Soursob

Soursob is a perennial bulbous weed that grows from a small brown mother bulb buried deep in the soil, which produces a cluster of tiny bulbils around it each season. The leaves are clover-like, made up of three heart-shaped leaflets on long stalks, often with pale flecking or a slightly folded appearance, and they have a sour, tangy taste and smell when crushed due to high oxalic acid content. Flowers appear from late winter through spring, held above the foliage on slender stems, bright yellow with five petals and a funnel shape, usually in small clusters. In South Australia and parts of Victoria the flowers are typically sterile and do not set viable seed, so the plant relies almost entirely on underground bulbils for reproduction and spread.

This weed is widespread across southern Australia, particularly in South Australia, Victoria, and parts of New South Wales and Western Australia, thriving in the cooler, wetter months of autumn, winter and early spring. It dies back completely over summer, with the mother bulb and bulbils lying dormant underground until the following autumn break, when new growth emerges rapidly from disturbed or undisturbed soil alike.

Soursob is a serious weed of pastures, gardens, roadsides and cultivated ground because the bulbils are extremely persistent, resistant to frost and drought, and easily spread by cultivation, mowing equipment, and moving soil or turf that contains them. Each mother plant can produce dozens of small bulbils, and even a small fragment left in the soil after removal is enough to establish a new infestation the following season.

Management relies on persistence over several seasons because the underground bulbils survive most single treatments. Physical removal by digging is difficult since bulbils sit at varying depths and easily detach and remain in the soil. Smothering with dense mulch or competitive planting can reduce vigour over time, and repeated removal of top growth before flowering helps prevent the mother bulb from replenishing its energy reserves, gradually weakening the infestation.