Photo guide to weeds

Salvinia (Salvinia spp.)

  • Photograph of a hand holding up a clump of Salvinia which shows the long filamentous root system
    Photograph of a hand holding up a clump of Salvinia which shows the long filamentous root system
  • Photograph of a thick mat of Salvinia on the surface of water
    Photograph of a thick mat of Salvinia on the surface of water

General information

Several species of salvinia occur naturally in America, Europe and Asia. Only one - Salvinia molesta, a native of Brazil - has become established in Queensland.

Salvinia molesta is a Class 2 declared pest plant under Queensland legislation and a Weed of National Significance (WONS). All other species of Salvinia not yet found in Queensland are declared Class 1 pest plants.

Scientific name

Salvinia molesta

Other names
  • Giant salvinia
Similar species
  • Other salvinia species
  • Free-floating aquatic fern.
  • Small green leaves are positioned in pairs along a common stem.
  • Leaf surfaces are covered with long, stiff, water-repellent hairs that are joined at the tip to form eggbeater like shapes.
  • As the plant matures, leaves become thick and fold at the mid-rib.
  • Young leaves are oval, about 12mm across and lie flat on the water, often resembling duckweed.
  • Roots trail from each pair of young leaves.
  • Prefers slowly-moving streams or still water ponds with high nutrient levels and water temperatures of 20oC to 30oC.
  • Native to Brazil.
  • Found in isolated water bodies from North Queensland to the New South Wales border, and west to Mt Isa.
  • Found in New South Wales and the Northern Territory.
Life cycle
  • Believed to be a sterile hybrid.
  • Does not produce flowers, reproducing by vegetative means
  • Produces little growth in winter.
  • Under optimal conditions can double in volume every 2-3 days.
  • Mainly spread by people who empty aquariums and ponds into waterways.
  • Spread by fouling of fishing equipment and boat trailers.
  • Can also spread by water currents.


  • Forms thick mats that can completely cover water storage areas in a short time.
  • Degrades water quality.
  • Destroys wildlife habitats.


  • Builds up and collects debris during flooding, causing bridges and fences to collapse.
  • Reduces water flow to irrigation equipment, increasing pumping times and costs.
  • Prevents access by stock to drinking water.


  • Endangers children and livestock who can become entangled in heavy infestations.
  • Creates mosquito breeding habitat.
  • Interferes with recreational activities such as boating, fishing and swimming.
  • Spoils the natural beauty of open water, such as dams and lakes.
  • The best form of weed control is prevention. Treat weed infestations when they are small - do not allow weeds to establish.
  • Ways to prevent weed spread

Mechanical control

  • Removal by hand or machine is a practical control method often used for small areas, or when weed numbers are low.
  • Mechanical control can take advantage of flooding or water flushes that deposit salvinia in dams, lagoons and calm waters of rivers and creeks. When using this method, it is essential to remove salvinia before rapid growth commences.
  • Salvinia can survive for long periods out of water when it is deposited on moist banks. To help prevent its re-introduction into a watercourse, it is essential that it be moved away from the water´s edge and, preferably, burnt.

Herbicide control

Biological control

  • The salvinia weevil Cyrtobagous salviniae has been extensively released. In Queensland, can give good biological control.
  • The salvinia weevil is 2-3mm long and dark brown to black. It is most effective when air temperatures are 27-35°C and nitrogen levels of the salvinia are high.
  • The salvinia weevil larvae feed on the new growth buds and tunnel into the rhizome. Tunnelling weakens the salvinia, reducing its ability to grow and compensate for bud loss. Adult weevils also affect plant growth by feeding on buds.
  • In the initial stages of weevil damage some salvinia leaves will turn brown. As the salvinia weevils continue their control efforts, the whole mat will turn brown, sink underwater and finally decompose.
  • Although effective in tropical areas, the salvinia weevil is not establishing as intensely in cooler southern areas of Queensland and may take several years to control infestations there.
  • Depending on the size of the infestation and the environmental conditions, the time taken for weevils to control salvinia varies from 1-3 years. They may take more than five years to establish in cooler areas.
  • To ensure maximum build up, it is best to release salvinia weevils in spring. When releasing weevil-infested salvinia, it is essential to choose warm, sunny positions where a drop in water level will not leave the salvinia stranded.
  • To establish an effective breeding population of weevils, infested plants should be placed in an area where the salvinia is concentrated.
Declaration details
  • All other species not yet found in Queensland are declared Class 1 species .
  • A declared Class 2 species under the Land Protection (Pest and Stock Route Management) Act 2002.
  • Taking for commercial use, introduction, keeping, releasing and supplying (including supplying things containing reproductive material of this pest) is prohibited without a permit issued by Biosecurity Queensland.
  • Landholders are required to control declared pests on their properties.

Last updated 03 May 2013