Quick Facts

Quick facts

• Originally from Brazil, Salvinia (Salvinia molesta), a Weeds of National Significance, is an aquatic free-floating fern with opposite leaf-like fronds with floating root-like frond, introduced to Australia for use in aquaria and ponds.
• It spreads vegetatively by stem fragments, and can rapidly form dense mats that completely cover still or slow-moving water bodies impacting access to water for animal and people, harming aquatic wildlife.
• It grows well in tropical and subtropical climates growing best when the water temperature is between 20°C and 30°, but is also found in southern Australia with growth is limited or absent under 10°C, with plants able to survive being frozen.
• Salvinia (Salvinia molesta), has distinctive egg-beater like hairs and their presence confirms identity this species.
• It is difficult to control but methods used are chemical and physical but long-term follow-up is required.
• Biological control is effective but can only used in warmer areas of Australia.
• Prevention is the most cost-effective form of weed control. Quarantine, early detection and good hygiene within infestations will prevent its spread.

What Does It Look Like?

What is it?

Salvinia (Salvinia molesta) is a small free-floating aquatic fern that can grow rapidly to cover the entire water surface with a thick mat of vegetation. The fronds are arranged along the stems in threes. Two fronds are leaf-like, more or less round in shape, 7-40 mm long and 2-25 mm wide, pale green to greenish brown, frequently overlapping and folded along the midrib (McCarthy 1998). The upper surfaces of the leaf-like fronds are covered with distinctive egg-beater-shaped hairs 2-4 mm long that diverge into four branches near the top and fuse together at the tips (DiTomaso & Healy 2003).

The third frond is root-like, submerged in water, and slender, up to 30 cm long and covered with opposite fine brown root-like hairs. A single plant, called a phenet, is made up of colonies of ramets (a ramet refers to each pair of leaves and associated bud on the rhizome). There are rarely more than 100 ramets in a phenet (Oosterhout 2006).

A single plant, called a phenet, is made up of colonies of ramets (a ramet refers to each pair of leaves and associated bud on the rhizome) that are all joined together. There are rarely more than 100 ramets in a phenet (Oosterhout 2006).

Salvinia plants currently present in Australia is incapable of sexual reproduction; therefore sporlings (‘seedlings’ of ferns) do not occur. If sporocarps (spore sacs) are formed they are 2-3 mm long and borne in clusters along the root-like leaves and spores are not viable (McCarthy 1998).

Plants float together, forming mats over the surface of the water. Oosterhout (2006) summarises three distinct stages of growth that occur as floating plants become more crowded on the surface. Primary stage – Primary growth occurs in the early stages of an infestation, when plants are not crowded. The small, flat, oval-shaped leaves lie in direct contact with the water surface with water surface is visible between plants. Leaves can be as small as 2 mm in width. The Salvinia recovering from damage also shows the primary growth form. Secondary stage – Secondary growth occurs when the leaves become slightly cupped, with their lower surface in contact with the water and leaves are larger. Tertiary stage – Tertiary growth occurs when plants become crowded in a mature infestation. Leaves become bigger and are tightly folded. Tertiary weed mats can become multi layered, displaying ridge-like thickening as layers build up. Up to 30 000 ramets per square metre have been recorded in nutrient-rich water (Oosterhout 2006).

Recognition: This species can normally be recognised by the combination of the following characters; the diagnostic character (a character used to tell this species from others) is the upper surfaces of the fronds are covered with distinctive egg-beater-shaped hairs, 2-4 mm long that diverge into four branches and fuse together at the tips; a free floating plant with two opposite leaf-like fronds; slender root-like fronds submerged in water covered with opposite fine brown root-like hairs.

For further information and assistance with identification of Salvinia contact the herbarium in your state or territory.

Flower colour

No flower

Growth form (weed type/habit)

Aquatic

Where it currently grows? Preferred habitat

Salvinia is usually found in still or slow-moving water bodies with high nutrient levels normally favouring warmer climates, but is found growing in cooler temperate southern Australia.

Are there similar species?

Salvinia is not likely to be mistaken for any indigenous Australian plant species or any other known weed species.

Why Is It A Weed?

What are its impacts?

