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Annual ragweed (Ambrosia artemisiifolia)

Annual ragweed is a serious health hazard because its pollen can cause hay fever and other respiratory allergies, and can aggravate asthma. It can form dense infestations in poorly managed pastures, reducing productivity.

Key facts

Type: Weed, herb.

Native to: North America.

Established in: QLD, NSW, SA, WA.

Other names: Common Ragweed, Ambrosia, Hay-fever Weed, Asthma Plant.

Best time to act: Look for seedlings from spring through summer and treat plants before they set seed, usually before flowering or at early flowering; flowering usually occurs from mid to late March.

Identification

  • This shallow-rooted, erect herb usually grows 1 to 2 metres tall and has fern-like, slightly rough, grey foliage.
  • Its deeply lobed leaves are oppositely arranged near the base and alternately arranged further up the stems.
  • Small green male flower-heads grow in cone-shaped spikes; female flower-heads are single or clustered and hidden by the upper leaf bases.
  • Its small, black, top-shaped fruits are rough and surrounded by a hard ring with 4 to 7 short spines.
  • Look-alike: Perennial ragweed (Ambrosia psilostachya) has a perennial habit, while annual ragweed has distinctly stalked leaves and more prominently spiny fruiting clusters.
  • Look-alike: Parthenium weed (Parthenium hysterophorus) has ribbed stems and masses of small white flower-heads at branch tips, unlike annual ragweed.

Where it grows

In Australia it is widely naturalised in south-eastern Queensland and northern New South Wales, and occurs in disturbed sites, roadsides, watercourse banks, waste areas and poorly managed pastures.

Why it matters

  • Its highly potent pollen causes respiratory allergies such as hay fever and can aggravate asthma.
  • Dense infestations can develop in degraded and overgrazed pastures, suppressing pasture plants and reducing productivity.
  • It is unpalatable to horses, and overgrazed horse paddocks often host large infestations.
  • It is also a weed of waterways, floodplains, roadsides, wasteland, vacant lots and gardens.

How it spreads

  • It spreads by seed, and some seeds can remain dormant but viable for more than 40 years.
  • Spiny seeds can attach to animals, wool, bags and clothing, or be carried in mud on animals, vehicles and machinery.
  • Seeds also spread in floodwater or as contaminants in fodder, topsoil, gravel and soil used in road-making or garden topdressing.

Control options

  • Use registered herbicides where appropriate, and rotate herbicides because resistance to at least one herbicide has developed in Europe.
  • Where feasible, pull plants by hand, but avoid contact with flowering plants and pollen if prone to allergies.
  • Slash or mow plants before they set seed, then check for flowering regrowth and control plants emerging from the soil seed bank.
  • Maintain healthy, dense pasture to suppress ragweed; opportunistic burning may help heavy infestations where paddocks are not overgrazed and soil moisture is adequate for grass to regrow.

Hygiene and washdown

  • Clean mud and seeds from vehicles, machinery, clothing and other gear after working in infested areas.
  • Avoid moving contaminated soil, gravel or fodder from infested 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: 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: 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: Declared weed. Weeds Australia lists it as a declared weed in the NT under the Weeds Management Act 2001. Check the NT declared weeds list for its class and what it requires.
  • 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
  • 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