Soursob is a bulbous perennial with conspicuous yellow flowers, and is a widespread weed in gardens, broadacre cropping and pasture




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Plant Policy
Soursob (Oxalis pes-caprae)

Soursob is a bulbous perennial with conspicuous yellow flowers, and is a widespread weed in gardens, broadacre cropping and pasture.



Weed Risk
Invasiveness
Since the forms of soursob naturalised in Australia do not produce seed, it is spread only as bulbs, which are moved in contaminated soil, nursery stock or attached to machinery. Consequently a control program by hygiene can be very effective at preventing the spread between properties.
Although it is widespread, many areas suited to its establishment are free of the weed.
Impacts
Soursob is no longer a major competitor with cereals and other broadacre crops, where it is readily controlled by herbicides.
Most local control programs involve extension as landholders see the value of keeping properties free of the weed. There is no longer a need for declaration to prevent spread in soil, pot plants, and for the eradication of infestations on non-crop areas, roadsides and gravel pits.
Although it grows in permanent pastures, it is very slow to spread without soil movement or cultivation.
Potential distribution
Soursob grows on a wide range of soil types, and in both heavy shade and exposed sites. It is adapted to winter rainfall and dry summers, and survives with an annual rainfall as low as 320 mm. It could potentially grow in most of the agricultural zone of South Australia apart from saline and rocky habitats.
Feasibility of Containment
Control costs
Soursob is now easily controlled in broadacre cropping by routinely used herbicides.
Persistence
Soursob can persist indefinitely at a site by vegetative reproduction.
Current distribution
Soursob is scattered throughout the southern parts of South Australia extending to the Flinders Ranges. It is commonest in the higher rainfall regions, including the Lower South East, Adelaide and Fleurieu region and Lower Eyre Peninsula, but more widely scattered on the west coast and Kangaroo Island.

State Level Risk Assessment

Assessment using the Biosecurity SA Weed Risk Management System gave the following comparative weed risk and feasibility of containment scores by land use:




Land use


Weed Risk

Feasibility of control

Response at State Level


Crop/pasture rotation

low

17


high

24


monitor

Grazing - southern

medium

90


medium

48


manage sites

Vegetables

low

25


high

21


monitor

Perennial horticulture

negligible

0


low

85


limited action

Native vegetation

low

28


low

76


limited action


Considerations

Soursob was introduced as a garden ornamental in the nineteenth century. It rapidly became a conspicuous weed in gardens, along roads and in the developing agricultural areas due to the lack of any hygiene controls on spread in soil and on machinery.


It is no longer a significant weed of cropping due to the introduction of sulfonyl-urea herbicides in the 1980s.
No Natural Resources Management Board has a current management plan to enforce control of soursob, or has found it necessary to use any of their powers under the Act to manage soursob in their region. Any control actions needed on properties are carried out at the discretion of land owners.
Therefore, soursob is not declared under the Natural Resources Management Act 2004 in South Australia.

Synonymy
Oxalis pes-caprae L., Sp. Pl. 1: 434 (1753)
Taxonomic synonyms:

Oxalis burmannii Jacq., Oxalis 41, t. 20 (1794).

Oxalis cernua Thunb., Dissert. Oxalide 14 (1781).

Oxalis grandiflora Arechav., Anales Mus. Nac. Montevideo 3: 238 (1900).

Hon Ian Hunter MLC

Minister for Sustainability, Environment and Conservation
Date: 3 January 2015



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