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Castor oil plant (Ricinus communis)

Castor oil plant is naturalised across mainland Australia and can invade pastures, reducing grazing productivity. Its seeds, leaves and flowers are poisonous to people and animals, and it can outcompete native plants.

Key facts

Type: Weed, shrub.

Native to: North-east tropical Africa.

Established in: QLD, NSW, ACT, VIC, SA, WA, NT.

Other names: Castor bean, Castor oil bush, Palma-christi, Mamona.

Best time to act: Seeds usually germinate in spring to early summer, and actively growing plants are susceptible to herbicide treatment.

Identification

  • A hairless annual or short-lived perennial shrub that can grow to 6 metres tall.
  • Its large, glossy leaves are divided into 7 to 9 pointed segments with toothed edges and conspicuous veins.
  • Male and female flowers grow on the same plant in upright clusters, with female flowers at the top and male flowers at the bottom.
  • Its spherical fruits are 1 to 3 cm across, have soft red or green spines, and usually contain three smooth, mottled grey and silvery-white seeds.
  • Look-alike: Bellyache bush (Jatropha gossypifolia) has stems and leaves covered in sticky hairs, leaves with 3 to 5 lobes, and smooth fruits 1.0 to 1.2 cm in diameter.

Where it grows

In Australia it occurs in scattered populations across all mainland states, commonly along watercourses and floodplains, and in neglected areas around towns, old fields and roadsides.

Why it matters

  • The seeds are extremely poisonous to people and livestock, and the plant is also poisonous to dogs and other animals.
  • It can invade pastures, especially those on alluvial soils, causing loss of grazing land and productivity.
  • Dense stands along river banks can outcompete native plants and reduce habitat and food for native animals.

How it spreads

  • Ripe capsules release seeds explosively, scattering them over several metres.
  • Seeds can travel longer distances in moving water or in soil carried by animals and machinery.

Control options

  • Hand pull or dig out seedlings and young plants; remove as much of the root as possible, especially when soil is damp.
  • Large areas can be slashed or mown and followed by shallow cultivation; check for regrowth and new seedlings.
  • Registered herbicides can be used on actively growing plants, including stump treatment for large individual plants.
  • Wear protective clothing, gloves and eye protection, and follow up control for 4 to 5 years.

Hygiene and washdown

  • Avoid moving soil that may contain castor oil plant seeds on machinery or other equipment.
  • 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 1 of 11 regions. Regional priority weed in Murray. 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: 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, 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)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • 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
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources