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Perennial ragweed (Ambrosia psilostachya)

Perennial ragweed competes strongly with crops and pasture, reducing crop yields and fodder availability. It also produces allergenic pollen that can cause severe hayfever in susceptible people.

Key facts

Type: Weed, herb.

Native to: Central Canada and the western United States.

Established in: QLD, NSW, SA, WA.

Other names: Dunbible Weed, Ragweed, Roman Wormwood, Western Ragweed.

Best time to act: Plants develop rapidly in winter and spring, flower from mid-summer to early autumn, and should be treated or slashed before setting seed.

Identification

  • It is an erect, robust perennial herb, usually about 1 metre tall but sometimes reaching 2 metres, and can form dense colonies.
  • Its grey-green, aromatic leaves are deeply lobed and may have no leaf stalk or a short one.
  • The hairy, grooved stems are woody near the base and branch in their upper half.
  • Cream to greenish-yellow flowers form in small heads, with male flower heads grouped in branching spikes near the stem tips.
  • The greyish-brown, woody fruits are about 3–4 mm long, with a short beak surrounded by 4 or 5 blunt projections.
  • Look-alike: Burr ragweed (Ambrosia confertiflora) is similar, but its fruits have 10–20 short hooked spines rather than the 4 or 5 blunt projections of perennial ragweed.
  • Look-alike: Annual ragweed (Ambrosia artemisiifolia) is similar, but it does not produce underground rhizomes, its fruiting involucre is smooth or nearly hairless, and its leaves always have stalks.

Where it grows

In Australia it occurs mainly in warmer temperate and subtropical areas, in pastures, open woodlands, roadsides, disturbed and waste areas, creek banks and riparian vegetation, with records in NSW, Queensland, South Australia and Western Australia.

Why it matters

  • It competes strongly with crops and pasture and can cause crop losses and reduced fodder availability.
  • It is unpalatable to cattle, so dense infestations can considerably reduce pasture productivity.
  • Its aerial parts produce chemicals that inhibit the germination and growth of other plant species.
  • Its allergenic pollen can cause severe hayfever and contact dermatitis in susceptible people.

How it spreads

  • It spreads locally through creeping underground rhizomes and root fragments, which can produce shoots and form large colonies.
  • Seeds can attach to animals, clothing and other fibrous material, and can also be dispersed by vehicles and water.
  • Seeds and rhizome fragments can be moved in soil, mud and gravel, including on vehicles and cultivation machinery.

Control options

  • Use registered herbicides according to their label; repeated applications over several years may be needed to prevent regrowth from dormant seeds.
  • Where practical, hand-pull young plants; avoid contact with flowering plants and pollen if prone to allergies.
  • Slash or mow before seed set, then check for and control regrowth.
  • Avoid cultivation, which can spread root fragments and stimulate new growth.

Hygiene and washdown

  • Clean vehicles and farm machinery after working in infested areas to avoid moving seeds in mud and rhizome fragments on equipment.
  • Avoid moving contaminated soil or gravel, which can carry seeds and rhizomes to new sites.
  • 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: State prohibited weed. State prohibited weeds are the highest priority: the Victorian Government is responsible for eradicating them, so report any sighting to Agriculture Victoria.
  • 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