Found in range of locations from gardens to roadsides, the noxious weed Oxalis pes-caprae or soursob is a concern primarily in pastures and amongst indigenous vegetation. It is distinguished by three heart-shaped leaflets with or without stalks that fold in dull days or at night. Flowers are bright yellow in colour and open in sunlight and close at night. There are masses of underground bulbs which are spread by water, birds, in dumped garden waste and during cultivation (not to be confused with native varieties of oxalis which have no bulbs).
Although primarily a weed affecting cultivated areas, soursob in pastures has the potential to replace almost all desirable plants from the time of autumn rains to well into the spring reducing carrying capacity. Stock will selectively graze more palatable species than the soursob with its high oxalate content. Soursob has severe impacts on indigenous ground-flora with dense infestations eradicating smaller plants.
In terms of its control, soursob would have to one of the most, (if not the most!) difficult weeds to eradicate. Control is made problematic for two main reasons; firstly it is the vigorous network of underground bulbs that render hand weeding or mechanical removal ineffective. Secondly it is the irritating life cycle of the plant that limits effective herbicide use to a one week window at the bulb exhaustion stage. A one week window that could open at any time between May and July. Suffice to say soursob is a weed that takes several years to control. Another suggested method of control involves using a combination of light exclusion and raking however this would be suited to a garden situation. Cultivation and grazing are generally ineffective means of control.The optimum time for herbicide control may have come and gone for 2010 but the weed is no more apparent than now. Take the time to check your priority areas of land. Soursob is one to keep in the back of your mind, as spring and summer pass you'll be well prepared to strike with vengeance come autumn next year. Information on how to determine the bulb exhaustion stage will be provided as part of weed talk next year (prior to the optimum control time). If you would like further information on soursob please contact Kate Williams, Project Officer with the South Gippsland Landcare on (03) 5613 5973 or email firstname.lastname@example.org