Soursob

Oxalis pes-caprae L.

Synonyms - Oxalis cernua.

Family: - Oxalidaceae.

Names:

Oxalis is from the Greek oksos meaning sour and refers to the taste of the leaves and stems.
Pes-caprae is from the Latin pes meaning foot and caprae meaning goat and refers to the goat foot shape of the folding and deeply lobed leaflets.
Soursob refers to its sour taste and may be derived from soursop, a North American fruit with an acid taste.

Other names:

African Wood Sorrel
Bermuda Buttercup (NZ, USA)
Buttercup Oxalis (UK, USA)
Cape Cowslip (UK)
Geelsuring (S. Africa)
Oxalis
Soursop
Sour Grass
Sorrel (S. Africa)
Variable Wood Sorrel
Wet The Bed
Yellow-flowered Oxalis
Yellow Sorrel.

Summary:

An almost hairless plant with long-stalked, trifoliate leaves that often have dark spots. The radiating clusters of drooping yellow, 5 petalled flowers are in clusters of 3-16 flowers on long slender stalks held well above the leaves from June to November. There are 10 stamens and 5 styles. The fruit is a narrow capsule. It has annual tops with a perennial root system with a vertical rhizome, bulbils and a bulb.

Description:

Cotyledons:

Two. Rarely seen in Australia.

First leaves:

Trifoliate, similar to older leaves. Arise from the bulb.

Leaves:

Forms a rosette of 10-40 leaves, each with 3 leaflets that are 10-45 mm diameter overall. The rosette may be up to 250 mm wide.
Stipules - None.
Petiole - 60-150 mm long, round, broadened and jointed near the base where it attaches to the tuber. Has a few fine hairs. Leaflets have a very short or no petiolule.
Blade - Of leaflet, heart shaped, 8-25 mm long by 9-35 mm wide, deeply 2 lobed, green and often with dark purple or brown spots on top, lighter green underneath. Few hairs on edges and underside, hairless on topside. Leaves fold up and droop at night and in dull situations.

Stems:

Underground rhizome which occasionally may be above ground in dense, shady infestations. When plants are growing in shade or are crowded it may be above ground.
Flower stem - Up to 350 mm tall, often drooping.

Flower head:

Flowers on 10-20 mm long, leafless, stalks (pedicels), in clusters (umbels) of 3-16(25) on the end of erect, 150-350 mm long, succulent stalks (peduncles), emerging from the centre of the plant, with a few fine hairs. Pedicels may have glandular hairs.

Flowers:

Yellow, stalked, drooping, trumpet shaped with 5 petals that open in sunlight, follow the sun and close as light fades.
Bracts - parallel sided, 2-3 mm long with orange calli near the tips.
Ovary - Oblong, hairy on the top half. Styles with simple and glandular hairs.
Sepals - Narrowly egg shaped, 5-8 mm long, hairy with 2 orange calli at the tip.
Petals - Yellow, trumpet shaped with 5 spreading lobes, 10-35 mm long by 40 mm wide. Close in dull conditions and at night.
Stamens - 10 in 2 rings. Outer ones shorter, 3-4.5 mm long and opposite the petals, others 5-7 mm long. Filaments joined at the base. Filaments with teeth and glandular hairs.
Anthers - Release pollen by a longitudinal slit.

Fruit:

Rarely produced.
Oblong capsule tapering to a fine tip that rarely matures to form viable seed. Seeds expelled explosively up to 2000 mm through longitudinal slits leaving the valves attached to the plant.

Seeds:

Rarely formed. Known from some sites in WA but not elsewhere in Australia.

Roots:

Soursob has a complex underground structure. Just below the ground is a carrot like, vertical rhizome that produces leaves from the top and bulbils on its sides. Below the rhizome a thread like root connects it to the parent bulb. Within the exhausted parent bulb the new season bulb forms on top of a long, fleshy, vertical contractile tuber or 'root'.
The rhizome is vertical, white, fleshy, 50-100 mm long by 2-10 mm diameter at the top and tapers to a thread like root joined to parent bulb. The rhizome has many fibrous true roots near its base. One to several small bulbils form in the axil of each rhizome bract. On average a total of 20 bulbils are produced. In shaded and crowded situations the rhizome may be above ground.
The bulb is egg shaped to cylindrical with a pointed top, 8-30 mm long by 10 mm diameter, made up of white segments that are initially fleshy and becoming hard. It is covered with thin brown scales.
Bulbils are similar to the bulb but much smaller and produced on the vertical rhizome.
The contractile tuber is fleshy, translucent, brittle, 100-200 mm long and 10-20 diameter and has the new seasons bulb attached to its top in spring. It has many fibrous roots also. As the root dries it contracts pulling the new bulb deeper into the soil.

