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Hymenachne (Hymenachne amplexicaulis)

A semi-aquatic grass that was planted as ponded pasture and now chokes wetlands, lagoons and drains in the tropics, displacing native wetland plants and fish habitat.

Key facts

Type: Weed, grass.

Native to: Tropical Central and South America.

Established in: QLD, NSW, NT.

Other names: Olive hymenachne.

Identification

  • Robust grass up to 2.5 m tall with spongy, pithy stems.
  • Broad leaves with heart-shaped bases that clasp the stem.
  • Dense, cylindrical, spike-like seed heads up to 40 cm long.
  • Look-alike: Native hymenachne (Hymenachne acutigluma) has leaves that do not clasp the stem as strongly.

Where it grows

Wetlands, lagoons, floodplains, drains and stream banks in coastal tropical and subtropical areas.

Why it matters

  • Chokes waterways and wetlands.
  • Reduces fish and waterbird habitat.
  • Blocks drainage and irrigation channels.

How it spreads

  • Seed and stem fragments carried by water and floods.
  • Machinery, boats and stock.

Control options

  • Registered herbicides suitable for use near water, applied at the right growth stage.
  • Grazing and fire in some situations, followed by herbicide.
  • Coordinated catchment-scale programs work best.

Hygiene and washdown

  • Clean machinery and boats used in infested wetlands and drains.
  • 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): Weed of National Significance. One of 32 nationally agreed priority weeds, managed through national strategies with all states and territories.
  • Queensland: Restricted matter, category 3. Do not give it away, sell or release it into the environment. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Sale prohibited; Regional priority in 4 of 11 regions. It must not be imported into NSW or sold anywhere in the state. Regional priority weed in 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: Restricted weed. Where it is a restricted weed, trade in it (including the plant, its seed or contaminated material) is prohibited.
  • 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: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • 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: Declared pest, prohibited. It is a declared pest throughout the ACT. Dealing with it, such as selling, moving or keeping it, is prohibited.

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
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources