Oxalis acetosella L., Wood-sorrel

Account Summary

Growth form and preferred habitats

This attractive, variable, low-growing, white-flowered, wintergreen, patch-forming, perennial with a slender creeping rhizome occurs in moist, shady situations and, locally, appears even in the smallest fragment of woodland cover. Wood-sorrel has beautiful, light- to dark-yellowish green, trefoil leaves, the colour varying with light levels. The leaflets, borne on long petioles up to 10 cm, fold down twice a day, carrying out 'nastic movements' in response to changing turgor pressure in the hinge-like pulvini cells at the leaf base. The rhizomes of O. acetosella run along the soil surface and the root system penetrates only the top few cm, thus although it can cope with shallow soils, it is very much restricted to permanently moist habitats (Packham 1978; Grime et al. 1988, 2007).

In some parts of S Britain, O. acetosella is confined to primary (ie ancient), relatively undisturbed woodland, or has a strong affinity for old woods, making it a useful indicator of these situations deserving conservation action (Rackham 1980, p. 54). This more restricted occurrence is closely related to species-limiting levels of moisture- stress which the plant experiences in the more continental climate of S & SE England in a way that is quite foreign to other wetter, cooler milder areas of B & I like Fermanagh. In SE England, the species lies closer to the southern margins of its distribution and, as is also the case elsewhere that enjoys a drier, more continental climate, O. acetosella does not grow well in dry soils (Packham 1978). In Fermanagh and other wetter parts of B & I, numerous other types of permanently damp, shaded situations which to some extent mimic woodland conditions, allow Wood-sorrel to occur much more widely in non-woodland situations and, occasionally, to absolutely thrive in them. This is especially the case in well-drained, yet moisture retaining organic soils, often with an insulating depth of leaf litter. Being a low-growing herb, however, excessive amounts of leaf litter inhibits the plant's growth. The latter situation confines O. acetosella to localised sites where less material accumulates, for instance on even slightly raised spots or mounds on the ground, on slopes and at the base of tree trunks.

Plants cannot tolerate any degree of prolonged trampling, but are little affected by grazing as they contain toxins that are sharp and distasteful to animals (see below).

The range of Wood-sorrel habitats additional to old, established woodland, include darker, damper areas in plantations, scrub thickets, hedgerows and along river and streamside banks. O. acetosella is one of the very few vascular plants which can penetrate conifer plantations, but even then it is only commonly found in marginal situations, or where the evergreen canopy density is partially opened or thinned by some form of damage (Packham 1978; Grime et al. 1988, 2007).

Locally, in Fermanagh, O. acetosella also occurs in damp shade on cliff and quarry ledges and under and among rocks in block scree or limestone grykes (± deep, shady crevices). It also grows in rough montane grassland and under bracken, gorse and ericaceous heath at relatively high altitudes.

The established growth strategy is described as intermediate between stress-tolerator and stress-tolerant ruderal (Grime et al. 1988, 2007). The rather low growth rate and small stature of O. acetosella creates stress for the individual plant when faced with shade and competition for resources from taller, shade-tolerant, woodland floor species.

While it occurs across a wide range of soil reaction and parent materials (pH 3.5 to >8.0), O. acetosella is most frequent in the acid end of the range, below pH 5.0, and it is predominantly found on infertile, undisturbed, rather deeply shaded situations, where the ground is permanently damp and never subject to drought (Packham 1978; Grime et al. 1988, 2007).

Toxicity

All Oxalis species contain oxalic acid and oxylates that can cause poisoning. However, it is unlikely that sufficient of the plant would be eaten in B & I to cause actual poisoning. A total oxylate content of 7% has been estimated in air-dried plants and sheep appear to be most at risk of poisoning, as little as 600 g of the plant leading to severe calcium deficiency that can lead to kidney failure and death. Stock animals appear aware of the toxins from their unpleasant, lemon-sharp, sour, sorrel-like (Rumex acetosa) taste, or their odour, and they quickly learn to avoid the species (Cooper & Johnson 1998).

Flowering reproduction

Beautiful, solitary, bisexual, pendulous bell-like, white flowers with parts in fives are produced on 5-15 cm long peduncles in April and May. On close inspection, the petals are observed to be exquisitely veined with fine, lilac to purple stripes, making it one of the most delicate and subtly decorated widespread, native wildflowers in B & I. Pink, lilac and purple petal colour variants do exist, but they are very rare and local: the current author (RSF) has never come across them anywhere. The spring flowers, which are so welcome every year, appear to attract few insects and they set little seed. However, later in the summer, an abundant crop of petal-less flowers are produced that self-pollinate and self-fertilize while still in the bud (ie they are cleistogamous). These later flowers produce most of the seed for the season in 3-4 mm ovoid capsules (Clapham et al. 1987). Up to ten seeds are produced per fruit capsule; their dispersal is explosive, the capsule rupturing and scattering the seeds up to a metre from the parent plant. Wet seeds are sticky and may occasionally enjoy secondary dispersal on leaves or other wind-blown litter (Ridley 1930, p. 22).

The survey of soil seed banks in NW Europe concluded that seeds are transient to short-term persistent in the soil seed-bank, ie their survival ranges from less than one year up to several years, but less than five years (Thompson et al. 1997)

Vegetative reproduction

O. acetosella tends to grow in patches, many of which represent a single clonal mat of growth, achieved by spread and branching of its horizontal surface-creeping rhizomes over the individual's several years lifetime. In darker areas of woodland in particular, clonal patches may measure up to 50 cm across, and they can meet and mingle with adjacent clones to create a larger mat or sheet of the species, potentially dominating the ground layer. In this way, it can, in some cases, oust other familiar woodland species such as Hyacinthoides non-scripta (Bluebell), by spreading amongst the adult plants and shading out their seedlings (Packham 1978). Reproduction by seed allows invasion of fresh habitat, but the extensive vegetative spread shown by Wood-sorrel is important in maintaining established communities.