Key Characters:

Trifoliate leaves with notched leaflets. No aerial stems. Leaves and peduncles radical. Yellow flowers, 10-25 mm long, in umbels of 3-16 flowers. Roots swollen into tubers. Many underground bulbils.

Biology:

Life cycle:

Annual top growth and perennial bulbs. Bulbs sprout over a 6 week period from autumn to winter and produce a slender underground stem which thickens to form the vertical rhizome. The top of the rhizome which is normally at ground level produces a rosette of 10-40 leaves low lying leaves that crowd companion plants. Flower stalks (peduncles) also arise from the top of the rhizome and emerge from June to September. Flowering may start in June and continue until October. During late winter and spring bulbils are formed on the rhizome and remain dormant until the following autumn. After flowering commences a new bulb starts to form. A contractile tuber forms underneath the new bulb. As moisture becomes limiting and temperatures rise in summer the top growth dies and the contractile tuber contracts drawing the new bulb deeper into the soil. The area is left relatively bare over summer allowing water flows and wind to disperse the bulbils. In mature stands many bulbs are up to 100 mm deep. When adult plants die, dormant corms will sprout making control more difficult.
Plants sprouting from bulbils produce a bulb in their first year and don't flower.

Physiology:

Top growth may contain 14.6% of oxalate on a dry weight basis.
Moderately frost sensitive tolerating temperatures to -50C.

Reproduction:

By bulbs and bulbils.

Flowering times:

Winter to spring in western NSW.
June to October in SA.
June to October in WA.
Mainly June to November in SE Australia.

Seed Biology and Germination:

Rarely produces seed.

Vegetative Propagules:

Produces many bulbs and bulbils. 10-50 bulbils per plant are produced each year.
Spread when contaminated soil is moved.
Bulbils and bulbs have no dormancy, that is all bulbs and bulbils germinate or die each year.
Bulbs can establish from 600 mm deep in sandy soils.

Hybrids:

Several forms exist. One produces some seed compared to others that are essentially sterile.
There are three forms based on style length. The short, mid and long style types and these vary in their sensitivity to herbicide.

Allelopathy:

Population Dynamics and Dispersal:

Spread by bulbils and bulbs.
It is capable of spreading rapidly.
Water flows carry bulbils downstream and along flood ways.
Road making and earth works spread contaminated soil.
Much spread has been due to contaminated nursery soil in pots and as soil around fruit trees and intentional planting as an ornamental.
Ants move some bulbils.
Rarely establishes in undisturbed bushland.
Sheep and birds also move bulbils in adhering mud and when the eat the bulbils New infestations are often found under trees.
If undisturbed, Soursob doesn't usually spread more than 100 mm per year.
Cultivating equipment, especially tyned machines cause rapid spread of the bulbils around the field.
Often spread by dumping of garden refuse.
Densities of 3000 plant m-2 have been recorded.
Up to 4500 kg/ha of bulb and bulbils can be produced in a year.

Origin and History:

South Africa.
Introduced to SA in 1841, recognised as a weed in the 1850's and by 1879 was a serious weed of wheat and gardens.
Probably introduced to WA around 1832 as a contaminant in soil around vines and fruit trees.

Distribution:

ACT, NSW, NT, QLD, SA, TAS, VIC, WA.
In SA 1.25 million hectares are infested.
It occurs in QLD but tends to die out with time.
Africa, England, India, Mediterranean Europe, New Zealand, Portugal, USA

Courtesy Australia's Virtual Herbarium.

Habitats:

Dry coastal vegetation, heathland, heathy grassland, lowland grassland, grassy woodland, dry sclerophyll forest, dry sclerophyll woodland, rocky outcrops, riparian areas.
Full sun to shady areas.

Climate:

Temperate. Mediterranean. Subhumid warm temperate. Semi-arid warm-temperate. Subtropical.
Prefers frost free areas.
Most abundant in areas with more than 330 mm rainfall.

