Members of the genus Oxalis, with their delicate, brightly coloured
flowers, clover-like leaves and corolla lobes twisted in the bud,
are mainly native to southern Africa and South America. Many people
will know them for their sour-tasting stalks. The name Oxalis,
comes from the Greek oxis, which means acid, referring to
the sour-tasting sap of some species.
Oxalis, established by Linnaeus in 1753, is characterized
by stemless (acaulescent) or stemmed (caulescent) herbs or subshrubs
with corms. This genus is the only one of the dicotyledonous genera
to produce corms (tuberous, bulb-like rootstocks). The species are
perennials and low-growing, reaching heights varying from 0.05 to
leaves, borne in basal rosettes, are also characteristic of the
genus. They are compound, usually trifoliolate (having three leaflets)
with leaflets varying from more or less heart-shaped (broadly obovate
and cuneate at the base) to narrowly obovate and predominantly stalked
(petiolate). Petioles long and slender. Occasionally the leaves
are sessile (without a stalk) or subsessile. The leaflets are often
flushed with purple below and maybe hairy or have calli (thickenings).
They display sleep movements, drooping or folding at night.
Flowers are solitary or in umbel-like clusters on long, erect peduncles
(stalks). They are radially symmetric (actinomorphic) with five
free sepals (calyx) and a tubular corolla with five lobes (petals).
In some species the tube is long and slender, whereas in others
it is rather short or either narrowly or broadly funnel-shaped.
Similar to the leaves, the flowers close at night and in dull weather.
The lobes of the corolla are prominently twisted in the bud, revealing
delicate, glossy, brightly coloured petals as the corolla opens.
White, rose-pink, pink, red, shades of purple and yellow are colours
observed in the genus . Oxalis is also characterized by obovate
(inversely ovate), broadly obovate or suborbicular (almost with
a ± circular outline), clawed corolla lobes.
Ten stamens, in two series, five long and five short, united at
the base, and often golden yellow, are characteristic of Oxalis.
Pollen grains produced in the anthers display different types. These
pollen types proved to be significant in distinguishing different
sections and species in the genus. The ovary is superior, 5-locular
with five free styles. The fruit of Oxalis is a 5-lobed capsule,
much longer than broad, with nearly parallel sides. The seeds, one
to many, are explosively ejected.
Oxalis belongs to the family Oxalidaceae, a family of six
genera and about 775 species, occurring mainly in the tropical but
also in temperate regions of both the New and Old World. The family
in southern Africa is represented by two genera: Oxalis and
Biophytum. Oxalis comprises about 700 species world-wide,
with centres of diversity in both southern Africa and South America.
About 270 species occur in southern Africa. The members are widespread,
but are mainly confined to Namibia and the Western and Eastern Cape
in South Africa.
Species of Oxalis are mainly confined to the winter rainfall
Fynbos Biome. However, members also occur in the Savanna and Grassland
Biomes where rain occurs in summer. The habitats where members of
Oxalis are found, vary from fynbos, grassland, forest, wetlands,
moist, disturbed places, waste places, along watercourses, roadsides,
rocky hillsides, clayey or sandy soils. The genus needs assessment
regarding rare and endangered species.
Economic and cultural value
Some southern African species of Oxalis are grown as ornamentals,
for example, O. flava, O. lanata and O. purpurea.
However, the horticultural potential of these delicate plants still
needs to be explored.
O. corniculata, a native of Europe, is a troublesome, cosmopolitan
weed of disturbed places or is often found along roads, in gardens
or in pots.
Wyk et al. (2002) reported that 'Oxalis species, for example,
O. pes-carpae, and other plants containing soluble oxalic
acid are not really poisonous but they can lead to human and animal
fatalities if excessive amounts are consumed. In southern Africa
(as in many other parts of the world), outbreaks of oxalate poisoning
have been reported, resulting from pastures infested with Oxalis
or Rumex species. Under field conditions, sheep are mostly
affected, but cattle and horses may suffer from degenerative conditions
of the bones after prolonged exposure to low levels of oxalates'.
