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Alligatorweed
(Alternanthera philoxeroides)
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Photo credit: © Sae Kinoi, https://flic.kr/p/py5x2i
Giant Salvinia
(Salvinia molesta)
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Species at a Glance
Giant salvinia, also known as water fern and kariba-weed, is a floating fern native to southern Brazil. This species is capable of very high growth rates depending on its habitat conditions, potentially doubling in volume every 7-10 days and quickly forming dense mats on the surface of the water.
Identification
Leaves: Because it’s a fern, leaves are referred to as fronds. Two floating fronds appear at each node of the stem with a third frond that dangles under the water. Floating fronds are oval, folded, green, about 2 cm (0.8 in) long, and are covered in 4-pronged hairs on the upper surface that join at their tips to resemble an egg-beater. The purpose of these hairs is to repel water and provide buoyancy. In the early stages of life, the fronds will lie flat on the water’s surface, but as they age they fold up and compress into chains. The underwater frond is brown, highly divided, and is sometimes mistaken for a root; however, the plant has no true roots. It also acts to conceal the spores.
Flowers: Salvinias have no flowers.
Spores: Round, nut-like sporocarps trail beneath the plant and produce infertile spores.
Stems/Roots: A horizontal rhizome, which lies just below the water’s surface, ranges from grass to olive-green and sometimes reddish-brown. Shoots are upturned extensions of the horizontal rhizomes and may reach lengths of up to 6m (20 ft).
Similar Species
Often confused with common salvinia (Salvinia minima), this species is the larger of the two, and can be distinguished by the hairs on the fronds that form the distinct “egg-beater” structure.
Habitat
Giant salvinia thrives in slow-moving, nutrient-rich, warm, fresh waters such as ditches, ponds, lakes, slow rivers, and canals. It will only tolerate freshwater and cannot grow in brackish or marine environments. While it can withstand freezing air temperatures, it will not survive under ice.
Spread
Giant salvinia was most likely introduced by intentional and unintentional releases due to its use in aquariums and water gardens. Once introduced, it can spread by vegetative fragments that hitchhike on recreational boats and equipment and on the feathers or fur of waterfowl and animals. Each small plant fragment is capable of growing into a new plant. In addition, each node has several lateral buds, which can remain dormant through times of stress and drought until conditions are right for growth.
Distribution
Native to southeastern Brazil and northern Argentina, giant salvinia was introduced as an ornamental aquatic plant and was first found in the United States in South Carolina in 1995. It is now scattered throughout the southern U.S. from California to Virginia.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Environmental Impacts
Giant salvinia forms dense mats of vegetation that reduce water flow and lower the light and oxygen conditions in the water. This stagnant dark environment shades out native plants and creates bare spots in the habitat below, altering the diversity and quality of the ecosystem. Invasions of this species also threaten socio-economic activities dependent on open flowing water such as hydro-electricity generation and fishing and boating transport.
Video
Note
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).