The water fern is aptly named. It grows on the surface of slow-moving water as free-floating plants. In some cases, it reproduces so prolifically that it can cover entire lakes. Because this Brazilian native can spread so rapidly, it is considered one of the most problematic invasive species in the world.
I. Appearance and Characteristics
Salvinia molesta, commonly known as giant salvinia, or as kariba weed after it infested a large portion of Lake Kariba between Zimbabwe and Zambia, is an aquatic fern, native to south-eastern Brazil.
Salvinia molesta is a complex of closely related floating ferns; they can be difficult to distinguish from each other. This water fern is often grown as an ornamental plant but has escaped and become a noxious pest in many regions worldwide. There are a few different growth forms for S. molesta. The primary growth form is an invading form with small flat leaves to the tertiary or mat form with large, crowded, folded leaves. Under the best conditions plants can form a two-foot-thick mat.
These mats can put a halt to recreational activities on lakes and waterways. S. molesta has been used to extract nutrients and pollutants from the water. When this plant is dried out, it is used as satisfactory mulch.
Salvinia molesta prefers to grow in slow-moving waters such as those found in lakes, ponds, billabongs (oxbows), streams, ditches, marshes, and rivers. It prefers nutrient-rich waters such as those found in eutrophic water or those polluted by waste water. It does not usually grow in brackish or salty waters, but has been reported in streams with a tidal flow in southeast Texas.
It copes well with dewatering, and while it prefers to grow in moderate temperatures, it will tolerate low or very high temperatures. The United States Geological Service believes that it could grow in zones 7a, 8, 9, and 10 of the USDA Plant Hardiness Map.
The rapid growth rate of Salvinia molesta has resulted in its classification as an invasive weed in some parts of the world such as Australia, United Kingdom, New Zealand, and parts of the United States. Surfaces of ponds, reservoirs, and lakes are covered by a floating mat 10–20 cm (in some rare cases up to 60 cm) thick. The plant’s growth clogs waterways and blocks sunlight needed by other aquatic plants and especially algae to carry out photosynthesis, thereby deoxygenating the water. As it dies and decays, decomposers use up the oxygen in the water. It also prevents the natural exchange of gases between the air and the body of water the plant has invaded, causing the waterway to stagnate. This can kill any plants, insects, or fish trapped underneath its growth.
Its ability to grow and cover a vast area makes it a threat to biodiversity. Large infestations covering a wide area may also pose a problem to migratory birds, as they may not be able to recognise an infested waterway when flying overhead, so may not stop at it. S. molesta provides ideal conditions for the breeding of mosquitoes that carry disease. The growth habit of Salvinia is problematic to human activities including flood mitigation, conservation of endangered species and threatened environments, boating, and irrigation.
II. How to Grow and Care
Water fern is an aquatic fern that thrives in still or slow-moving freshwater environments. Key care points include maintaining high humidity and ensuring consistent water conditions free from pollution. Water fern has a high growth rate and can be invasive; thus, controlling its spread is imperative. Regularly removing excess plants helps to manage growth and prevent ecological imbalance in aquatic systems.
Sunlight
Water fern’s optimal light condition is in full sun, which encourages robust growth and health. While water fern tolerates a range of light levels, too much deviation from intense light exposure can lead to suboptimal growth or reduced vigor. Water fern possesses adaptive traits such as leaf floatation that maximize sunlight capture. Ideally, water fern thrives outdoors in open water bodies where it can receive unfiltered sunlight throughout the day. An understanding of these preferences ensures that water fern maintains its vitality and characteristic rapid expansion when in the proper light setting.
Watering
Originating from tropical locales, water fern thrives in humid environments and has a high tolerance for moisture. This species is adept at managing its water content, requiring a balance that mimics its native habitat’s conditions. For optimal growth, water fern should be watered every week. As an aquatic fern commonly grown outdoors, water fern’s ability to float on water surfaces makes it uniquely suited to garden ponds or water features where it can contribute to the ecosystem by providing shade and shelter for aquatic life.
Fertilizing
For water fern, a balanced nutrition fertilizer suits regular maintenance, while high-nitrogen formulations bolster growth. Fertilize monthly, using sparing quantities to prevent water quality issues. Fertilization enhances water fern’s vigor and leaf health. Seasonal adjustments are key; reduce frequency in cooler months. When applying, distribute evenly and avoid over-concentration, protecting the aquatic ecosystem. For novice and seasoned gardeners, these tips promise a flourishing water fern.
Propagation
Water fern, an aquatic floating fern, thrives in calm water bodies. It has an aggressive growth habit, often forming dense mats. Propagation is straightforward: fragments containing at least one node simply require placement on the water surface to develop roots and grow. Such fragments break naturally from parent plants, so even unintentional disturbance can result in spread. Hence, care should be exercised to prevent unchecked growth and potential invasiveness. For successful propagation, ensure adequate light and avoid overcrowded conditions to foster healthy growth.
Transplanting
The prime time for relocating water fern is during the burgeoning warmth of late spring to early summer, ensuring vibrant growth. Optimal conditions include sunny spots with calm waters. If necessary, limit handling to preserve delicate fronds.
III. How to Control
- Biological control
A tiny weevil, Cyrtobagous salviniae, found in the native distribution of S. molesta, is being studied as a biocontrol. C. salvinae was first used as a biological control in Australia at Lake Moondarra, a recreational lake in Mount Isa, Queensland in 1980. By mid-1981, the weevil had reduced the population to a few small patches. It ate the leaves of the weed, but preferred the buds. Its larvae ate the roots, rhizomes, and the buds.
As the plant died, it turned brown and sank to the bottom of the waterway and decomposed. This weevil was used with success in other parts of the world (13 tropical countries) such as the Sepik River in Papua, Sri Lanka, Lake Ossa in Cameroon as part of an AMMCO project to restore African manatee; Wappa Dam in Queensland, and lagoons (e.g. Kakadu National Park) in the Northern Territory, Australia.
In Australia, the moth, Samea multiplicalis was also released in the hopes that it would reduce the size of the weed population. While this moth did become established in Australia and spread, it was not effective as a biological control. A third species, the grasshopper Paulinia acuminata, was considered, but not released in Australia.
However, this grasshopper has been deliberately released for control of Salvinia molesta in parts of Africa, Sri Lanka and India, and accidentally in Puerto Rico. The giant gourami has long been known to prefer S. molesta and feed on it voraciously. It has been successfully used to control S. molesta in reservoirs of Sri Lanka.
- Mechanical control
Plants are removed by machine or harvesting equipment and by hand; the latter is suitable only for small infestations. Harvesting equipment also can encounter difficulties, as it cannot remove all of the infestation, it cannot access shallow areas, and equipment can be inhibited by large masses of the plant. Once removed, the plant must be dried, burnt, or disposed of in a manner that ensures it will not re-enter the waterway. Other methods to control the aquatic weed are dredging and chaining.
- Chemical control
Chemical control on S. molesta is difficult as it does have some resistance to herbicides. The chemical fluridon has been successfully used, but it requires prolonged contact and is not effective if it is suddenly diluted by rainwater or any other influx of water. Other chemicals, such as hexazinone and diquat and double-chelated copper are used together to kill S. molesta.