Fermanagh occurrence

As a result of its wide ecological tolerances, in NW Ireland O. acetosella is recorded from no less than 392 Fermanagh tetrads, 74.2% of those in the VC. Locally, in addition to damp, shady situations in woods, scrub, hedge- and river-banks, cave-mouths, screes, rock ledges in quarries and in limestone crevices, O. acetosella can occur in upland grassland under bracken, gorse and ericaceous heath on boggy peat at relatively high altitudes. This accounts for the fact that it has been recorded from all the tetrads which cover the long summit ridge of Cuilcagh, the highest mountain in the VC, although the species does not actually reach the more exposed areas of the ridge itself.

British and Irish occurrence

In Ireland, O. acetosella is widespread and common in most of the country and, while recorded in every VC (Scannell & Synnott 1987), it is less prevalent and more scattered in Connemara and in the Irish Midlands. In Britain, it is even more widespread, common and occasionally abundant, except in the drier parts of the country with the lowest rainfall in S & SE of England, and especially around the Wash and the Humber estuary where the mean annual rainfall is less than 635 mm (Packham 1978; Preston et al. 2002).

European and world occurrence

O. acetosella s.l. occurs from Iceland, Faeroes and N Scandinavia, S & SE-wards across most of Europe to the mountains of C Spain, Italy and Greece. It occurs at over 2000 m in European alpine heath and Krummholz communities just beyond the timberline (Packham 1978). It becomes rarer towards the Mediterranean coast and Corsica is the only Mediterranean island it reaches.

O. acetosella is also native in large areas of N & C Asia, although further east there are a number of closely related taxa that can be confused with it, and a similar situation involving allied species exists in N America. Previously some of these taxa were considered subspecies of O. acetosella and the latter was then reckoned to be a single species, circumpolar in distribution (Hultén & Fries 1986, Map 1260). In mainland Asia, the species is typically subalpine and alpine in habitat at altitudes from 2130-4110 m (Packham 1978).

O. acetosella is a member of the Eurasian boreo-temperate phytogeographic element (Sell & Murrell 2009).

Uses

Despite their sharp, sorrel-like flavour and oxalate toxin, some people still consider Wood-sorrel a desirable salad vegetable or suitable for addition to cream-cheese sandwiches (Mabey 1996)! The 'Sour Docks' or Sorrels cultivated for centuries as pot-herbs were Rumex acetosa (Common Sorrel) and R. scutatus (French Sorrel or Rubble Dock), as well as the smaller R. acetosella (Sheep's Sorrel) and the totally unrelated Oxalis acetosella (Wood Sorrel) on account of the 'grateful acidity' of their herbage due to the bioxalate of potash, also present in Rhubarb. The foliage of O. acetosella has been eaten as a spring salad "from time immemorial, their sharpness taking the place of vinegar" (Grieve 1931).

In herbal medicine, O. acetosella was used as a diuretic, an antiscorbutic and for its 'refrigerant action'. It was used to treat high fever, to quench thirst and allay the fever. As Allen & Hatfield (2004) point out, the medicinal benefits of this species have been often confused with those of Rumex acetosa, due to the similarity between their vernacular names, and therefore care must be taken in applying their use. There are always suspicions voiced whenever a medicinal use is confined to a particular region of B & I, for instance the application of O. acetosella for healing bruises only in Devon, or for making a poultice for scrofula in Colonsay in the Inner Hebrides. In Ireland, Wood-sorrel has been used to treat diarrhoea in Cos Mayo and Wicklow, as a blood tonic in Cavan, as a heart tonic in Wicklow and to treat palsy in Limerick. It was also prescribed for stomach cancer in an unknown part of Ulster (Allen & Hatfield 2004).

While used by herbalists to treat haemorrhages and urinary disorders, on account of its oxalic salts content, Grieve (1931) cautions against its general use for all constitutions, "especially those of a gouty and rheumatic tendency".

Names

Both botanical names, 'Oxalis' and 'acetosella' refer to the acidity of the foliage, the genus name being derived from the Greek 'oxys' meaning 'sour' or 'acid' and the Latin specific epithet 'acetosella' meaning 'vinegar salts'.

O. acetosella must be one of the species most frequently given English common names, since Grigson (1987) lists no less than 58 and Vickery (2019) lists almost the same number, although there are very many minor variants along the same lines. Many of the English names refer to the fact that people (especially children), consume the leaves either as a salad, or as a sharp tasty snack, eg 'Egg and cheese', 'Bread and cheese', or 'Cuckoo bread and cheese', the latter one of several that give an indication of springtime, when young leaves are at their most mild flavour (and oxalate content), and therefore safest to eat. Warnings are given, however, since too many leaves could lead to poisoning!

Other animals are mentioned, including 'Fox's meat', 'Rabbit meat', 'Hare's meat' and 'Sheep's sorrel'. References to 'lady' or 'woman', such as 'Lady cakes', 'Lady's meat' and 'Woman's nightcap' bring religion into the subject as they mean Mary or 'Our lady'. Easter is another topic, with names such as 'Alleluiah' and 'Hallelujah' referring to the period of the year when the flowers first appear and the cuckoo is about. The word 'sour' is often a name element also, such as 'Sour sab' and 'Sookie Sourach' (Grigson 1987).

Threats

None.

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