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Soursob (Oxalis pes-caprae)

Soursob is widespread across Australia and can form dense stands in crops, pastures and bushland. It competes with crops and pasture plants, harms native ground flora and is poisonous to stock; it is mildly toxic to people if eaten in large quantities.

Key facts

Type: Weed, herb.

Native to: South Africa.

Established in: NSW, VIC, TAS, SA, WA.

Other names: Bermuda Buttercup, Buttercup Oxalis, Cape Cowslip, Yellow Sorrel.

Best time to act: Look for it during its autumn-to-spring growth, with flowering from June to October, and target treatment just before or around flowering when the bulbs are at the exhaustion stage.

Identification

  • A small, upright perennial herb that grows to about 350–400 mm high.
  • Its dull to bright green leaves have three heart-shaped leaflets, often with purplish-brown flecks or markings.
  • Bright yellow, trumpet-shaped flowers grow in clusters on a single leafless stalk above the leaves, usually from June to October.
  • Underground, it has a white fleshy bulb and rhizome that produce small bulbils.
  • Look-alike: It can be mistaken for clover when not in flower or for other weedy Oxalis species.
  • Look-alike: Oxalis flava has solitary yellow flowers and fan-like leaves with 4–7 narrow leaflets; Oxalis compressa has similar flower clusters but flattened, hairy leaf stalks.

Where it grows

In Australia it occurs in all states and territories, in coastal heath, grassland, woodland and dry forest, as well as roadsides, gardens, orchards, crops and pastures.

Why it matters

  • Dense infestations compete with cereal crops; recorded yield reductions include up to 75% in wheat and 87% in oats in Western Australia, and up to 50% in wheat in South Australia.
  • In pastures, it can replace desirable plants from autumn rains into spring and reduce carrying capacity.
  • In bushland, dense growth can smother plants, prevent other seedlings from germinating and eradicate much of the smaller indigenous ground flora.
  • All parts contain oxalate and are poisonous to stock, especially sheep not accustomed to the plant; it is mildly toxic to people if eaten in large quantities.

How it spreads

  • It is spread mainly by bulbs and bulbils, as seed rarely develops in Australia.
  • Bulbs and bulbils can move in contaminated soil, nursery stock and pots, on machinery, during cultivation or roadworks, and in dumped garden waste; they can also be transported by wind, water and birds.
  • An undisturbed patch may spread about 10 cm a year, but spread can be rapid after disturbance moves the loosely attached bulbs and bulbils.

Control options

  • Time treatment for the bulb exhaustion stage, generally just before or around flowering; this stage is difficult to judge without digging up and inspecting plants.
  • Use registered herbicides according to the label; spot-spraying at the right stage is a practical option for dense infestations.
  • Large populations usually need herbicide treatment over several years to exhaust the bulbs; avoid soil disturbance that can spread bulbils.
  • Carefully dig out occasional plants before bulbils form, or use repeated pulling or tillage; these approaches may take years, and grazing and burning are not considered effective controls.

Hygiene and washdown

  • Clean machinery and equipment after working in an infestation, particularly after cultivation or roadworks, to avoid carrying bulbs and bulbils to new sites.
  • Avoid moving contaminated soil, nursery stock or pots, and dispose of infested garden waste so it cannot spread the plant.
  • 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: Restricted weed. Where it is a restricted weed, trade in it (including the plant, its seed or contaminated material) is prohibited.
  • 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: 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.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • 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

Sources