Quick Facts

Quick facts

• Originally from South Afica, Soursob (Oxalis pes-caprae) is a small, upright, perennial herb, growing to 400 mm high, with bright yellow flowers and leaves of 3 heart-shaped leaflets.
• It is a weed of agriculture (orchards, crops, pastures) and bushland, also occurring on roadsides, gardens, and and is widespread throughout all states of Australia.
• Soursob form dense stands (monocultures) and  is spread by movement of its bulbs and bulbils.
• It is an important weed of cereal crops, pasture and bushland and is poisonous to stock and mildly toxic to humans especially if eaten in large quantities.
• Control methods include hand removal and herbicides, but must be timed carefully to be effective.
• Large populations are normally controlled by herbicide application over several years to exhaust the bulbs and avoid soil disturbance and further spread of bulbils.

What Does It Look Like?

What is it?

Soursob (Oxalis pes-caprae) is a small, upright, perennial herb, growing to 350–400 mm high with an underground bulb producing many small bulbils (a bulbil is a small bulb that will produce a new plant). The leaves are dull to bright green, often with purplish brown flecks, speckles or markings on the upper surface. Leaves consist of 3 heart-shaped (bilobed) leaflets, each about 30 mm across and long (similar to clover leaf). Leaflets fold back at night and in low light (Parsons & Cuthbertson 2001). Each leaf is on a long (usually 50-200 mm long) round, hairless to sparsely hairy leaf stalk. Leaf stalks emerge from a vertical rhizome at the soil surface, or from the rhizome above the soil surface when the plants grow in shaded or  dense situations. The rhizome is vertical, white, fleshy, 50–100 mm long, x 2-10 mm diameter at the top, and tapers to a thread like root joined to the parent bulb. The white fleshy bulbs have a dark brown papery cover, are conical in shape and grow to 250 mm long (Brooks 2001). One to several small bulbils form in the axil of each rhizome bract (where the leaf joins the stem). The bulbils are similar to the bulb, but much smaller, white, developing on the rhizome above the bulb. On average a total of 20 bulbils are produced. The rhizome produces many fibrous true roots, some almost horizontally just under the soil surface, and others along and down the rhizome above the bulb, and  below the bulb from a tuber from which many other fibrous roots are produced (Parsons & Cuthbertson 2001).

Flowers occur in drooping clusters of 3-25 flowers on top of a single leafless stalk (Blood 2001; Parsons & Cuthbertson 2001) that rises just above the leaves. The flowers are bright yellow and trumpet-shaped with 5 petals united at the base. Each flower grows to 25 mm long and 40 mm diameter, opening in sunlight, following the sun and closing in low light.

Fruits and seeds (seed capsules) rarely develop in Australia (Blood 2001), although several records do exist o fruits developing in South Australia.

For further information and assistance with identification of Soursob contact the herbarium in your state or territory.

Flower colour

Yellow

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Soursob prefers heavier, fertile, well drained soils (Parsons & Cuthbertson 2001) with autumn, winter and spring rains. In Australia, it invades coastal heath vegetation, grassland, woodland and dry forest. It also occurs along roadsides, in gardens, orchards, crops and pastures (Thorp & Wilson 1998 – ). It tolerates temperatures down to -5^oC, sun and shade and prefers annual rainfall of over 330 mm (Blood 2001). It rarely invades undisturbed plant communities (Parsons & Cuthbertson 2001).

Are there similar species?

Soursob can be mistaken for clover when not in flower or for other weedy Oxalis species (Blood 2001). Oxalis flava also has yellow flowers but these are solitary and its leaves are fan-like with 4-7 narrow leaflets (Hussey et al. 1997). Oxalis compressa bears clusters of bright yellow flowers on long leafless stalks that are very similar to Soursob but it can be distinguished by its flattened hairy leaf stalk (Hussey et al. 1997).

Why Is It A Weed?

What are its impacts?

Soursob (Oxalis pes-caprae) is primarily a weed rapidly establishing and affecting cool to warm temperate, to semi arid, and sub-tropical cultivated or disturbed areas. It is not uncommon for infestations to have up to 3000 plants per square metre.

Agriculture: At large densities, Soursob is strongly competitive with cereal crops and has been measured in Western Australia to reduce the yield of wheat by 75% and oats by 87% (Parsons & Cuthbertson 2001). Yield losses in wheat of up to 50% have been recorded in South Australia. However, Soursob is no longer a major competitor with cereals and other broadacre crops, where it is readily controlled by herbicides and most landholders see the value of keeping properties free of the weed. Soursob is also important in pastures where it may replace almost all desirable plants from the time of the autumn rains to well into the spring potentially seriously reducing in carrying capacity. Although it grows in permanent pastures, it is very slow to spread without soil movement or cultivation.

Soursob is poisonous to stock due to a high concentration of oxalate in all its parts. The oxalate ion precipitates as insoluble calcium oxalate crystals in the brain or kidneys. Sheep are most susceptible at the start of the season when they are not accustomed to the plant. Sheep that have been exposed to Soursob are seldom seriously affected as they build up a bacterial population in the rumen capable of breaking down oxalates (Parsons & Cuthbertson 2001).

Some orchardists and vine growers value Soursob for the ability of its roots and rhizomes in helping break up clay soils, improving aeration and water penetration. It is also thought to be useful in honey production (Parsons & Cuthbertson 2001).

Native ecosystems: It is also an important weed of bush land, where its low, spreading growth in autumn quickly covers the ground, preventing germination of other seedlings and smothering surrounding plants (Blood 2001). Soursob has a severe impact on indigenous ground-flora with dense infestations eradicating most smaller plants. It occurs in medium to large populations in a number of different vegetative communities including; dry coastal vegetation and some dunes, heathland & woody heathland, lowland grassland & grassy woodland; dry sclerophyll forest & woodland, riparian vegetation, and rock outcrop vegetation, no doubt also impacting on the habitat of native fauna.

