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Water soldier (Stratiotes aloides)

Water soldier is not known to be naturalised in Victoria, but it has the potential to become a serious weed if introduced to suitable Australian waterways. Dense growth can cover the surface of small water bodies, obstruct access and recreation, and alter water quality and habitat.

Key facts

Type: Weed, aquatic.

Native to: Europe and parts of western Asia.

Established in: .

Other names: Crab's claw.

Best time to act: Inspect through the warmer months when plants may rise to the surface and flowering shoots may be visible, and treat promptly before propagules can spread.

Identification

  • Look for a dense rosette of stiff, sword-shaped leaves growing in or just above the water.
  • The leaf margins are sharply toothed, giving the rosette a prickly appearance.
  • Plants may be submerged or rise to the surface, where they can form dense floating stands.
  • In summer, flowering shoots emerge above the water; the flowers are white.
  • Look-alike: It can be confused with other rosette-forming aquatic plants; check for the stiff, sharply toothed leaves and seek expert identification if uncertain.

Where it grows

Where introduced in Australia, it could grow in shallow, still or slow-moving, nutrient-rich water such as ponds, ditches, wetlands, oxbow lakes and sheltered river or lake margins.

Why it matters

  • Dense stands can cover the water surface and obstruct people, boats and machinery, reducing access and recreation.
  • Growth can affect water flow and alter water chemistry, light and nutrient conditions.
  • It can form a monoculture that changes aquatic habitat and may exclude native wetland plants and affect fauna.
  • The plant can provide habitat for some aquatic invertebrates, but its effects on fauna depend on the site and the resulting habitat changes.

How it spreads

  • Seeds may be dispersed by animals, and germination can be enhanced after seeds pass through a digestive tract.
  • Vegetative propagules, including offsets and turions, can be moved by currents and wave action.
  • Plant material or water moved on boats, machinery and other equipment can risk spreading it to new waterways.

Control options

  • Before acting, contact the relevant state or territory biosecurity agency and waterway manager to confirm identification and obtain advice on permitted methods.
  • Where approved and practical, physically remove plants and propagules while containing the material so it cannot drift or escape downstream; dispose of it securely away from waterways.
  • Use only herbicides registered for the target and aquatic setting, and follow label directions and any required permits or approvals.
  • Reinspect treated sites and nearby waterways, and report suspected new infestations to the relevant state or territory biosecurity agency.

Hygiene and washdown

  • After working in an infested waterway, remove plant fragments from boats, trailers, machinery, footwear and gear, and contain the material for secure disposal.
  • Drain water and clean and dry equipment before moving between waterways; do not transfer water or plant material to another site.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Prohibited matter. Report it to Biosecurity Queensland on 13 25 23 within 24 hours and do not move or deal with it. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Prohibited matter. It is an offence to deal with it, and you must notify NSW DPIRD as soon as you become aware of it.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Declared pest (C1), prohibited. C1 Exclusion: to be kept out of WA, or out of the parts of WA where it is not established. Report any sighting to DPIRD. It must not be brought into WA (prohibited organism).
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

Sources