This is a native plant, found mostly in woodlands. I see it sometimes in hollowed-out tree stumps and even in quarries and on cliffs. It’s a perennial, flowering from Spring to Midsummer. The flowers are white; their violet venation serving to guide small insects to the nectar. The leaves are trifoliate, like clover, and the plant is sometimes considered erroneously to be the Irish Shamrock. Unlike Shamrock (which is one of several species of clover) the leaflets are heart-shaped. Like clovers, the leaflets have ‘sleep movements’, folding down at night.
Oxalis acetosella L. by Otto Wilhelm Thomé, (1840–1925), German botanist and botanical artist from Cologne. Public domain, via Wikimedia Commons
Floral biology is unusual. There are two phases. Phase 1: in the Spring the flowers are open and there is nectar to attract insects; however, most of the seeds are produced later from summer flowers (Phase 2) which do not open and are produced close to the ground. Self-pollination occurs ‘behind closed doors’ (i.e. the flowers are ‘cleistogamous’). It is a mystery as to why the plant produces nectar in the spring when seeds are produced by self-pollination from cleistogamous flowers later in the year. I searched the literature using Google Scholar. A useful paper on this topic comes from researchers at Uppsala University showing that both phases can produce seeds although the relative proportions vary (Berg and Redbo-torstensson 2002). They conclude that, in evolutionary terms, this is ‘bet-hedging’, ensuring seed production in a range of different environments and weather conditions.
I found that several unrelated groups of flowers behave in the same way (violets, some grasses)
Wood Sorrel, flowers and leaves wide open. Image: Chris Jeffree.
These plants live for several years, sometimes dying back in winter and sprouting from reddish creeping rhizomes. More often they stay green over winter, and even grow, forming conspicuous patches on the woodland floor. It isn’t as tall as most of the ground-dwellers, and this may be why it often appears away from the rest of the ground flora, avoiding competition by growing in raised areas (hummocks and tree stumps). It may not compete well with the invasive alien Allium paradoxum which covers woodland floors in the Spring, just at the time when Oxalis comes into flower.
Wood Sorrel, showing venation of the petals. Here the leaves are folded down. Image: Chris Jeffree.
Wood sorrel has a wide ecological range, tolerating very low temperatures and soils that can be strongly acidic or highly calcareous with a pH covering a very wide range from 3 to 8 (Grime et al 1988). Its wide tolerance of soil conditions may be partly the result of its association with vesicular-arbuscular mycorrhiza (currently a rapidly-developing research topic). It is however a shade plant, and it performs best in shady and damp conditions, being one of a few species that grow in the deep shade of conifer plantations. Packham (1978) describes experiments in which it grew best at 27% of natural light, and needed frequent irrigation over the dry summer months. Although widespread in Britain, it is less frequent in the warmer and drier south-east of Britain.
Wood Sorrel emerging from litter on the woodland floor. Image: Chris Jeffree.
World-wide it is found in woodlands and also in sheltered spots on mountains. It is common in alpine regions, the Caucasian mountains, the subalpine forests of Japan, and it reaches to over 4000 metres in Yunnan, China (Packham 1978).
The plant has a connection with Christian tradition. The flowering was said occur between Easter and Whitsun (Pentecost), and so the plant was called Alleluia. However, Easter and Whitsun are variable dates and Spring is warmer than it once was, so the name no longer seems appropriate.
Wood Sorrel growing with the invasive alien Allium paradoxum. Both above-ground and below-ground competition may lead to the demise of Wood Sorrell. Image: Chris Jeffree.
The genus Oxalis belongs to the Family Oxalidaceae, and it is the only representative of its Family in Britain. There are 14 other species of Oxalis in Britain (Stace 2019) most of them somewhat rare and derived from imported garden plants. The most common in Scotland is O. exilis, a native of New Zealand and Tasmania. Another is O. corniculata, native in China and India and occurring in Britain as a garden weed.
Oxalis exilis, a New Zealand species often found in urban situations in Britain. Image: Chris Jeffree.
The name Oxalis comes from the Greek oxýs = acid, sharp, and hális = salt. Oxalis contains oxalic acid, a low molecular weight compound that is more acidic than acetic acid, and poisonous. Oxalic acid was first isolated in the form of a salt (calcium oxalate?) from Wood Sorrel by the Dutch botanist-chemist and physician Herman Boerhaave in 1745. In the case of Oxalis, the acid may be a very effective chemical defence against herbivore attack. Many other plant species contain oxalic acid, most notably Rhubarb, and to a lesser extent the cabbage/mustard Family Brassicaceae. Wood Sorrel is popular with plant foragers, one web site saying it has “a delicious lemony-apple flavour in leaves and stems” but warning not to eat too much. Foragers are also warned to avoid plants with a high oxalic acid content if they suffer from rheumatic disorders, as it reacts with calcium to form calcium oxalate, thus causing a shortage of that important bone-forming element.
The British distribution of three species of Oxalis in Britain. From BSBI Maps.
Wood Sorrel is regarded by some people as a medicinal plant. Culpepper’s Complete Herbal written in 1653 says it is good for:
“hindering putrefaction of the blood, and ulcers of the mouth and body, and to quench thirst, to strengthen a weak stomach, to procure an appetite, to stay vomiting and very excellent for any contagious disease or pestilent fevers”.
Nowadays, it is used in Chinese medicine and it can be purchased online in Western countries. One recent source says:
“Despite safety concerns, people take wood sorrel for liver and digestive disorders, a condition caused by lack of vitamin C scurvy, wounds, and swollen gums”.
Global distribution of Oxalis acetosella, from GBIF. It is native to Europe and much of Asia.
References
Berg H and Redbo-torstensson P (2002) Cleistogamy as a bet-hedging strategy in Oxalis acetosella, a perennial herb. Journal of Ecology 86, 491-500. https://doi.org/10.1046/j.1365-2745.1998.00272.x
Grime JP et al (1988) Comparative plant ecology : a functional approach to common British species. Unwin Hyman, London.
Packham JR (1978) Oxalis acetosella L. Journal of Ecology 66, 669-693.
Stace C (2019) New Flora of the British Isles. C&M Floristics.
©John Grace
Botany in Scotland 