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Water star grass (Heteranthera zosterifolia)

Water star grass can form dense mats in warm, still freshwater, smothering native plants and reducing water quality. It is found at a few locations in NSW, and should not be sold in parts of the state.

Key facts

Type: Weed, aquatic.

Native to: South America.

Established in: NSW.

Identification

  • It is a perennial aquatic plant that usually starts growing underwater, rooted in mud, then sends long stems to the water’s surface.
  • Underwater leaves are grass-like with pointed tips, grow in a star-shaped rosette, and are up to 50 mm long and 7 mm wide.
  • Floating leaves are often oval with rounded tips, about 4 cm long, and grow on stalks up to 8 cm long.
  • Flowers are white to pale blue, darker at the centre, with a 1 cm tube that spreads into six lobes or petals; they usually grow in pairs but can occur in groups of up to 10.
  • Plants form roots at nodes; their fruit is a 0.5–1 mm capsule containing 8–14 winged seeds.
  • Look-alike: Kidney-leaf mud-plantain (Heteranthera reniformis) has kidney-shaped leaves and no underwater leaves.

Where it grows

In NSW, it grows near Port Macquarie on the North Coast and at Morriset in the Hunter region; cultivated plants were found and removed in Greater Sydney, and naturalised plants have not been found elsewhere in NSW.

Why it matters

  • Thick surface mats smother and outcompete native plants and reduce light, oxygen levels and water quality.
  • They reduce food and habitat for fish and other aquatic animals.
  • They can limit water flow, increasing flood risk, and restrict access to water for recreation.

How it spreads

  • Plants produce seeds, which may spread when pond or aquarium waste is dumped in wetlands and waterways; seeds may also be carried by wind, water flowing out of ponds or animals, especially water birds.
  • Stem fragments can break off adult plants and start new infestations.
  • Floods, birds, boats, machinery and people can move broken stem fragments.

Control options

  • Avoid running boat motors or paddling through water plants, prevent nutrient-rich runoff entering infested waterways, and never dump unwanted water plants.
  • Remove individual plants or small infestations, taking care to remove all roots and stem fragments and avoid breaking stems; contact the local council for disposal advice.
  • Larger infestations can be controlled with registered herbicides approved for aquatic use, applied by tablets, injection or spot spraying.
  • Follow the product label and any permit directions.

Hygiene and washdown

  • Before leaving waterways, check and remove all plant material from boats, canoes, trailers, vehicles, machinery and fishing gear.
  • Never dump unwanted water plants.
  • 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 3 of 11 regions. Regional priority weed in Hunter, North Coast, South East. 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: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • 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

Sources