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Spinyhead sida (Sida acuta)

Spinyhead sida is a serious weed of crops and pastures in the Top End of the Northern Territory, and is also a weed of pastures and sugarcane elsewhere in Australia. It is a prolific seeder, and infested areas are often ignored by stock, allowing it to persist and spread.

Key facts

Type: Weed, shrub.

Native to: Central America.

Established in: QLD, NSW, WA, NT.

Other names: Spiny-head Sida, Spiny-headed Sida, Sida.

Best time to act: In northern Australia, seed mostly germinates in October–November and plants mostly flower in mid to late summer, so look for plants and carry out control before flowering.

Identification

  • It is an erect annual or perennial shrub, usually about 1 metre tall, with woody stems that branch several times and a well-developed taproot.
  • Its leaves are lance-shaped, tapered at both ends, with serrated margins.
  • Yellow flowers usually grow singly or in pairs in the leaf axils.
  • The seed capsules split into five to eight portions called mericarps, each with two sharp awns about 1.5 mm long.
  • Look-alike: It may be confused with Flannel Weed (Sida cordifolia), Sida rhombifolia and Sida spinosa; S. cordifolia has retrorsely barbed awns, while S. rhombifolia has 7–11 mericarps and flower stalks longer than 7 mm.
  • Look-alike: Sida acuta has 5–7 glabrous mericarps and quickly loses its hairs as it matures; adult S. spinosa plants and their mericarps are generally hairy.

Where it grows

In Australia it is widespread in tropical and subtropical northern regions, especially in open scrublands and disturbed places such as roadsides, campsites, degraded pastures, tree plantations and cultivated areas.

Why it matters

  • It is a serious weed of crops and pastures in the Top End and a weed of pastures and sugarcane in Australia.
  • Stock graze it occasionally but usually ignore infested areas, eating more of the remaining plant species and favouring the weed's spread and persistence.
  • It invades open woodlands and competes strongly with other plants, in part through its tough woody stems and deep taproots.

How it spreads

  • Seeds are enclosed in two-awned mericarps that readily attach to fibrous material, including clothing and animal fur.
  • It also spreads in hay and pasture seed, in mud on footwear, machinery and vehicles, and through animal hooves and manure.

Control options

  • Grub small outbreaks before flowering, cutting the root well below the surface to prevent or minimise regrowth.
  • Repeated cultivation can control larger areas, and slashing before flowering helps reduce seed production.
  • Well-fertilised improved pastures can suppress its growth; choose pasture species carefully because Buffel Grass is also a serious environmental weed.
  • The leaf-feeding beetle Calligrapha pantherina provides substantial control in coastal and near-coastal areas of the Northern Territory; consult the APVMA for chemical information and use only registered herbicides.

Hygiene and washdown

  • Check and clean footwear, machinery and vehicles to remove seeds or seed-bearing mud before moving from infested areas.
  • Avoid moving contaminated hay or pasture seed, which can carry Sida seed.
  • 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: 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
  • 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