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Giant bluetop (Praxelis clematidea)

Praxelis is an introduced weed spreading in northern and eastern Queensland, with the potential to seriously degrade native ecosystems. It could also threaten crops such as bananas, other fruits and sugar cane, and increase management costs.

Key facts

Type: Weed, herb.

Native to: Subtropical South America, including southern Brazil, Venezuela, Bolivia and northern Argentina.

Established in: QLD, NSW, WA, NT.

Other names: Praxelis.

Best time to act: Look for flowering mainly from November to May and act early, before seeds drop from the plant.

Identification

  • An annual or perennial herb, 0.2–1 m tall, with brittle, cylindrical stems covered in short soft hairs.
  • Leaves grow in opposite pairs and are tear-shaped to diamond-shaped, with conspicuously toothed edges.
  • Crushed leaves have a pungent smell similar to cat's urine.
  • Lilac or bluish florets grow in groups at stem ends in conical to bell-shaped heads, with 30–50 florets in each head.
  • Seeds are black achenes 2–3 mm long, topped with 15–40 white pappus bristles.
  • Look-alike: Ageratum conyzoides is similar, but has a flat or slightly dome-shaped flower-head receptacle and persistent bracts; Praxelis has a conical receptacle and its bracts drop off.
  • Look-alike: Ageratum houstonianum can have flower colour and size closer to Praxelis; Praxelis leaves are strongly pungent when crushed and its seeds have 15–40 pappus bristles.

Where it grows

In Australia, infestations are recorded in Queensland, especially along the northern coast from Townsville to Cairns, on the Atherton Tablelands, and in scattered parts of Cape York Peninsula, the Torres Strait islands and near Gympie.

Why it matters

  • Praxelis can invade both disturbed and relatively undisturbed areas and has the potential to seriously degrade Australian ecosystems.
  • It could infest pastoral grasslands and conservation areas, particularly open eucalypt woodlands.
  • It could threaten bananas, other fruits and sugar cane, and significantly increase management costs.
  • There is some evidence it may be poisonous to stock and humans if ingested.

How it spreads

  • Praxelis spreads mainly by seed and can produce large numbers of seeds within three or four months of germinating.
  • The seeds can spread by wind or water, attach to animal fur, feathers, clothing or machinery, and travel on vehicles or in building and landscaping materials.
  • Roots and new plantlets can also form along branches in contact with the soil.

Control options

  • Early detection and eradication are important; small infestations can be eradicated more easily when found early, but follow-up is needed.
  • Contact your state or territory weed management agency or local council, and do not attempt control without expert assistance.
  • No herbicide products are specifically registered for Praxelis control in Queensland; a permit allows some products to be used for control as an environmental weed in various situations.
  • Hand pulling small areas is not recommended because mature seed can drop and enlarge the infestation.

Hygiene and washdown

  • Prevent seed from being carried away on vehicles, machinery or clothing.
  • Take care not to move Praxelis seeds in building or landscaping materials 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): National Environmental Alert List. One of 28 weeds named on the National Environmental Alert List as early-stage invaders with serious potential to harm the environment.
  • 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: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Declared pest (C1, C2), 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. C2 Eradication: present in limited areas and targeted for eradication. 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
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources