This site and its content are under development.

Aesculus hippocastanum L., Horse-chestnut

Account Summary

Introduction, neophyte, deliberately planted, frequent.

1976; Dawson, Miss N.; Necarne Forest.

Throughout the year.

Growth form and Fermanagh occurrence

As in most lowland parts of these islands, in Fermanagh Horse-chestnuts, 25(-40)m tall, deciduous, broad-crowned specimen trees, are very commonly planted in town and country parks, landed estates, larger gardens, school playgrounds and along roadside hedgerows. From these sites the tree readily seeds itself into adjacent areas, including the shade of semi-native deciduous woodland and scrub, or in more open conditions on waste ground or rough grassland. The Fermanagh flora survey found that the tree is widespread in lowland areas that are neither permanently too wet nor subject to seasonal flooding and it has been recorded across 156 tetrads, 29.6% of those in the VC total.

British and Irish occurrence, status and introduction

Despite its undoubted widespread, common and familiar nature and presence throughout lowland B & I, apart that is from peaty areas in the far north of Scotland, the Scottish isles and down the W coast of Ireland, where it is either rare or absent (New Atlas), Horse-chestnut is an introduced alien and it is rarely completely naturalised anywhere on these islands. Its present distribution and species population size definitely appears to rely on man's assistance – both active and passive. Further evidence of the alien nature of the tree is provided by the fact that in Britain it supports a total of just nine species of leaf-eating insects and mites, compared, for instance to the 98 supported by the native Beech (Fagus sylvatica) (Kennedy & Southwood 1984).

In the wild, as a native tree, A. hippocastanum is confined to three small mountain populations on the converging political boundaries of Greece, the former Yugoslavia and Albania. Botanists did not discover it until 1557, when a Flemish physician in Constantinople sent a specimen to the Italian herbalist Pietro Mattioli, indicating at the same time its use by Turks as a medicine for horses (Nelson & Walsh 1993, p. 131). Another, differing account, has it that seeds were sent from Constantinople to the Dutch botanist Charles de L'Ecluse (Clusius) who was working in Vienna, and he forwarded samples to England and to France early in the 17th century (Sell & Murrell 2009).

Irrespective of who first received Horse Chestnut seed in W Europe, samples were first dispatched to England around 1612 or 1615 and, despite its southern origin, the species has proved sufficiently variable and perfectly adaptable to the range of environmental conditions throughout these islands that specimens very often grow to large proportions and seed themselves abundantly.

Fuelled by the unusually large starting capital of the very substantial seed, A. hippocastanum saplings grow extremely rapidly in their first few decades. They first fruit when around 20 years old and reach their optimum seed-bearing stage by the time they are only 30 (Thomas 2000, p. 183). The seed must germinate soon after it is produced, as it is non-persistent, quickly loses viability and rots in damp conditions either on or under the soil surface.

Toxicity

The glossy, mahogany-coloured chestnut seed, like the bark, young leaves and flowers, is very bitter and distasteful to horses and other stock animals, although starving beasts obviously will eat anything. Cattle are reported to have been killed in the USA by a related Aesculus species, while dogs have also suffered and sometimes died from eating Horse-chestnuts. The poisonous principle is generally agreed to be a saponic glycoside named esculin (or aesculin), although it has been suggested that the tree may also contain alkaloids (Cooper & Johnson 1998). Soaking Horse-chestnuts in lime-water, or soaking and then boiling and grinding them to flour, makes them more palatable (and safer) fodder.

Seed dispersal

Since the familiar fruit, the 'conker', is relatively large and decidedly bitter and toxic, it is not easy to explain how the tree achieves natural dispersal. Ridley (1930, p.374) mentions rats nibbling and transporting chestnuts without killing them and rooks have at least once been observed taking and burying them, as also have squirrels in London, although they were the introduced American Grey Squirrel (Ridley 1930, p. 451; Nelson & Walsh 1993, p. 133).

A very quirky correspondence began in BSBI News in 1989 and continued over four issues of the newsletter, being triggered by a query from a schoolgirl to Mary Briggs, the late secretary of the Botanical Society of the British Isles (Briggs 1989). The original topic was conker dispersal, which gave rise to a number of imaginative suggested vectors, including extinct dwarf elephants and hippopotami – featuring rapid passage of large quantities of vegetable matter through the gut – thus allowing the possibility of some chestnuts surviving an incomplete digestion process. Alternative transport mechanisms considered were wild boar, bears and mountain torrents (Akeroyd 1990a, b; Whitehead 1990).

An unusual use for conkers

Additional chestnut topics, aired in BSBI News during late 1989-1990, included the widespread harvesting of the fruit in Britain during both World Wars, when Chaim Weizmann, a Jewish chemistry professor, discovered that the solvent acetone required for the production of cordite (the smokeless powder propellant for bullets and shells, superior to traditional gunpowder), could be manufactured by bacterial fermentation of starch (Ounsted 1990; Stearn 1990; Akeroyd 1990b).

Maize transported from America was used at first in place of the original source, wood, but when the German blockade of the Atlantic convoys threatened the maize supply, an appeal went out for Horse-chestnuts, which were collected across the country by school children. While a small quantity were, indeed, used by the acetone factory, it transpired that the vast bulk of the harvested conkers rotted away in railway sidings across the country (see the related Internet pages of the-tree.org.uk and the Imperial War Museum (http://collections.iwm.org.uk/server/show/ConWebDoc.1267), accessed 2004.

Timber and tree longevity

Horse-chestnut timber is soft, weak and fractures smoothly, so that large branches have a tendency to break off when deluged with heavy rain during thunderstorms. This makes it unsafe to shelter under the tree during such weather conditions (Mitchell 1996, p. 193). Largely on account of the weak timber, it is difficult or impossible to estimate the age of large, old trees, but many specimens in parts of England at least were reckoned by the famous British tree expert, the late Alan Mitchell, to be around 300 years, and he gives examples of some of these in his posthumously published book Alan Mitchell's Trees of Britain (Mitchell 1996).

Names

The genus name 'Aesculus' was a Latin name originally given to a variety of Oak (possibly even Quercus petraea), but it was applied by Carl Linnaeus to the Horse-chestnut instead. The Latinized specific epithet is a combination of two Greek words 'hippo' meaning 'horse' and 'kastanos' meaning 'chestnut'. The specific epithet (and by translation, the English common name) was coined by the French botanist, Tournefort, possibly to suggest chestnuts unfit for human consumption (Gilbert-Carter 1964), the fruits of the tree closely resembling, but only in appearance, the edible fruits of the Sweet Chestnut, Castanea sativa. 'Horse' and 'Dog' applied as elements to common names are usually intended to be derogatory.

Apart from this, the tree has two other horse connections: according to early medical lore, Horse-chestnuts were fed by Turks to their horses as a stimulant and to make their coats shine; and the large shield-shaped leaf-scars on the twigs are curved rather like a horseshoe, the vascular traces representing the nail holes!

Threats

Being an introduced species, A. hippocastanum does not qualify in terms of conservation threat assessment, but since it is so widely planted, doubtless most people would nevertheless greatly regret any dwindling of its development. Horse-chestnuts are prone to several pest and pathogen problems, undoubtedly intensified by the species limited genetic base (the founder effect). At present, the species is under severe, potentially lethal attack by the combination of a leaf mining micro-moth (Cameraria ohridella) and a bacterium (Pseudomonas syringae pathovar aesculi), that together cause defoliation and an ugly bleeding canker on the trunk and major branches. This attack on the species is spreading rapidly across both B & I and is all too easily observed now in NI. Whether it kills the tree or not remains to be seen.