the water surface and produce, at each node, two floating to emergent leaves, and a third, submersed leaf that is dissected into filaments. Floating leaves are orbicular to oval in shape, with heart shaped bases and rounded to notched tips. Leaf length ranges from 0.4 to 2.0 cm. Smaller, orbicular leaves lie flat on the water surface; larger leaves become elongated and fold upright on the midrib. Shade grown leaves remain broadly orbicular, and emerald green. Leaves grown in full sun become larger and elongated and often change from emerald green to rusty brown with maturity and senescence. The upper surfaces of floating leaves are uniformly covered with rows of white, bristly hairs. The stalks of each hair divide into four thin branches that are spreading and free at the tips. The branching hairs create a water repellent shield. Long chestnut colored hair coats the underside of floating leaves, the submersed filaments, buds and the rhizome.

Salvinia minima is believed to be a sterile species. It is not known to produce fertile spores and is postulated to be of hybrid origin (Schneller 1980). Regardless, sporocarps are common among the submersed leaves of large plants.  Sporocarps are sacs, which enclose smaller sacs (sporangia) that are formed to hold microscopic spores. Shaped like small lemons (~1mm wide) sporocarps are attached in spirals along the main axis of the submersed filaments.

Identification Keys: Nauman 1993;Wunderlin 2000; Mickel and Beitel 1988; Stoltze 1983.

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.

Native Range:  Central and South America; common and wide-ranging from southern Mexico to northern Argentina and Brazil (Mickel ad Beitel 1988, Stolze 1983). De la Sota (1976) remarked that, in Argentina, the natural range of Salvinia minima could not be precisely determined due to its frequency in the watergarden and aquarium trade.

Global Range:  Introduced to Bermuda, Puerto Rico, Spain and North America (Weatherby 1937; Proctor 1989; Lawalree 1964; Jacono et al 2001).

Habitat: Shallow backwaters of bayous, lakes and ponds, oxbows, ditches, slow flowing streams, cypress swamps and marshes.

Mode of Introduction: Cultivated in greenhouses and gardens in the United States since the late 1880s (Weatherby 1921, 1937; Fernald 1950). Early plants in Florida likely entered natural areas from flooding of cultivated pools or through intentional release (Jacono et al 2001). Salvinia minima is still widely available in the water garden trade, either as a sale item or a contaminant. Although it continues to infest new regions, it is not included on the Federal Noxious Weed List and is prohibited only in the states of Texas and Louisiana.

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 ride over 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).

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.).

Author: C.C. Jacono
25 Feb. 2003