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Sicklepod (Senna obtusifolia)

Sicklepod is an aggressive pasture weed that can dominate grasses and reduce carrying capacity by as much as 85%. It is toxic to cattle if eaten and is also a concern in native grassland and other open ecosystems in Far North Queensland.

Key facts

Type: Weed, shrub.

Native to: Caribbean region and tropical America.

Established in: QLD, WA, NT.

Other names: Java Bean, Sicklepod Senna, Sickle Senna, Chinese Senna.

Best time to act: Flowering and fruiting occurs from February to August; look for plants then and slash before seed set, preferably during flowering, while targeting herbicide to seedlings or new regrowth.

Identification

  • It is a vigorously growing woody shrub, usually 1.5 to 2.5 m tall and about 1 m wide.
  • Its compound leaves have two to four opposite pairs of leaflets, with rounded tips and wedge-shaped bases.
  • Look for one or two erect glands between the lowest pairs of leaflets.
  • The small yellow flowers are about 1 to 1.5 cm across.
  • The brownish-green mature pods are sickle-shaped, 10 to 25 cm long and 2 to 6 mm wide; when ripe, they burst open and shed shiny, flattened dark brown seeds.
  • Look-alike: Coffee Senna has pointed leaflets and upward-curving seed pods that are square in cross-section.
  • Look-alike: Candle Bush has leaves up to 60 cm long with eight to fourteen pairs of leaflets, and upward-angled pods with prominent wings along the sides.

Where it grows

In Australia it occurs mainly in coastal and sub-coastal northern Queensland and the Northern Territory, and also in central and southern Queensland and northern Western Australia, especially at disturbed sites, roadsides, waterways, waste areas, crops and pastures in wetter tropical and subtropical environments.

Why it matters

  • It can completely dominate pasture grasses, reduce carrying capacity by as much as 85%, and leave properties unproductive.
  • Sicklepod is generally unpalatable to stock but is toxic to cattle if eaten.
  • In sugar cane, it can significantly affect yield, and woody plants in harvested cane can cause machinery breakdowns at sugar mills.
  • In Far North Queensland it is a concern in native ecosystems and is threatening small remnant areas of native grassland in the Wet Tropics.

How it spreads

  • Cattle and horses may eat mature seeds, which can pass through the animals and germinate in dung; this is the most common way seed spreads between properties.
  • Ripe pods release seeds explosively, and longer-distance spread occurs mainly by water and mud, and in harvested sugar cane carried by bins on tramways and by harvesters, vehicles and machinery.
  • A plant can produce about 8,000 seeds, which may remain viable for up to 10 years and build up a large soil seed bank.

Control options

  • Use registered herbicides on seedlings and new regrowth; the thick cuticle on mature plants limits herbicide effectiveness.
  • Slash before seed set, preferably while plants are flowering; blunt blades help shatter stems, and spraying regrowth after slashing has been successful.
  • Rotary hoeing or discing followed immediately by sowing improved pasture can be effective if the grasses are well managed; ploughing can scarify seeds and cause a flush of plants.
  • Use fire only as part of a controlled, strategic management plan, as it can scarify seeds and trigger many seedlings.

Hygiene and washdown

  • Clean all machinery on site after mechanical control to reduce the risk of carrying seed into clean areas.
  • Be alert to seed spread in mud and on vehicles and machinery.
  • 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: 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. 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)
  • 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