Plant of the Week – February 23rd 2026 – Least Yellow-sorrel (Oxalis exilis) – Botany in Scotland

This small creeping plant is a native of Australasia. Its first record in the whole of Britain was from ‘Slateford Quarry tip’ in Edinburgh, in July 1906. How did it get there? Here’s the clue: the second record in Britain was by Ida M Hayward, who recorded it ‘under a rose bush, Galashiels’ in 1913.

Oxalis exilis – just one flower per stem, unlike O. corniculata which has 2-8 flowers per stem. Image: Chris Jeffree.

Ida Hayward was a young woman living at her uncle’s woollen mill in Galashiels and the uncle encouraged her to take an interest in the strange plants that were appearing in the vicinity of the woollen mill. Helped by the eminent English botanist George Claridge Druce she was able to identify many species and together they published a book The Adventive Flora of Tweedside. In the index of the book, this plant is recorded as the more familiar Oxalis corniculata. It seems that Oxalis exilis had already been described from New Zealand in 1839 by Allan Cunningham (1791-1839), botanist and plant collector, but evidently no-one told Hayward and Druce.

Oxalis exilis – five functional stamens and five whithered ones lacking anthers. Image: Chris Jeffree.

Hayward’s rose-bush discovery was almost certainly Oxalis exilis (as the database of the BSBI now states). The seeds must have been in wool from Australia or New Zealand, and these seeds had a free ride to the Scottish town of Galashiels. Now, the plant appears throughout the British Isles, occupying very similar habitats to O. corniculata. Both are increasing rather rapidly at present.

In Britain we know Oxalis corniculata as the Procumbent Yellow Sorrel and Oxalis exilis as the Least Yellow Sorrel. The two species have often been confused. Both are small creeping perennials with yellow flowers and leaves resembling clover. They differ in size: O. exilis is smaller but the best way to tell the difference is that exilis has only a single flower per stem whereas corniculata has 2-8 flowers on each stem. Also, exilis has five stamens and corniculata usually has ten. Reid (1975) noted: “a further difference is that the flowers of O. corniculata open and close about one hour earlier than those of O. exilis” (only useful if you have both of them side-by-side). The two species are so similar that earlier authors thought the plant was merely a small-leaved variety of corniculata, calling it Oxalis corniculata var. microphylla.

O. exilis forms a cushion, with rather few flowers. Image: Chris Jeffree.

Slateford Quarry, where the species was first recorded, was at the south of Edinburgh and was probably on the waggon route to Edinburgh from the Borders town of Galashiels. There were not one but three busy quarries –  Hailes Quarry, Old Redhall Quarry and New Redhall Quarry, providing sandstone for much of Edinburgh’s buildings between the 17th and 20th centuries.

Cushions of Oxalis exilis rooted in the cracks of a pavement. Image: Chris Jeffree.

We have found Oxalis exilis many times in our survey of Scotland’s urban flora. It is sometimes seen on walls (Dundee), in a cemetery (Piershill Edinburgh), along roads and within private gardens (as a weed). The map below gives some impression of where it is currently to be found. Rather surprisingly, it has not been recorded from Galashiels since the 1960s, and nor has Oxalis corniculata.

Oxalis exilis – distribution in Britain and Ireland. Image from BSBI/Maps

The botanical word ‘sorrel’ causes confusion because there are two unrelated groups of plants called sorrels. Oxalis belongs to the wood-sorrel Family, called the Oxalidaceae, whereas the sorrels of culinary fame (including Rumex acetosa) belong to the Polygonaceae, the Family of docks. The specific name exilis is Latin and means thin, slender, meagre, or weak.

Oxalis exilis – global distribution from GBIF.

The word ‘sorrel’ is probably from the Old English sur “sharp and acidic to the taste, tart, acid (and similar words in many European languages). All sorrels taste ‘sour’ and acidic. Oxalis gives its name to oxalic acid (COOH)₂, a strong organic acid that all plants use to regulate their biochemical reactions. The acidity of Rumex acetosa comes mainly from acetic acid (CH₃COOH) which when diluted and flavoured we all know as vinegar.

The specific name, exilis, means thin, slender, meagre, or weak (Latin).

References

Groom QJ, Van der Straeten J. and Hoste I (2019) The origin of Oxalis corniculata L. PeerJ 7, e6384 doi:10.7717/peerj.6384

Reid JA (1975)  The distinction between Oxalis corniculata L. and O. exilis A. Cunn. Watsonia 10, 290-291.

Sykes WR (2009) The Oxalis corniculata group. New Zealand Journal of Botany, 47,107-113.

Thompson J (1982). Oxalis in Australia. Australian Systematic Botany Society Newsletter, 32, pp.4-6.

©John Grace