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Oxalis corniculata L., Procumbent Yellow-sorrel

Account Summary

Introduction, neophyte, invasive garden escape, so far very rare.

August 1997; Northridge, R.H.; Enniskillen Town.

Growth form, variation and Irish occurrence

This yellow-flowered, small, creeping Oxalis sp., 15(-20) cm tall, is so very variable, phenotypically and genetically, that local forms have often been described as new species (Lovett Doust et al. 1985). It has only twice been recorded in Fermanagh, once as var. atropurpurea (see below) and once in this guise as the green-leaved form. Almost all forms of this species, however, demonstrate some degree of purplish tinge in the leaflets and, again, this character varies in response to the prevailing light levels. Most B & I introduced Oxalis species originate in warmer climes, they are not very competitive and typically they only occupy very open, often disturbed habitats, sometimes ± confined to greenhouses (Louvett Doust et al. 1985).

The more-or-less green-leaved form of O. corniculata was found naturalised in Enniskillen town as a persistent (perhaps pernicious) garden weed, rather than in a truly 'wild' situation. The Cat Alien Pl Ir lists records from seven other Irish VCs. The records from NI (Cos Tyrone (H36), Down (H38) and Antrim (H39)) all report this species as being confined to gardens, greenhouses (where it can easily run riot and become a real nuisance weed) or nurseries (FNEI 3). The survey of urban Belfast found it in seven 1-km squares, mainly as a weed of flowerbeds (Urban Flora of Belfast).

The status of introduced plants

It is often difficult to decide whether or not introduced plants like Oxalis in gardens should ever be considered 'wild', or, if so, in what sense they are wild or feral? The ability of a planted garden subject to perpetuate itself without the continuing assistance of human maintenance is insufficient by itself. As D.P. Young (1958, p. 53) pointed out in an article specific to Oxalis growing in B & I, the real criterion hinges on the plant's capacity to withstand all attempts to eradicate it!

In areas with mild winters and at least indoors, O. corniculata is a freely seeding, short-lived perennial with a tough taproot. In cooler conditions, it behaves as an annual, overwintering exclusively by seed. The plant produces procumbent, creeping stems or stolons that root at the nodes and it can prove invasive and persistent, but it is not quite as pernicious as those species that also produce bulbils (Louvett Doust et al. 1985).

Introduction and spread in Britain and Ireland

O. corniculata is believed by Eiten (1959) to have originated somewhere in tropical Asia, Malaysia, Australasia, or in high ground on islands in the W Pacific. In other words, it is an 'Old World' species and has been introduced to northern temperate areas of the world in historic times (Louvett Doust et al. 1985). It was introduced to gardens in Britain prior to 1585 when it was first reported as a garden weed in Somerset (Raven 1953; Stace & Crawley 2015). It was first recorded beyond the garden confines in 1770.

O. corniculata did not begin to spread much until the 20th century, but since the 1960s has become increasingly invasive; having consolidated in the S & W of England, it is now spreading north and in addition to gardens, is colonising paths, walls and waste ground (Sell & Murrell 2009). The New Atlas hectad map shows O. corniculata has reached Inverness, although the occurrence is only rather scattered north of a line between Hull and Liverpool (M.F. Watson, in: Preston et al. 2002).

In the rather warmer climatic conditions of S Ireland (eg Mid-Cork (H4) and Cos Waterford (H6), Dublin (H21) and Louth (H31)), a solitary plant has been found at a seaside bay in Mid-Cork, while the other VCs reported habitats 'beyond the garden wall' included streamside and roadside banks and on waste ground.

Reproduction and dispersal

The small yellow flowers, petals 4-7 mm, are borne in 1-7 flowered umbels. They successfully self-pollinate if insect visitors fail to oblige and are completely self-fertile (Louvett Doust et al. 1985). Although the plant can spread vegetatively to a limited, local extent, dispersal is mainly by seed which is explosively shed from the capsules, travelling a distance of approximately 2 m from the parent plant. The seeds are also sticky and may be spread further afield by rodents. Since the species has shown itself capable of spreading quite rapidly in Britain, and has gradually become a pernicious weed, quite often it may also be transported with waste as a garden discard. Thanks to the characteristic generosity of gardeners sharing plants with their friends, it very probably is also accidentally passed in pots from garden to garden, as well as through the horticultural trade (Young 1958).

Threats

Colonising behaviour, spread and persistence by this small species is not really a problem in Ireland as yet, but it might eventually become one if it follows the long-delayed invasive pattern it has displayed in Britain.