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Lacy ragweed (Ambrosia tenuifolia)

Lacy ragweed is an introduced perennial weed recorded in several parts of New South Wales and in Victoria, Western Australia and South Australia. Its long runner roots help it persist, and its pollen may affect people sensitive to ragweeds.

Key facts

Type: Weed, herb.

Native to: South America.

Established in: NSW, SA, WA.

Best time to act: Look for the plant while it is actively growing and treat it before its heads mature.

Identification

  • An erect perennial herb growing to about 75 cm high, with long runner roots.
  • Leaves are opposite or alternate and divided twice into narrow, linear to oblong lobes; long white hairs give them a grey, lacy appearance.
  • The leaves are usually 2–8 cm long and 4–5 cm wide.
  • The small heads are light green, with female heads occurring singly or in small groups and containing one floret.
  • The bracts form a fruit about 2 mm long, with a few very short teeth and a central beak.
  • Look-alike: Annual ragweed (Ambrosia artemisiifolia) is a similar ragweed; lacy ragweed is perennial and has long runner roots.

Where it grows

In Australia it is a weed of roadsides and wasteland, recorded in New South Wales south from Coffs Harbour, across the NSW subdivisions NC, CC, SC, CT, NWS, CWS, SWP and NFWP, and also in Victoria, Western Australia and South Australia.

Why it matters

  • It can persist and spread through its long runner roots, making complete removal difficult.
  • It occupies disturbed sites such as roadsides and wasteland, where it may compete with desirable vegetation.
  • Ragweed pollen may trigger hay fever or other allergic reactions in people who are sensitive to it.
  • It is declared a noxious weed in some shires.

How it spreads

  • Long runner roots allow the plant to spread vegetatively.
  • Moving soil or root fragments from an infestation can carry the plant to new sites.

Control options

  • For small infestations, dig out the whole plant and its runner roots, taking care not to leave root fragments behind.
  • Use only herbicides registered for the species and site, and follow the product label and relevant local advice.
  • Revisit treated sites to check for regrowth and new plants, and control them promptly.
  • Check with the local council or state biosecurity agency about current requirements, including any reporting obligations.

Hygiene and washdown

  • Clean soil and plant fragments from boots, tools and machinery before leaving an infested site.
  • Avoid moving soil that may contain runner-root fragments to clean areas.
  • 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: 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: 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.

  • 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