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Calotrope (Calotropis procera)

Calotrope forms dense thickets that reduce rangeland pasture productivity and make mustering more difficult. It competes with native plants and can change the appearance of grassland communities.

Key facts

Type: Weed, shrub.

Native to: Northern Africa, the Arabian Peninsula, the Middle East and parts of southern Asia.

Established in: QLD, WA, NT.

Other names: Rubber Bush, Rubber Plant, Cabbage Tree, Giant Milkweed.

Best time to act: In monsoonal tropical Australia, seeds germinate with the onset of the wet season, usually October to December, and flowering occurs mainly in winter, with the first flowers opening in July.

Identification

  • An upright, branched shrub or small tree, usually 1 to 4 m tall, with milky sap and waxy-looking leaves and stems.
  • Young stems are smooth, greyish-green and covered in small whitish hairs; mature stems have light-brown, deeply fissured, cork-like bark.
  • The large, thick leaves are greyish-green, oppositely arranged and have smooth margins.
  • Clusters of white or pinkish flowers have darker purple or purplish-brown tips and a purplish, crown-shaped centre.
  • The greyish-green fruit pods are rounded to egg-shaped and split open at maturity, releasing flattened brown seeds with long, white, silky hairs.
  • Look-alike: Giant Calotrope (Calotropis gigantea) is similar but often grows taller than 4.5 m; its flowers have longer, narrower petals that are uniformly pink or mauve, unlike Calotrope’s shorter, broader whitish petals with dark-purple tips.

Where it grows

In Australia it is widespread in the drier parts of northern and central Australia, especially northern Queensland, north-western Western Australia and northern parts of the Northern Territory, and occurs in habitats including inland watercourses, grasslands, open woodlands and pastures.

Why it matters

  • It reduces the productivity of rangeland pastures and makes mustering more difficult.
  • It forms dense thickets on alluvial floodplains and spreads into adjacent over-grazed pastures.
  • It competes with native plant species and can transform the appearance of grassland communities.
  • Its milky sap is toxic to humans and sometimes to livestock.

How it spreads

  • It reproduces mainly by seed, and populations can also increase locally through suckering from the roots.
  • Seeds can travel long distances by wind and water.
  • Seeds may also be dispersed in mud sticking to animals and vehicles.

Control options

  • Established plants are difficult to eradicate because deep, large, spongy roots readily produce suckers and survive almost any treatment.
  • Mechanical control, including fire, is relatively ineffective; the plant is susceptible to regular cultivation.
  • Check with the local council or state or territory weed management agency about Calotrope control requirements.

Hygiene and washdown

  • Seeds may be dispersed in mud sticking to animals and vehicles.
  • 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: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Declared pest (no control category). Declared pest in WA without a control category, so there is no state-wide control requirement, but movement and keeping rules can still apply.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Declared weed, Class B. Class B: growth and spread must be controlled. All declared weeds are also Class C, so they must not be brought into the NT.
  • 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)
  • 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