The hidden diversity of Oxalis

Oxalis, also known as sorrels or “surings”, is the most diverse genus of bulbs in the Cape Flora. Of the ±220 species, about 180 are found at the Stellenbosch University Botanical Garden.

Their flowers vary widely in shape, size, and colour, adding to the richness of the Cape’s bulb diversity. Most species open during the day and close at night, following a familiar rhythm of light. A notable exception is Oxalis noctiflora, which flowers at night and is likely to attract moths.

Each plant carries one of three flower forms, with stigmas positioned at different heights. Stigmas only accept pollen from anthers at the same level, encouraging cross-pollination and supporting genetic diversity across populations. After pollination, seed capsules release seeds explosively, often scattering them several metres away from the parent plant!

Oxalis produces two types of seeds: orthodox seeds, which dry out and remain dormant until conditions are suitable, and recalcitrant seeds, which germinate immediately as small, ready-formed seedlings.

When recalcitrant seeds come into contact with water, a sticky gel forms at the base of the seedling, helping it anchor and retain moisture. This gel is associated with bacteria that feed on oxalic acid and have been recorded as capable of converting atmospheric nitrogen into a usable form, an advantage in the nutrient-poor soils of the Cape.

The Oxalis fragilis is among the species conserved at SUBG and is known from only two wild populations. The Garden’s ex situ conservation work supports possible future reintroduction efforts, particularly in the face of ongoing pressures such as development, agriculture, invasive species, and climate change.

The Oxalis collection, founded by Stellenbosch University botanist Prof. Léanne Dreyer, is considered the largest of its kind globally. It has supported decades of research and allows visitors to experience the remarkable diversity of this genus within a living collection.

Once listed as Critically Endangered (Possibly extinct), it had not been seen since 1936 until its rediscovery in the Swartland in 2018 by Rupert Koopman. The finding was confirmed by Prof. Léanne Dreyer and Dr Kenneth Oberlander.

At the time, the known population was under threat from a proposed dam development. As a precaution, plants were collected to establish a genetically representative ex situ population at SUBG. Around 3000 seedlings were later produced through controlled hand-pollinations to support conservation and future reintroduction.

Since then, the dam development has been relocated, and in 2024, a second wild population was discovered by the Custodians of Rare and Endangered Wildflowers (CREW). However, less than 5% of suitable habitat remains, with ongoing pressures from agriculture, invasive species, and climate change.

Ex situ conservation remains an important safeguard for the long-term survival of these fragile survivor species.