Soil:

Most abundant on sandy soils and red granitic loams.
Well drained soils

Plant Associations:

Significance:

Beneficial:

Ornamental.
Honey plant.
Flowering stems chewed by children for the sour taste.
On heavy soils in vineyards and orchards the rhizomes and tubers are reported to improve aeration and water infiltration and reduce the growth of other weeds.

Detrimental:

Major weed of crop reducing yield due to competition.
Weed of pastures, orchards, vineyards, vegetables, fallows, gardens, roadsides and disturbed areas.
Invasive weed of bushland often forming monoculture patches that can displace most native species after disturbance.
Unpalatable, sour taste.

Toxicity:

High oxalate content. May be toxic to stock causing oxalate poisoning. Breeding ewes are most likely to be affected.
Most reports are from situations where hungry, unaccustomed stock are exposed to dense infestations.
Acute toxicity is due to low blood calcium levels. Chronic toxicity is due to sharp oxalate crystals piercing the kidneys and causing scarring.

Symptoms:

Usually found dead lying on brisket with head stretched forward and back legs stretched backwards with slime from nostrils. Otherwise stiff gait, loss of control of hindquarters then forequarters, muscle trembling and rigidity especially around the neck and head, heavy breathing, collapse and death.

Treatment:

Calcium borogluconate 40% injected into the veins or under the skin. 60 mL for sheep and 300 mL for cattle. Must be applied as soon as possible before kidneys are damaged.
This may be supplemented with 250-1000 mL lime water or 2-5 g chalk in water or 60-100 g Epsom salts given by mouth for sheep.
Recovery may take weeks or not occur.

Legislation:

Noxious weed of SA, VIC, TAS and WA.

Management and Control:

Soursob is difficult to control and will take several years.
Herbicides provide the most effective method of control.
Cultivation is generally ineffective and if tyned implements are used it normally spreads the infestation. Ploughing at the bulb exhaustion stage and repeated 4 weeks later can result in partial control if a vigorous crop is planted after the last ploughing.
Bulb exhaustion stage can only be determined by digging up and inspecting plants. At this stage the parent bulb will have flaky segments rather than fleshy segments and the new bulb will be less than 2 mm wide with little or no contractile tuber or root below them. It usually occurs from mid May to late July and only lasts for about a week.
Grazing is usually an ineffective means of control and often leads to denser infestations because stock selectively graze more palatable species.

Thresholds:

Cereal yield losses of 50-75% are common in dense stands in SA and WA. Growth rate appears to be more important than density in determining the yield loss.

Eradication strategies:

Plants can't be hand pulled successfully because small bulbils break off.
Dig out small areas taking the plant and the soil around each plant containing bulbils. This is best done at bulb exhaustion stage which is usually just before flowering. Remove bulbs and bulbils or fumigate the soil or bury the soil more than 1000 mm deep.
To determine the bulb exhaustion stage, dig up some plants and check that;
a) the adult bulb is shrivelled
b) the contractile root under the bulb has not formed or is very thin and
c) the bulbils on the rhizome above the bulb are less than 1 mm diameter.
Larger areas require repeated spraying with diuron and sulfonyl urea herbicides over several years.
Apply 1 L/ha glyphosate 450g/L in July or August and "hay freeze" the pasture in the season before crop. After the break in the following season use 1 L/ha glyphosate 450g/L plus 500 g/ha Diuron 850g/kg plus 250 mL Pulse Penetrant plus 2 kg ammonium sulphate per 100 L of spray mix. Plant to wheat. Apply 20 g/ha chlorsulfuron 750g/kg when the wheat is at the 4 leaf stage. In the following year use 1 L/ha glyphosate 450g/L plus 1500 g/ha Diuron 850g/kg plus 250 mL Pulse Penetrant plus 2 kg ammonium sulphate per 100 L of spray mix. Plant to lupins, peas or faba beans.
In bush land areas

Herbicide resistance:

Resistance is not expected to develop because it doesn't produce seed.

