Large-flowered Wood Sorrel

Oxalis purpurea L.

Synonyms - Oxalis variabilis Jacq.

Family: Oxalidaceae.

Names:

Oxalis is from the Greek oksos meaning sour and refers to the taste of the leaves and stems.
Purpurea
Large-flowered Wood Sorrel

Other names:

Four o'clock (WA)
One o'clock (SA)
Purple Wood Sorrel.

Summary:

A rosette forming, perennial, low lying herb with trifoliate leaves and almost circular leaflets that are often purplish underneath and green and spotted or streaked on the upper side. The flowers occur singly and are trumpet shaped, pink-purple or sometimes white and 5 petalled that with a yellow throat from May to October. There are 10 stamens and 5 styles. The fruit is a narrow capsule.
It has a perennial bulb that produces annual tops.
Native to South Africa it has become a weed in gardens, crops, pasture, and along roadsides.

Description:

Cotyledons:

Two.

Leaves:

3 leaflets. Tend to lie close to the ground. All arise from the centre of the plant to form a rosette.
Stipules - None.
Petiole - 20-80 mm long, cylindrical and tapering. Hairy.
Blade - Circular, 4-40 mm long x 4-30 mm wide, green to reddish and often purple on the underside with lighter dots and streaks on the upper surface that turn black as the plant dries. Central leaflet often larger and wedge shaped towards the base. No leaflet stalks. No hairs on top and hairy underneath.

Stems:

None.

Flower head:

Single flowers on 30-120 mm long, often hairy stalks with 2 alternate bracts below the middle.

Flowers:

Ovary - Hairy ovary with a more or less hairy style.
Sepals - 5, narrowly egg shaped sepals, 5-10 mm long, striped with almost transparent lines that turn dark on drying. Hairy with simple and tiny flat topped hairs.
Petals - 5, 10-40 mm long, purple, mauve or white with a wide, yellow tube
Stamens - 10 in 2 rings. Outer ones shorter and opposite the petals. Filaments joined at the base with glandular hairs on the upper half.
Anthers - Release pollen by a longitudinal slit.

Fruit:

It rarely, if ever, forms fruit or sets seed in WA.
When formed it is a capsule which expels seeds explosively on ripening through longitudinal slits leaving the valves attached to the plant.

Seeds:

Small, 1 mm long, brown (with white spots on inner faces), triangular pyramid.

Roots:

Extensive with bulbs.
Bulb, egg shaped, 15-30 mm long, pointed at top but only rarely beaked, protected by dark scales.

Key Characters:

Single white or purple flowers with a yellow throat on a peduncle that is barely taller than the leaves. Circular leaflets. No aerial stem, not creeping. Leaves basal with translucent dots that turn black on drying.

Biology:

Life cycle:

Perennial with annual top growth up to 300 mm. Germinates autumn to winter. Flowers winter to spring. Under ideal conditions it will flower in 7-8 weeks.
White flowered varieties tend to flower earlier than the purple flowered ones.

Physiology:

Optimum conditions for bulb regeneration are 25 degrees C for several weeks followed by 2 weeks at 5 degrees C (Hertogh, 1987).
Optimum growth conditions are 16-18 degrees C with medium light intensities, natural photo periods and moderate to high nutrient status. Under these conditions it will flower in 7 to 8 weeks (Hertogh, 1987).

Reproduction:

By bulbs.

Flowering times:

June to October in SA.
May to August and occasionally in October in Perth.
Late autumn to spring in WA.

Seed Biology and Germination:

No seed produced in Australia.

Vegetative Propagules:

Bulbs, bulbils and rhizomes.

Hybrids:

At least three forms are naturalised; the white, mauve and purple flowered forms.
White flowered forms have larger flowers, shorter peduncles, hairier leaves and they flower earlier.

Allelopathy:

Population Dynamics and Dispersal:

Water flows carry bulbils downstream or along flood ways.
Ants move some bulbils. Sheep, birds and machinery also move bulbils in adhering mud.

Origin and History:

South Africa.

Distribution:

NSW, SA, TAS, VIC, WA.
Widespread through southern and eastern Australia.

Courtesy Australia's Virtual Herbarium.

Habitats:

Climate:

Temperate. Mediterranean.

Soil:

In WA it is common on the gravel and loamy to heavy soils in the medium rainfall zone. It is often associated with water ways due to its main method of spread.

Plant Associations:

Significance:

Beneficial:

Ornamental plant used as a ground cover, potted plant or in hanging baskets.

Detrimental:

Weed of crops causing yield reductions due to competition.
Weed of pastures displacing sown species.
Weed of roadsides, lawns, gardens, cemeteries and disturbed areas.

Toxicity:

High oxalate content. May be toxic to stock causing oxalate poisoning. Breeding ewes are most likely to be affected.
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 the 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:

None.

Management and Control:

It is difficult to achieve highly selective control in legume based pastures. 300 mL/ha Spinnaker® applied in late autumn to early winter has provided reasonable control. It will reduce pasture growth.
Defoliation causes some reduction in bulb production (Pierce, 1990).
Cultivation is generally ineffective.

Thresholds:

Eradication strategies:

Glyphosate and metsulfuron are the most effective herbicides for control and reducing bulb production (Pierce, 1990). Infested paddocks can be sprayed with glyphosate then planted to barley or a glyphosate plus metsulfuron mix then planted to wheat. A post emergence application of metsulfuron can be applied 6 weeks later. Two to three years of this treatment will almost eliminate the weed. Reinfestation is usually associated with free flowing water distributing bulbs. Ants collect and distribute the bulbs over short distances. Old home sites, fence lines and waste areas are common sources of infestations. These areas can be treated with metsulfuron if it is desirable to leave grass cover or a glyphosate plus metsulfuron mix for total vegetation control.
Because it does not set seed in WA and the bulbs are short lived, it should be possible to nearly eradicate this plant at the farm level.
In bush land areas Logran and Spinnaker are also useful and generally cause less damage to native species where overall spraying is necessary.
It usually takes 3 or more years to achieve high levels of control.
300 mL/ha Spinnaker® controls O. purpurea in pasture and 50 g/ha Logran® controls O. glabra in wheat. These products will probably also kill other Oxalis species.
Fumigation with metham sodium is useful for limited areas such as seed beds and glasshouses.

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.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P484-485.

Hertogh (1987).

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. Photo.

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). #919.16.

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). P495.

Pierce (1990).

Acknowledgments:

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