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Dune onion weed (Trachyandra divaricata)

Trachyandra divaricata is a naturalised weed in several Australian coastal regions, including parts of Western Australia, South Australia and New South Wales. It can compete with native plants on coastal dunes, form monocultures and is toxic to livestock.

Key facts

Type: Weed, herb.

Native to: Southern Cape Province, South Africa.

Established in: NSW, SA, WA.

Other names: False Onion Weed, Strapweed, Branched Onion Weed, Trachyandra.

Best time to act: Look for and treat plants before seed capsules mature; flowering is mainly from mid-winter to late spring, with a possible second flowering season in autumn.

Identification

  • A robust perennial herb that grows in tufts.
  • Its leaves can reach 60 cm long and 8 mm wide, with smooth or irregularly papillose margins and tubular scales around their bases.
  • The flower stalk can reach 70 cm and is branched in a widely spreading pattern.
  • Flowers are white, with a conspicuous green stripe and a yellow spot near the base of each segment.
  • The fruit is a capsule up to 1 cm long; its smooth, tetrahedral seeds are about 1.5 mm long.

Where it grows

In Australia it grows in sandy soils in coastal areas, including coastal dunes and regions of southwestern Western Australia, Eyre Peninsula, the Adelaide region and the Newcastle–Sydney–Wollongong region.

Why it matters

  • It competes with native species for space and other resources and has the potential to form monocultures.
  • It is a weed of coastal sand dunes.
  • It is toxic to livestock.

How it spreads

  • When the fruiting stems dry, their tangled crown can break away and tumble in the wind, dispersing seeds.
  • Seed-bearing plants and contaminated soil or equipment can potentially move seeds to new sites.

Control options

  • For small infestations, remove plants before seed capsules mature, taking care to collect the plant material and any seed-bearing stems.
  • Where chemical control is needed, use only a herbicide registered for the species and site, and follow its label and local requirements.
  • Revisit treated areas and remove regrowth and new seedlings before they set seed.
  • Check with the relevant state or territory biosecurity agency or local land manager about reporting and any applicable management requirements.

Hygiene and washdown

  • Before leaving an infested site, check and clean boots, tools, vehicles and machinery to remove seed-bearing plant material and contaminated soil.
  • Keep collected seed-bearing material contained during transport and dispose of it in accordance with local guidance.
  • 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
  • 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