Urban areas: A common and persistent weed in gardens, lawns, on roads sides, and in amenity plantings, and neglected and abandoned areas. Known to be mildly toxic to humans, especially when consumed in larger quantities.

How does it spread?

Soursob is spread by movement of its bulbs and bulbils. Since the vast majority of 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. If left undisturbed, a patch of Soursob will spread perhaps 10 cm per year, which is due solely to sideways formation of tubers by the marginal plants. Spread is very rapid after disturbance due to translocation of bulbs and bulbils, which are only loosely attached to the rhizomes (Parsons & Cuthbertson 2001). The bulbs and bulbils can be transported by wind, water, birds, on machinery particularly during road works, during cultivation, in dumped garden waste and in contaminated soil and pots from nurseries (Blood 2001).

What is its history in Australia?

The first record of Soursob occurring in Australia was in 1841 in the Adelaide Botanic Gardens, where it was grown as an ornamental plant. It was recognized as a potential weed in the 1850s and by 1879 it was known as a serious weed of gardens and wheat fields in parts of South Australia. It may have arrived in Western Australia earlier than this, with vines and fruit trees which came from South Africa and were planted in the Swan Valley in 1832 (Parsons & Cuthbertson 2001).

How To Manage It?

Best practice management

Soursob (Oxalis pes-caprae) is difficult to eradicate unless control methods are well timed and persistent over several years. The plant must be attacked at a critical stage in its life cycle called the bulb exhaustion stage. This is when the food material of the  bulb is exhausted and any new bulbils are not sufficiently developed to survive. Unfortunately this stage cannot be accurately determined by looking at the aerial growth, only by digging up plants and inspecting the bulbs. The bulb exhaustion stage is just at, or just prior, to flowering. Unfortunately, not all plants in an infestation will be at the same stage at the same time. Any control method, be it cultivation or chemical, should be aimed at this critical stage (Muyt 2001; Parsons & Cuthbertson 2001; Brown & Brooks 2002).

Chemical control: Chemical control of Soursob is often the most practical option available for dense infestations in bushland and in cropping areas. It is practical on both a large and small scale, avoids soil disturbance and prevents soil erosion (Parsons & Cuthbertson 2001; Brown & Brooks 2002).

Foliar sprays:  Spot spray is the method of application. Apply at bulb exhaustion stage, generally just on flowering. Read the manufacturers' labels and material safety data sheets before using herbicides.

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au . NOTE: Training is normally a requirement in most states and territories for all or some of the methods.

Non-chemical control: Physical control: Occasional Soursob plants may be carefully dug out, but hand removal after bulbil formation will dislodge bulbils and contribute to their spread (Muyt 2001; Brown & Brooks 2002). Exercise care if manually removing as physical removal can result in further spread of bulbils. Repeated pulling of the tops will deplete the bulb’s carbohydrate reserves, but these efforts take years to be successful.

Mechanical control: Cultivation can provide control on new infestations. Repeated tillage is required to effectively control the bulbs.

Cultural: Grazing is not considered an effective control option. Plants contain variable quantities of soluble oxalates and can be lethally toxic to livestock when ingested in quantity.

Fire: Burning is also not considered to be an effective control option.

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Soursob bulbs sprout in autumn before or after soaking rains. The bulb first produces a slender stem or rhizome which thickens as it reaches the soil surface. Aerial growth commences as a rosette of leaves, the leaf stems becoming erect during early winter, at which time flowering stems are also produced. Flowering occurs from June to October after which rising temperatures kill the aerial growth (Blood 2001; Parsons & Cuthbertson 2001).

During early growth, one of the roots at the base of the bulb swells to form a translucent, brittle, contractile tuber several centimetres long. A new bulb is formed at the top of this tuber and, in developing, draws on moisture and minerals from the tuber which eventually shrivels and contracts, drawing the new bulb deeper into the soil (Parsons & Cuthbertson 2001). During the growing season it is common for two bulbs to develop inside the old bulb and one on the tuber. Many bulbils, often more than 20, develop on the rhizomatous stem above the bulb (Brooks 2001).

Where Is It Found?

Which states and territories is it found?

Found in all Australian states and territories

What areas within states and territories is it found?

Soursob occurs in all States and Territories of Australia. In South Australia it occurs in the Adelaide Hills, the northern wheatbelt to the Flinders Ranges, central Eyre Peninsula, and the South East. In Western Australia it is associated particularly with the Avon, Dale, Moore and Greenough River systems. It is common throughout Victoria and in southern New South Wales. It is less widespread and more scattered (mostly in urban areas) in Tasmania, Queensland and the Northern Territory (Parsons & Cuthbertson 2001).

Where does it originate?

Soursob originated in South Africa and, with several other species of Oxalis, was taken to Europe in the 18^th century for promotion as an ornamental plant. It soon became established as a weed in southern Europe where it is common in orchards, vineyards and cereal crops, particularly in Italy, Greece, Malta and Portugal. From Europe it has spread all over the world including Australia and it is also a troublesome weed in other including North and South America, New Zealand, Morocco, India, Israel, Lebanon, Tunisia and even in its native South Africa (where it is proclaimed a noxious weed) (Parsons & Cuthbertson 2001).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

VIC

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Oxalis pes-caprae

Other scientific names (synonyms)?

Oxalis cernua Thunb.

Does it have other known common name(s)?

Bermuda Buttercup, Buttercup Oxalis,  Cape Cowslip, Geelsuring, Oxalis, Sorrel, Sourgrass, Yellow-Flowered Oxalis, Yellow Sorrel