Giant Salvinia
Salvinia molesta
NEMBA category
English: Giant Salvinia, Kariba Weed, Aquarium Watermoss
Afrikaans: Karibawier, Salvinia
Salvinia molesta, commonly known as Salvinia or Giant Salvinia, is a free-floating, aquatic fern that forms dense mats on the surface of still or slow-moving freshwater bodies. It has two distinctive oval-shaped leaves that float on the water, covered in stiff, bristly hairs that resemble tiny egg-beaters, helping the plant repel water. A third, submerged, root-like leaf hangs down into the water. It doesn't produce traditional flowers or seeds.
This plant is a notorious alien invasive species in South Africa, capable of incredibly rapid growth, completely covering water bodies, blocking out sunlight, depleting oxygen, and suffocating aquatic life. It's considered one of the world's worst aquatic weeds.
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Origin:
Native to southeastern Brazil.
Distinguishing features:
Free-floating aquatic plant: It lives entirely on the water's surface, not rooted in the soil.
Unique 'egg-beater' hairs: The upper surface of its floating leaves is covered with stiff, erect, bristly hairs fused at their tips to form cage-like structures, resembling tiny egg-beaters, which repel water.
Two floating leaves, one submerged root-like leaf: Each node on the stem typically has a whorl of three leaves: two rounded or oval leaves floating on the surface, and one highly modified, filamentous, root-like leaf hanging submerged in the water.
Rapid mat formation: Grows incredibly fast, doubling its biomass in just a few days under ideal conditions, quickly forming thick, dense mats that completely cover the water surface.
No true roots: The plant does not have true roots; the submerged leaf performs a root-like function.
Overall appearance:
A low-lying, free-floating aquatic fern that forms dense, green to brownish mats on the water surface. Its leaves are distinctly covered in tiny, bristly hairs.
Height:
As a floating plant, its height above the water is typically only a few centimetres (2-10 cm), but the submerged leaf can hang down up to 25-30 cm.
Width:
Forms sprawling, interlocking mats that can extend across entire water bodies, covering many square metres. Individual plants can spread horizontally as they grow and branch.
Leaves:
Typically arranged in whorls of three at each node along the stem. Two leaves are oval to broadly elliptical, 1 to 4 centimetres long, floating on the surface. Their upper surface is covered with characteristic stiff, bristly, 'egg-beater' shaped hairs that are not wettable. The third leaf is finely dissected and submerged, acting like a root. Young plants have flat leaves, which become more folded or cupped as the mat thickens.
Flowers:
Not applicable, as Salvinia molesta is a fern and does not produce flowers. It reproduces by spores, but primarily through vegetative fragmentation.
Flowering season (South Africa):
Not applicable.
Fruits:
Not applicable, as it is a fern.
Seeds:
Not applicable, as it is a fern. It produces spores, but these are often sterile or not a major means of reproduction.
Bark:
Not applicable, as it is an aquatic fern.
Thorns:
This species does not have thorns.
Wood:
Not applicable, as it is an aquatic fern.
Roots:
Does not have true roots. The submerged leaf acts as a root-like structure, absorbing nutrients.
Soil type:
Not applicable, as it is a free-floating aquatic plant. It thrives in nutrient-rich freshwater, often preferring still or slow-moving water bodies.
Aspect:
Prefers full sun but can tolerate partial shade. It thrives in warm, tropical to subtropical freshwater environments.
Edibility:
Not considered edible.
Toxicity:
Not known to be toxic to humans or animals upon ingestion, but its presence severely harms the aquatic environment.
Uses:
Salvinia molesta has no known beneficial uses. It is generally considered a harmful weed. It is sometimes illegally sold as an ornamental plant for aquariums or ponds, which contributes to its spread.
Ecology:
As an alien invasive plant in South Africa, Salvinia molesta is one of the most destructive aquatic weeds. It rapidly covers the surface of still or slow-moving freshwater bodies (dams, rivers, canals, ponds, wetlands), forming thick, dense, interlocking mats. These mats block out sunlight, preventing photosynthesis by submerged plants, which then die. This leads to a severe depletion of oxygen in the water, causing fish, insects, and other aquatic animals to die. The dense mats also trap sediment, alter water flow, and obstruct water-based activities like irrigation, recreation, and fishing. Its primary mode of reproduction is vegetative fragmentation: tiny pieces of the plant can break off and grow into new plants, allowing it to spread incredibly rapidly. It also spreads via contaminated boats, fishing gear, or even by wind.
Distribution and habitat:
Widespread and invasive throughout many freshwater systems in South Africa, particularly in the warmer, eastern and northern provinces. It is a major problem in KwaZulu-Natal, Mpumalanga, Limpopo, Eastern Cape (found in dams and slow-moving rivers around Kenton-on-Sea), and parts of Western Cape and Free State. It invades dams, lakes, rivers, canals, irrigation ditches, ponds, and wetlands.
Derivation of name:
The genus name Salvinia honours Antonio Maria Salvini (1653–1729), an Italian professor of Greek and botanist.
The species epithet molesta is Latin for "troublesome" or "annoying," a very fitting description for this highly invasive weed due to its problematic nature.
"Giant Salvinia" and "Kariba Weed" refer to its large size and its notorious infestation of Lake Kariba.
Historical aspects:
Salvinia molesta was accidentally introduced to South Africa (and other parts of the world) as an ornamental plant for ponds and aquariums, likely in the mid-20th century. Its extremely rapid growth rate and ability to reproduce vegetatively from tiny fragments meant it quickly escaped cultivation. It famously caused a catastrophic infestation of Lake Kariba in the 1950s, leading to its common name "Kariba Weed," highlighting its devastating impact on major water bodies. It continues to be a severe environmental and economic threat to freshwater resources across the country.
Interesting facts:
This plant has a super cool secret weapon: its leaves are covered in tiny, water-repellent hairs shaped like mini 'egg-beaters' – so water just beads right off them!
It's a real speed demon! Under the right conditions, this plant can double its size in just a few days, literally covering entire dams and rivers with a thick green carpet!
Even though it looks like it has roots, it actually doesn't! The stringy bits hanging underwater are just modified leaves that act like roots.
This plant is so notorious for taking over water bodies that it's sometimes called 'Kariba Weed' after it caused a massive problem in Lake Kariba in the 1950s!
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Plant family:
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