Oxalis is a large genus of annual and perennial herbs in the family Oxalidaceae, comprising somewhere between 550 and 700 accepted species depending on the authority — NCSU counts 582, SANBI references approximately 700, and GBIF records 953 total descendants (including infraspecific taxa). Linnaeus formally described the genus in 1753 in Species Plantarum, giving it the name derived from the Greek word for "sour," a reference to the sharp, lemony taste imparted by oxalic acid present throughout the plants.
Plants in the genus are characteristically low-growing, typically reaching 0.05–0.3 m in height. Most produce trifoliolate leaves — three heart-shaped leaflets arranged in a clover-like form — though some species have up to ten leaflets. A distinctive feature is nyctinasty: the leaflets fold downward at night or in dim light. Flowers are five-petalled and appear in white, pink, red, purple, or yellow. The genus is notable as the only dicotyledonous genus known to produce true corms, though many species also spread via rhizomes, stolons, bulbils, or seed capsules.
The genus is distributed on every continent except Antarctica, with the greatest species richness concentrated in two regions: southern Africa (approximately 270 species, mostly in the winter-rainfall Fynbos Biome of South Africa's Cape provinces and Namibia) and South America (especially Brazil). Additional diversity is found across North America, Europe, and Asia. Species occupy a wide range of habitats including woodlands, grasslands, alpine meadows, and disturbed ground.
Several Oxalis species have become significant weeds beyond their native ranges. Oxalis corniculata (creeping woodsorrel) is now cosmopolitan, appearing on every inhabited continent. Oxalis pes-caprae (Bermuda-buttercup) is invasive in Mediterranean-climate regions worldwide. Various species also have considerable cultural importance: O. tuberosa (oca) is cultivated as a root vegetable in the Andes and is sold internationally as "New Zealand yam," while leaves of many species have long been consumed as a sour, edible herb. In South Africa, the tangy leaves of O. pes-caprae are a traditional ingredient in waterblommetjie stew. Medicinally, leaf decoctions have been used to treat fevers and as diuretics.
Etymology
The genus name Oxalis comes from the Greek word oxys, meaning "sour" or "sharp," referring to the distinctive acidic taste of the plant's leaves and stems. The sourness is caused by oxalic acid — notably, the acid was itself named after the genus rather than the other way around. Linnaeus published the genus in 1753 in Species Plantarum, and the name has remained unchanged since.
Distribution
Oxalis has a near-worldwide distribution, occurring on every continent except Antarctica. The genus reaches its greatest diversity in two distinct centres of origin: southern Africa, where roughly 270 species are concentrated mainly in the winter-rainfall Fynbos Biome of South Africa's Western and Eastern Cape provinces and Namibia, with additional species in summer-rainfall Savanna and Grassland Biomes; and South America, particularly tropical Brazil. Further species are native to North America, Europe (including at least five species documented in Switzerland alone), and Asia.
Through human activity, several species have spread far beyond these ranges. O. corniculata is now a cosmopolitan weed found across all inhabited continents. O. pes-caprae is established as an invasive plant across much of the Mediterranean basin, California, and other regions with similar climates. The species O. acetosella is native across Europe from Iceland to Spain and east through Asia to Japan.
Ecology
Oxalis species occupy a broad range of ecological niches, from moist temperate woodland floors to arid Mediterranean scrubland and alpine zones. Many woodland species, including the type species O. acetosella, grow under closed-canopy forest in humus-rich soils and are well adapted to deep shade. In contrast, South African Fynbos species are adapted to the winter-rainfall, fire-prone shrubland of the Cape Floristic Region.
Leaf folding (nyctinasty) — the closing of leaflets at night or in response to cloud cover and temperature changes — is a characteristic feature of the genus. Species provide food and shelter to a range of invertebrates, including beetles, caterpillars, and moths. Some research suggests that symbiotic associations with nitrogen-fixing soil bacteria contribute to the success of weedy species in nutrient-poor or disturbed soils.
Notable invasive species include O. corniculata and O. pes-caprae, both of which reproduce prolifically via vegetative propagation and seed. O. pes-caprae in particular has proven difficult to control in invaded regions due to deep corm formation. Oxalate-containing tissues can cause livestock toxicity if consumed in large quantities.
Cultivation
Oxalis species are cultivated as ornamentals, houseplants, and ground covers. Most prefer well-drained, loamy soil with a slightly acidic pH and grow well in full sun to bright indirect light, though woodland species (such as O. acetosella) require shade or dappled sunlight and humus-rich, consistently moist soil. Optimal temperatures for common cultivated species are 50–75°F (10–24°C).
Many species are geophytes that undergo a natural dormancy period. After flowering, watering should be reduced and bulbs or rhizomes stored in cool, dry, dark conditions for one to three months before resuming growth. When planting, rhizomes or corms are typically buried approximately 5 cm (2 inches) deep. South African species such as O. flava, O. lanata, and O. purpurea are noted as having significant unexplored horticultural potential. O. triangularis (purple shamrock) and O. adenophylla are among the most widely grown ornamental species.
Propagation
Oxalis is propagated by seed, division, and vegetative structures. Seeds should be sown fresh (when ripe) in a cold frame and transplanted once established. Division in spring is the most reliable method for perennial species; smaller divisions should be potted individually until they develop sufficient root systems before planting out. Many species produce corms, bulbils, or rhizomes that naturally detach and establish new plants, contributing to both their garden spread and their weedy character in some climates.
Cultural Uses
Oxalis has a long history of human use across many cultures. The leaves and stems of most species are edible in small quantities, delivering a tart, lemon-like flavour due to their oxalic acid content; they have been used raw in salads and as a pot herb in soups. Oxalis tuberosa (oca) is an important Andean food crop cultivated for its starchy tubers, which are marketed internationally as "New Zealand yam."
In South Africa, the sour leaves of O. pes-caprae are a traditional and widely used ingredient in waterblommetjie bredie (water-lily flower stew), a Cape Malay dish. Medicinally, leaf decoctions have been used historically to treat fevers (quenching thirst and reducing temperature), and fresh or dried leaves have been applied externally to boils and abscesses. The leaves also have astringent and diuretic properties in traditional medicine. Because of their resemblance to shamrock, several three-leaflet species — particularly O. acetosella, O. deppei, and O. triangularis — are sold commercially as "shamrock" houseplants, especially around St Patrick's Day.
Consumption should remain moderate: oxalic acid in quantity interferes with calcium absorption, and individuals with rheumatism, arthritis, gout, or kidney stones are advised to avoid significant intake.
Taxonomy Notes
Oxalis L. was described by Carl Linnaeus in 1753 in Species Plantarum (Sp. Pl. 433) and placed in the family Oxalidaceae within the order Oxalidales, class Magnoliopsida. The genus is the type genus of the family. GBIF recognises 953 total descendants (including infraspecific taxa); NCSU cites 582 accepted species; SANBI references approximately 700 species globally. The discrepancy reflects ongoing taxonomic revision, with many historical genera now segregated within or associated with Oxalidaceae.
A notable morphological distinction is that Oxalis is the only dicotyledonous genus known to produce true corms. Soluble calcium oxalates are present throughout the plant, which is both a taxonomic character and a significant toxicological consideration. Several species are treated as serious agricultural or environmental weeds and their taxonomy is subject to revision at the species level.
