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Narrow-leaved water plantain (Alisma lanceolatum)

This introduced aquatic weed is naturalised in Australia and has been reported as spreading widely in South Australia's Onkaparinga River catchment. It is also a weed of rice fields in New South Wales, and accurate identification is important because it can be confused with native water plantain.

Key facts

Type: Weed, aquatic.

Native to: Europe, North Africa and Asia.

Established in: NSW, VIC, SA, WA.

Other names: Alisma lanceolatum, Lanceolate water plantain.

Best time to act: Survey and treat during active growth, and aim to remove plants before flowering and fruiting from December to March.

Identification

  • A perennial aquatic herb growing to about 1.5 m tall, with a thick, corm-like rhizome.
  • Emergent leaves have lance-shaped blades, usually 3–25 cm long and 1–6.5 cm wide, with an acute to tapering tip and a tapering, wedge-shaped base.
  • Its erect, branched flower stalk can reach about 1.2 m and bears clusters of small flowers.
  • Flowers are purplish-white to pink, with petals that often have pointed tips; flowering and fruiting occur from December to March.
  • Look-alike: Native Alisma plantago-aquatica has broader, ovate to broadly lance-shaped leaves with a blunt or rounded tip and a rounded to heart-shaped base; its petals are usually broadly rounded.
  • Look-alike: Alisma plantago-aquatica typically has strongly ladder-like tertiary leaf veins, while those of A. lanceolatum are reticulate.

Where it grows

It grows in damp ground or shallow water, including creek, lake and swamp margins and ditches.

Why it matters

  • It is a naturalised weed that appears to be spreading widely through the Onkaparinga River catchment in South Australia.
  • Alisma lanceolatum is reported among weeds of rice fields in New South Wales.
  • Confusion with the native A. plantago-aquatica can lead to the native species being mistakenly targeted for removal.

How it spreads

  • Its fruits can be carried by water, allowing the plant to spread along waterways.
  • Plant material and contaminated mud on boats, machinery, footwear or other equipment can move between wetland sites.

Control options

  • Before removal, confirm the identification and check with the relevant land manager or biosecurity authority about local requirements and suitable methods.
  • Where practical, remove plants and the rhizome by hand or digging, and collect flowering or fruiting material so it is not left in the water or moved to another site.
  • For larger infestations, use only herbicides registered for the intended aquatic setting, and follow the product label and any applicable permit or waterway requirements.
  • Revisit treated areas to check for regrowth and new plants, and contact the state biosecurity agency for advice or reporting guidance where relevant.

Hygiene and washdown

  • After working in an infested site, remove plant material and mud from boats, machinery, boots and gear, and contain the waste so it cannot wash into another waterway.
  • Clean and dry equipment before moving between wetlands, and avoid carrying contaminated water or soil to uninfested sites.
  • 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: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • 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: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Check NT list. There is no NT weed profile for it. Check the NT declared weeds list for its current status.
  • 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.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development

Sources