Invasiveness Assessment - Soursob (Oxalis pes-caprae) in Victoria (Nox)

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Plant invasiveness is determined by evaluating a plant’s biological and ecological characteristics against criteria that encompass establishment requirements, growth rate and competitive ability, methods of reproduction, and dispersal mechanisms.

Each characteristic, or criterion, is assessed against a list of intensity ratings. Depending upon information found, a rating of Low, Medium Low, Medium High or High is assigned to that criterion. Where no data is available to answer a criterion, a rating of medium (M) is applied. A description of the invasiveness criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the invasiveness of Soursob.

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
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Common Name: Soursob
Scientific name: Oxalis pes-caprae

Question	Comments	Reference	Rating
Establishment
Germination requirements?	Most bulbs germinate after autumn rains.	P & C (1992)	MH
Establishment requirements?	Can establish under moderate canopy - occurs in olive, citrus and vineyards, as well as woodlands.	Pierce (1997) Carr et al (1992)	MH
How much disturbance is required?	Establishes in minor disturbed natural ecosystems, (See Car et al 1992), as well as vigorously growing crops (e.g. cereal) and pastures.	Carr et al (1992) P & C (1992)	MH
Growth/Competitive
Life form?	Erect perennial: geophyte.	P & C (1992)	ML
Allelopathic properties?	None described.		L
Tolerates herb pressure?	“Owing to a high oxalic acid content – the cause of the sour taste – soursob is not eaten readily by stock”.	Meadly (1965)	MH
Normal growth rate?	Its rapid growth rate is a key feature of the plant's competitiveness.	P & C (1992)	H
Stress tolerance to frost, drought, w/logg, sal. etc?	Tolerant of frost, water logging (infests drainage ditches), drought (occurs in semi arid parts of Morocco).	P & C (1992) Hildreth & Agamalian (1985) – Tanji (1994)	MH
Reproduction
Reproductive system	Reproducing from bulbs and bulbils. Viable seed is produced on some plants in Western Australia.	P & C (1992)	H
Number of propagules produced?	20 flowers per plant x 3/5 seeds per flower = 60/100 seeds per plant. (estimate using illustrations). Seeds not commonly produced in Victoria. (DCFL 1987, Pierce).	Meadley (1965)	ML
Propagule longevity?	?		M
Reproductive period?	Forms dense monocultures. (See picture P & C (1992) p. 532).	P & C (1992)	H
Time to reproductive maturity?	“Plants arising from bulbils produce bulbs in their first year but do not flower”. So flower in second year.	P & C (1992)	MH
Dispersal
Number of mechanisms?	Bulbs/bulbils-machinery, water movement, birds eat bulbs.	P & C (1992)	H
How far do they disperse?	Ejection of seeds up to 2 m from the parent plant. “Because they are light. Clark (1934) considers that wind plays a major role. Bulbs float on water so this would be significant, particularly in Western Australia where Oxalis pes-caprae is associated with some major rivers and drainage systems”. (Pierce 1997). Above mechanisms could transport many seeds/bulbs > 200 m.	Pierce (1997)	H

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