Oxalis hirta (tropical woodsorrel)
The late autumn flowering tropical woodsorrel was introduced into cultivation in the 18th century, but is not widely grown due to its untidy habit and flowers that only open in the sun, which can be rare in November.
About this species
Oxalis is the seventh largest genus (group of related species) in the Cape Floral Region of South Africa, with 118 species, of which 94 are restricted to this area.
Oxalis hirta was introduced into cultivation in 1793 but has never become a popular plant despite its glorious, sometimes vivid pink, early winter flowers. This is probably because it is not always hardy and often grows untidily.
The renowned alpine plantsman and nurseryman, Will Ingwersen, wrote of this plant: ‘…but the flowers, great goblets of glowing portwine ruby, are produced just when winter is in the air, the days at their dullest and the air so full of moisture that the loose, lax stems flop over, and those buds will only unfurl and display their gorgeous beauty when the sun shines during the few scattered hours in November.’
Geography & Distribution
This species occurs naturally in south-west South Africa, from the Bokkeveld Mountains south to the Cape Peninsula.
Like all South African Oxalis species (except the cosmopolitan weed O. corniculata), O. hirta is a bulbous species (it grows from a bulb). It is dormant throughout the hot, dry summers characteristic of the mediterranean-type climate in Western Cape, and growth begins with the onset of the winter rains.
The branched, leafy shoots are initially upright but tend to flop over as they lengthen. They can eventually reach over 30 cm long. The leaves are almost sessile (without a stalk) and divided into three linear-oblong (longer than wide with parallel margins) leaflets. The whole plant is softly hairy.
The peduncles (flower-stalks) arise from the upper leaf axils in autumn and early winter and hold single flowers in shades of deep magenta-red to violet, purple or rarely white. The flowers have a yellow throat (part where the tubular petals widen into the mouth) and rarely the whole flower can be yellow.
In a revision of South African Oxalis by Terrence Salter in 1944, O. hirta was divided into seven varieties but these groups are somewhat arbitrary, distinguished by characteristics such as hairiness, leaflet shape and flower colour.
Oxalis hirta is cultivated as an ornamental.
Although no hazards are known for O. hirta, other species of Oxalis contain oxalic acid and are potentially poisonous to humans and livestock if consumed in large quantities. The name Oxalis comes from the Greek oxis, which means acid, referring to the sour-tasting sap of some species.
As with many bulbous species of Oxalis, O. hirta seems to grow better if pot-bound, as long as the bulbs do not actually touch. After the first watering let the soil dry out before watering again, but once growth appears the soil is best kept moist and regular watering is necessary. The leafy stems are initially erect and the flowers open in autumn and early winter.
Good light is needed to prevent etiolation (elongation of the plant stem as it grows towards the light). The stems continue to elongate during and after flowering and all too easily flop over and become straggly. By late spring they begin to go brown and watering should be reduced. Once the stems have dried they can be cut or carefully pulled away and no more water will be needed until the autumn.
Propagation is usually from bulb offsets that are separated during repotting and grown on. Seeds will only be produced if two clones are present.
The plant illustrated in Curtis’s Botanical Magazine is the form that gained a Preliminary Commendation from the Royal Horticultural Society when exhibited by Kew in October 1984. The flowers are larger and a deeper pink than other cultivated forms. The original material came from Henrik Zetterlund at Göteborg Botanical Garden (in Gothenburg, Sweden) and was given the cultivar name ‘Gothenburg’. It received an Award of Merit in November 1996.
This species at Kew
Pressed and dried specimens of Oxalis hirta are held in Kew’s Herbarium, where they are available to researchers, by appointment. The details, including images, of three of these specimens can be seen online in the Herbarium Catalogue.
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