Biological Control:

Related plants:

There are six Oxalis species that are native to Australia.
Bowie's Wood Sorrel (Oxalis bowiei)
Chilean Wood Sorrel (Oxalis perdicaria)
Coastal Oxalis (Oxalis radicosa) is an Australian native species.
Finger-leaf Oxalis (Oxalis glabra) is a herb with an erect thin leafy stem and single large flowers held above the leaves. The leaflets are small and narrower than those of the other species. The flowers are pink to purple (or occasionally white) with a yellow throat. It is a weed of heavier soils in disturbed woodland and occurs from Perth to York, south to Augusta and around Kojonup.
Fishtail Oxalis (Oxalis latifolia) has about 12 purplish pink flowers and fishtail shaped leaves.
Hairy Wood Sorrel (Oxalis hirta)
Large-flowered Wood Sorrel or Four O'clock (Oxalis purpurea) has rosettes of leaves that may be tinged purple on the underside, and arise from a bulb. The flowers occur singly and are usually pink to purple with a yellow throat but sometimes white with a yellow throat.
Native Oxalis (Oxalis perennans) similar to O. corniculata but is a native species that has yellow flowers, woody stems and a stout, tuberous, brown rootstock.
Pale-flowered Oxalis (Oxalis incarnata) is a delicate sprawling herb with clusters of green leaves at the ends of the stems and single white to pale pink flowers held above the leaves. It is often a weed of woodland or Karri forest.
Pink Bulb Soursob (Oxalis flava)
Pink Shamrock (Oxalis corymbosa or Oxalis debilis)
Shamrock Oxalis (Oxalis articulata)
Soursob (Oxalis pes-caprae) is a herb which grows from tubers and bulbs, with clusters of yellow flowers radiating from a tall stalk held above the tuft of long-stalked leaves. The leaflets sometimes have dark markings.
Yellow Wood Sorrel (Oxalis corniculata) is a creeping herb with much-branched and sometimes reddish stems that root at many points. Young stems usually very hairy. It has single flowers or small clusters of yellow flowers occurring among the leaves. Style is at the same level as the anthers.
Oxalis bifurca
Oxalis brasiliensis
Oxalis caprina
Oxalis compressa
Oxalis chnoodes is an Australian native species.
Oxalis depressa an environmental weed.
Oxalis exilis is an Australian native species often misidentified as O. corniculata. It has a white to brown carrot shaped rhizome, large yellow flowers with long styles that exceed the anthers
Oxalis lactea
Oxalis rubens is an Australian native species.
Oxalis violacea is often incorrectly identified as Oxalis latifolia.
Oxalis tetraphylla
Oxalis thompsoniae is an Australian native species.
No native Oxalis species have round tubers in WA.

Plants of similar appearance:

Clover, Trefoils, Medics.

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P194, 196. Photo.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P484. Diagram.

Blood, K. (2001). Environmental weeds: a field guide for SE Australia. (CH Jerram & Associates, Australia). P156-157. Photo.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P435-436. Photo.

Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney). P560-562. Photo.

Gardner, G.A. and Bennetts, H.W. (1956). The toxic plants of Western Australia. (West Australian Newspapers Ltd, Perth). P116-119. Diagram.

Gilbey, D. (1989). Identification of weeds in cereal and legume crops. Bulletin 4107. (Western Australian Department of Agriculture , Perth). P55. Photos.

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P192-193. Photo.

Hyde-Wyatt, B.H. and Morris, D.I. (1975). Tasmanian weed handbook. (Tasmanian Department of Agriculture, Hobart, Tasmania). P100-101. Diagram.

Hyde-Wyatt, B.H. and Morris, D.I. (1980) The Noxious and Secondary Weeds of Tasmania. (Tasmanian Department of Agriculture, Hobart, Tasmania). P74-75. Diagram.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #919.15.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P494-495. Diagram.

McBarron, E.J. (1983). Poisonous plants. (Inkata Press, Melbourne). P52. Diagram.

Meadly, G.R.W. (1965). Weeds of Western Australia. (Department of Agriculture - Western Australia). P100-103. Diagrams. Photo.

Moore, J.H. and Wheeler, J.R. (2002). Southern Weeds and their Control. Photos.

Muyt, A. (2001). Bush Invaders of South-East Australia: a guide to the identification and control of environmental weeds found in South East Australia. (R.G and F.J. Richardson, Australia). P111-112. Photos.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P529-533.

Peirce, J.R. (1997) The Biology of Australian Weeds. 31. Oxalis pes-caprae L. Plant Protection Quarterly, 12:110-119. Diagrams.

Wilding, J.L. et al. (1987). Crop weeds. (Inkata Press, Melbourne). P126-127. Diagrams. Photos.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.