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Indian swampweed (Hygrophila polysperma)

Introduced to Australia through the aquarium trade, Indian Swampweed has become naturalised in waterways after aquarium contents were discarded. It grows quickly and can form dense mats that shade out other submerged plants and restrict water flow.

Key facts

Type: Weed, aquatic.

Native to: Sri Lanka, India and south-eastern Asia through to Vietnam.

Established in: QLD, NSW.

Other names: Indian Swamp Weed, East Indian Hygrophila, Eastern Indian Hygrophila, Miramar Weed.

Best time to act: In New South Wales, flowering is reported around June, but the plant does not appear to flower when submerged.

Identification

  • This is a low-growing, creeping herb that may grow up to 20 cm high on dry land.
  • Young stem sections are white, while older lower sections become reddish and leafless; roots can form at the lower stem nodes.
  • Leaves are stalkless, oval to egg-shaped, 5–20 mm long and 3–5 mm wide, with smooth edges.
  • Pale mauve to blue flowers form spike-like clusters among the upper leaves and have a tubular corolla 4.5–5 mm long.
  • The 4.5–5.5 mm capsule has a hairy tip and contains 24–30 seeds covered with hairs that expand when wet.
  • Look-alike: The native Hygrophila angustifolia does not grow submerged and has hooks under its seeds and four stamens per flower; Indian Swampweed has no seed hooks and has two stamens.
  • Look-alike: Hygrophila costata (Glush Weed) has white flowers about 3.5 mm long with a two-lobed upper lip, unlike the pale mauve to blue, 4.5–5 mm flowers and unlobed upper lip of Indian Swampweed.

Where it grows

In Australia it occurs in freshwater waterways, including flowing and slower-flowing water, lakes, dams and ponds, and can grow in water up to 3 m deep.

Why it matters

  • Dense mats can out-compete submerged plants for light and shade out other species.
  • Mats can obstruct water flow, impair water quality and impede navigation.
  • Dense stands can change habitat suitability for native fauna, disrupt ecosystems and contribute to biodiversity loss.
  • When large stands die and decompose, they can create toxic, low-oxygen conditions that may kill fish.

How it spreads

  • It was introduced into waterways through the discarding of aquarium contents.
  • Plant fragments can attach to animals, machinery, watercraft and fishing gear, or float freely downstream.
  • Seed dispersal may occur, but its importance in spreading the weed is uncertain.

Control options

  • Small infestations can be removed by hand, taking care to collect all plant fragments.
  • Mechanical harvesting is possible for larger infestations, but it can create fragments that spread by boats, wildlife or downstream flow.
  • No biological control agent is reported.

Hygiene and washdown

  • Check boats, machinery and fishing gear for plant fragments and remove them before moving between waterways, as fragments can attach to these items and spread.
  • 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: Regional priority in 5 of 11 regions. Regional priority weed in Central West, Greater Sydney, Hunter, North Coast, North West. 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: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • 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: 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.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development

Sources