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Burr ragweed (Ambrosia confertiflora)

Burr Ragweed competes strongly with pasture plants and can reduce pasture productivity. Its hooked burrs contaminate wool, interfere with orchard harvests, and its highly allergenic pollen can cause severe hay fever and contact dermatitis in susceptible people.

Key facts

Type: Weed, herb.

Native to: Semi-arid plains of the southern United States and Mexico.

Established in: QLD, NSW.

Other names: Slimleaf Bursage.

Best time to act: Growth increases in spring, flower stems appear in early summer, and herbicide spraying is usually performed at the budding stage.

Identification

  • An erect perennial herb that grows to 2 m high and forms large colonies from creeping, runner-like roots.
  • Its grey-green leaves are fern-like, twice divided, 12–16 cm long and 10–15 cm wide.
  • Yellow-green male flowers form branching spikes at stem ends, while female flowers are clustered between upper leaves and stems.
  • The fruit is about 4 mm long and covered with 10–20 short, hooked spines.
  • Look-alike: Perennial Ragweed (Ambrosia psilostachya) is similar, but its seeds have a short pointed beak surrounded by 4–5 blunt projections rather than 10–20 short hooked spines.
  • Look-alike: Chinese Mugwort (Artemisia verlotiorum) can also look similar, but Artemisia flower heads contain clusters of flowers of both sexes.

Where it grows

In Australia it occurs on dry plains in New South Wales and Queensland, especially in run-down pastures, orchards, roadsides, wastelands and other disturbed areas.

Why it matters

  • It competes strongly with pasture species, including Rhodes Grass, and can suppress them.
  • Its burrs contaminate wool and are difficult to remove during scouring, adding to processing costs.
  • In orchards it competes for nutrients and interferes with harvest; its pollen and burrs can cause discomfort to pickers.
  • Its allergenic pollen can cause severe hay fever and contact dermatitis in susceptible people.

How it spreads

  • Hooked seeds attach to sheep and other animals, clothing and other fibrous material, enabling long-distance spread.
  • Water, especially floodwater, spreads the buoyant woody burrs; the plant is also spread in soil and gravel used for road building and top dressing.
  • Creeping roots maintain existing colonies, and root fragments severed during cultivation can grow into whole plants and colonise new areas.

Control options

  • Registered herbicides can give good control in pastures by suppressing Burr Ragweed and allowing pasture species to compete; spraying is usually done at the budding stage.
  • Cultivation is not effective and can worsen an infestation by spreading perennial root pieces and stimulating growth from root buds.
  • No biological control agent is listed.

Hygiene and washdown

  • Hooked burrs attach to clothing and other fibrous material, and the plant can also be spread in soil and gravel used for road building or top dressing.
  • 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 Central Tablelands, Central West, Western. 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.

  • 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