Salvinia (Salvinia molesta) is a Weed of National Significance. It is regarded as one of the worst aquatic weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts.

Recreational activities: An infestation can cover the entire surface of a water body, reducing aesthetic values and preventing most recreational activities such as swimming, boating, water sports, and recreational fishing (Oosterhout 2006).

Agriculture and fisheries: Commercial fisheries suffers dramatically with Salvinia plants blocking nets and impeding the passage of boats (Agriculture & Resource Management Council of Australia & New Zealand et al. 2003; CRC 2003). Dense mats of Salvinia can be mistaken for solid ground by people and animals with reported instances of animals falling into the water body beneath. The dense mats impede water flow, restrict stock access and provide favourable conditions for the breeding of disease-carrying mosquitoes. Salvinia can clog pump intakes and cause water losses from storage areas by increasing levels of evapo-transpiration (increased water evaporating from the plant leaves).

Native ecosystems: Salvinia forms dense mats over the water surface, severely modifying aquatic habitat, shading out and creating unfavourable conditions for all aquatic organisms of flora and fauna, including birds, fish, invertebrates and other aquatic animals and aquatic plants. Light is excluded and oxygen levels are reduced leading to water stagnation and pollution. Decaying plant material can produce foul odours.

How does it spread?

Salvinia molesta is probably of hybrid origin and is usually sterile (McCarthy 1998). When spores are produced they are not viable. Stem fragments are spread by various means including water movement. The dumping of the unwanted contents of ponds and aquaria is a major reason for the spread of Salvinia (CRC 2003). Each ramet (pair of frond-like leaves) with free floating submerged root-like leaf can exist independently, and therefore form strings of new plants vegetatively whenever a rhizome breaks. Rhizomes break with age or damage adding spread. Growth occurs at the ends of the rhizomes, where apical buds are present. New rhizome branches can develop from any ramet where axillary buds are present. Each bud can form a new branch of ramets, with the oldest branches eventually rotting and falling away. A growing salvinia plant displays this process in a zigzag pattern, giving rise to the wedge-shaped plants. The continuing production of new plant material makes Salvinia a self-perpetuating perennial under favourable conditions. A new plant can form from a piece of rhizome bearing a bud (Oosterhout 2006) .

What is its history in Australia?

Salvinia was probably first introduced to Australia by the aquarium trade. It was first reported as a weed near Sydney in 1952 and from Queensland in 1953. It is known to have been planted as an ornamental near Perth between 1950 and 1954 and has subsequently been grown in garden ponds in many other places including Melbourne, Adelaide and Alice Springs (Parsons & Cuthbertson 1992).

How To Manage It?

Best practice management

Successful management is achieved when control methods are combined (an integrated control method) in order to put greater pressure on the weed, or to treat the weed.

Chemical control: Salvinia can be sprayed with various herbicides including Diqaut, glyphosate, and Carfentrazone-ethyl. Results can be variable as upper leaf surfaces are largely non-wettable making absorption of herbicides difficult (Muyt 2001). There are no situations where a single application of herbicide will provide ongoing control of Salvinia. Initial treatments will always need to be followed up with further treatments. The correct application and timing is important and may vary across Australia. 

Warning: Herbicide use in and around water may require a licence and each State and Territory has regulations for the use of herbicides in and near waterways. Please check with your relevant government department and always check the product label information for safety information to people and wildlife and for irrigation water withholding periods. Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au .

Non-chemical control: Physical control: Manual (hand) removal plant can be removed by hand from small ponds but every plant needs to be removed as vegetative growth will occur plants will multiply eventually covering the pond. Large infestations are sometimes gathered with mechanical harvesters and scoops, although the effectiveness of this method is limited by the ease with which the plants break into fragments (Muyt 2001).

Booms and containment fences: Containing infestations with booms or meshed containment fences should be used to prevent further spread and keep intake or other areas free of Salvinia. Containment is important for infestations, where an infestation has the ability to spread and invade new areas;  keeping certain areas Salvinia free; contain sections of Salvinia in one area to minimise costs and the time required to carry out herbicide treatments or physical removal; separate areas for different control treatments (i.e. different herbicides, herbicide and biological control, mechanical removal, and biological control. it is important to Identify the source of the infestation  to avoid further spread and re-infestation.