O. pes-caprae occurs mainly in the Western Cape and has become
a troublesome weed in many parts of the world. However, the sour
leaves of O. pes-caprae are an essential ingredient of waterblommetjie
stew and other stews (Van Wyk & Gericke 2000).
Characters of the inflorescence (type), peduncular bracts, stem,
leaves and leaflets (number, shape and texture) are used to distinguish
different groups of species in the southern African members of Oxalis
(Bayer 2000). A few examples are given below. The two main sections
are shown in table below.
1a Species with a peduncle more than 1-flowered:
O. pes-caprae L. Geelsuring.
Stemless; leaves usually basal, trifoliolate, leaflets heart-shaped, glabrous above, hairy beneath; flowers yellow, peduncle 3-20-flowered.
b Species with peduncle 1-flowered. The majority of species have 1-flowered stalks and may be grouped as follows:
2a Leaves unifoliolate (1) or leaves 4- or more-foliolate:
O. tomentosa L.f. Vingersuring.
Stemless; densely silky hairy; leaflets 10-20, much longer than broad, base wedge-shaped; flowers white with a yellow tube [slide]
2b Leaves trifoliolate (with 3 leaflets):
3a Peduncular bracts at an articulation:
O. zeekoevleyensis R.Kunth .
Stemless; leaflets heart-shaped, sparsely hairy below; flowers rosy lilac with a yellow tube
3b Peduncular bracts not at an articulation or lacking:
4a Plants with leafy stems, lower leaves sessile (without a stalk) or subsessile (with a short stalk)
O. hirta L.
Stems 0.05-0.3 m high; leaflets grey-green, long and narrow, inversely ovate, hairy below; flowers mauve, magenta or white with a yellow tube.
4b Leaves apically congested and distinctly petiolate:
5a Leaflets linear, folded together lengthwise:
O. versicolor L.
Stems up to 0.2 m tall, sometimes branching; leaflets often with marginal thickenings; flowers white with a yellow tube and reddish purple margins.
5b Leaflets oblong to heart-shaped, succulent (fleshy):
O. pulchella Jacq.
Stemless; leaflets fleshy, almost flat with a ± circular outline; hairy below; flowers salmon or rose.
5c Leaflets non-succulent:
O. purpurea L.
Stemless; leaflets transversely obovate , ciliate, hairy and purple beneath, black-streaked when dry; flowers purple, yellow or white with a yellow tube.
In the Garden
Members of the genus are propagated by their corms and seed. There
are species to suit any situation; good or bad soil, full sunshine
or complete shade or semi-shade, and they grow well provided they
get enough water. The plants spread rapidly and can be used as seasonal
ground covers, but containers keep growth in check and prevent plants
becoming weedy. Many of the species are dormant in the dry season.
The genus is not known to be subject to pests or diseases in the
garden, but the only reason for this may be that it has not yet
been significantly cultivated (Du Plessis & Duncan 1989).
References and further reading
- Bayer, B. 2000. In P. Goldblatt & J. Manning, Cape plants:
a conspectus of the Cape Flora of South Africa. Strelitzia
- Dreyer, L.L. & Makwarela, A.M. 2000. Oxalidaceae. In O.A.
Leistner, Seed plants of southern Africa: families and genera.
Strelitzia 10: 432, 433.
- Du Plessis, N. & Duncan, G. 1989. Bulbous plants of southern
Africa: a guide to their cultivation and propagation. Tafelberg,
- Van Wyk, B-E. & Gericke, N. 2000. People's plants: a
guide to useful plants of southern Africa. Briza Publications,
- Van Wyk, B-E., Van Heerden, F. & Van Oudtshoorn, B. 2002.
Poisonous plants of South Africa. Briza Publications, Pretoria.