Journal of the Department of Agriculture, Western Australia, Series 4

Volume 1 Number 10 October, 1960 Article 8

1-1-1960

Soursob (Oxalis pes-caprae L.)

G R W Meadly

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Recommended Citation Meadly, G R W (1960) "Soursob (Oxalis pes-caprae L.)," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 1 : No. 10 , Article 8. Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol1/iss10/8

This article is brought to you for free and open access by Research Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Research Library. For more information, please contact [email protected]. SOURSOB 'Oxalis pes-caprae L.J

Soursob is another introduction from South Africa and has become established in a number of districts, especially in the Avon Valley. Although the leaves have the appearance of a clover. Soursob is unpalatable and is not closely related to the clovers.

Journal of Agriculture Vol 1 No 10 1960 •WeffMjtSA

By G.R.W. MEADLY M.Sc. Officer in Charge, Weeds and Seeds Branch

SOURSOB (Oxalis pes-caprae L.) URING winter and spring, the yellow colouration of paddocks is not always due to the D flowers of the widely distributed Capeweed. In some districts, particularly in the Avon Valley, the yellow-flowered soursob (Oxalis pes-caprae L.), has become firmly established. The genus Oxalis contains 800 or more different species, the two main natural centres of distribution being South Africa and tropical and extra-tropical South America. The two species native to Australia have DESCRIPTION been supplemented by a number from The name "soursob" is a variation of South Africa. The commonest of these is "soursobs" more frequently used when it O. pes-caprae, to which the name "sour- first appeared in Australia and un­ sob" is most frequently applied. The other doubtedly is derived from the sour taste species include yellow, pink and red- of the leaves. Until recently O. pes-caprae flowered types, having a similar her­ was known as O. cernua. Oxalis is the baceous perennial habit. Greco-Latin name for some plants with Soursob has its strongest development bitter leaves. Cernua describes the droop­ in South Australia where, in many parts, ing flowers while pes-caprae means the it dominates the landscape. It occurs foot of a goat and alludes to the shape extensively in Victoria, and also infests of the leaflets. appreciable tracts of land in New South It is an almost glabrous perennial with Wales and this State. Species of Oxalis a vertical subterranean rhizome up to 4 in. were introduced to England as garden thick at ground level, but tapering into a subjects as early as 1658, while about a fine thread attached to the parent bulb century later other species were arriving or root. The rhizome has numerous ad­ from the Cape of Good Hope. There is ventitious roots and also produces one to several bulbils in the axils of small scale- little doubt that the common soursob leaves. A cluster of typical leaves is formed found its way to this country as an in­ at ground level. Each has a short flat­ tentionally-introduced garden bulb, and tened base, a long cylindrical stalk and almost certainly was brought from the three terminal, clover-like leaflets, often Cape Province. It appears to have been marked with small purple spots. The introduced prior to the establishment of flowers are yellow, drooping and clustered the Adelaide Botanical Gardens in 1855 in umbels of 3-16 on long peduncles. and was recorded in the first catalogue of The capsule is pointed-oblong but rarely cultivated plants issued in 1859. matures in Australia. Flowering usually The date and method of introduction to occurs from June to October. Western Australia are uncertain but it is Although the leaves could be confused now widespread throughout the South­ with those of clovers the flowers are very west and also occurs in the Avon Valley different and the plants are not closely and parts of the Great Southern district. related. 881

Journal of Agriculture Vol 1 No 10 1960 SOUBSOB tOzalis pes-caprae L.J A—Complete plant showing bulb; B—Leaf; C—Flower; D—Section of flower; E—Stamens (From a pen-drawing by Mr. C. A. Gardner) 882

Journal of Agriculture Vol 1 No 10 1960 SIGNIFICANCE trees. "Balled" trees and shrubs should In this State, soursob occurs in many be examined prior to planting to ensure orchards and is also a troublesome weed of that they are not carrying bulbs of soursob crop land and pastures, especially in moist or, for that matter, other undesirable situations. Despite the delicacy of its weeds. If only a few plants appear they structure the plant provides keen competi­ should be dug out immediately. tion for other herbage, aided by the fact The above-ground parts of soursob can that development from bulbs and corms is be destroyed by the application of a 5 per usually more rapid than from seed. The cent, solution (1 lb. per 2 gallons) of prolific bulb formation gives rise to a dense sodium chlorate, but, owing to the cost, mass of plants which tend to smother the this method is only practicable for limited less vigorous pasture species and can also areas. In order to prevent the formation affect the yield of cereal crops. of new bulbs, spraying must be carried Owing to a high oxalic acid content— out early in the season when a general the cause of the sour taste—soursob is not emergence of the plants has occurred but eaten readily by stock. On occasions, how­ before flowering has commenced. Even if ever, because of the absence of more done at the optimum growth stage, eradi­ palatable herbage, sheep have eaten this cation cannot be expected from a single weed in quantity and cases of chronic oxalic application. acid poisoning have occurred. Wethers As already mentioned, grazing does not have remained healthy when trouble has provide an effective method of control but been experienced with ewes apparently a substantial reduction can be brought associated with the greater calcium re­ about by ploughing and cultivation. Some quirement of ewes, especially when preg­ work undertaken by Michael in South nant. All classes of sheep, however, can Australia showed that after a favourable be affected. season, there could be three tons of bulbs The poisoning is considered to be due to per acre while areas subjected to two the absorption of the oxalic acid into the workings produced only 7 cwts. per acre. animal's system to form calcium oxalate In South Australia, where the weed making the body deficient in available occurs freely on wheat land, ploughing is calcium. The disease usually does not ap­ recommended when the old bulbs are pear until the animals have been grazing nearing exhaustion—usually between late on the soursob for some time. Sheep reared May and the middle of June. A further in districts in which the plant is prevalent cultivation is carried out in late July or are less likely to be affected than animals early August to prevent the formation of introduced from an area where the weed new bulbs on plants that survived the does not occur. ploughing. These two workings can often In some cases the animals have died be followed by the sowing of an early suddenly, in others they have lingered for maturing cereal. The procedure outlined some time. A stiffness in gait is followed does not eradicate soursob but reduces its by loss of control of the hind quarters intensity so that a more satisfactory crop and at times, also the forequarters. may be grown. Once the weed is estab­ lished, eradication cannot be expected un­ CONTROL less control measures are repeated in a As soursob is very difficult to eradicate, thorough manner for several years. it is important to prevent it from becom­ Soursob is resistant to the commonly ing established. It is spread by a number used hormone-like herbicides but the of means, including stock, farm imple­ search is being continued for an effective ments and running water, but has also chemical treatment. The possibilities of been distributed both as a garden plant biological control have not been over­ and in the soil around shrubs and fruit looked.

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Journal of Agriculture Vol 1 No 10 1960