Management: Salvinia grows faster and produces more growth the higher the nutrient levels in a water body. Reducing  the growth and biomass of Salvina involves reducing nutrient levels in waterways. This can be achieved by  managing erosion on cultivated land, controlling stock access to banks, diverting effluent before it enters the waterway, and not washing waste from cattle yards or dairies into waterways (DPI NSW 2019).

Disposal: Small infestations in home ponds and aquaria can be gathered with fine-woven scoops, placed in sealed plastic bags and destroyed by leaving the bags in a warm, sunny place for a few days (Muyt 2001). Plants deposited on the banks of water-bodies can be moved away from the water's edge, and preferably burnt, to prevent reintroduction (Queensland Department of Natural Resources and Water 2006).

Biological control: Salvinia has also been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. The Salvinia Weevil (Cyrtobagous salviniae) has undergone CSIRO research and has been released in Australia as a biological control agent. Although effective in tropical areas it has little impact in more temperate climates (Queensland Department of Natural Resources and Water 2006). The Salvinia weevil is a small, black insect which grows to about 2 mm long. Adult weevils feed on the growing tips, suppressing growth. Larvae tunnel through the horizontal stems, particularly in younger parts of the plant causing the weed to break apart, sink and decompose on the bottom of the waterway.  The Salvinia weevil does most damage when Salvinia is healthy and green and the temperature is around 30°C. Weevils breed very slowly at temperatures less than 20 °C and ceases breeding below about 17 °C.  The best time to introduce the weevil is early spring as the warmer weather helps populations establish (DPI NSW 2019).

For further information on all control methods see the Salvinia Control Manual (Oosterhout 2006) and the NSW Weedwise website (DPI NSW 2019).

Does it have a biological control agent?

YES. Two agents released: the salvinia weevil (Cyrtobagous salviniae) and moth (Samea multiplicalis). Ongoing mass rearing and repeated releases from mass rearing facilities in Queensland and NSW; and post release monitoring by University of Wollongong (Harvey,  et al 2023).

When does it grow? (lifecycle/growth calendar)

Salvinia grows best when the water temperature is between 20°C and 30°C  and these conditions will occur at different times in different regions. It grows faster when nutrients are abundant, often after rainfall has washed nutrients into water bodies (CRC 2003). Plants can survive to temperatures up to 43°C. Growth is limited or absent under 10°C with plants can survive being frozen and salinities up to one tenth of sea water (CRC 2003). Salvinia can regenerate after being drought stressed, heat stressed or frosted, with new green leaves appearing on a dead-looking brown weed mat. Frost kills exposed leaves and buds, but leaves and buds within the weed mat can survive provided that they do not freeze. Buds will also remain viable in dry or extremely hot conditions if they are protected inside the weed mat (Oosterhout 2006).

Where Is It Found?

Which states and territories is it found?

NSW, NT, QLD, SA, VIC, WA

What areas within states and territories is it found?

Salvinia has been introduced to coastal Western Australia, northern Northern Territory, southern South Australia, eastern Queensland, the central coast of New South Wales (McCarthy 1998) and at one site in north-east Victoria, and one site from South Australia (Chris Brodie pers comm 2020). It is naturalised in most coastal streams from Cairns in northern Queensland to Moruya on the south coast of New South Wales. It potential distribution includes water bodies in every Australian state and territory (CRC 2003). 

Where does it originate?

Salvinia is native to south-eastern Brazil (CRC 2003).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

YES

Where is it a declared weed?

Declared in all Australian states and territories.

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Salvinia molesta

Other scientific names (synonyms)?

• Salvinia auriculata Aubl. (misapplied by Aston, H.I. 1973, Aquatic Plants of Australia. 43-45.)
• Salvinia cucullata Bory (misapplied by Mueller, F.J.H. von 1889, Second Systematic Census of Australian Plants. 228.)
• Salvina adnata Desv., Mém. Soc. Linn. Paris 6(2): 177 (1827).

Does it have other known common name(s)?

Giant Salvinia, Aquarium Watermoss, Kariba Weed