Aesculus hippocastanum – May 20th 2024- (Horse Chestnut)
There is no finer sight in the month of May than a fully-flowering Horse Chestnut tree. No wonder the tree has been widely planted in parklands, town squares, suburban streets and especially on village greens in the shires of England. It brings good cheer at a time of year when warmth finally arrives.
Inflorescence, a spire-like panicle held on upturned shoots. Photo: John Grace
It is not native to Britain. Its origin was a mystery for some time, several people claiming it came from but North America, but the consensus now is that it originates from mountainous areas of the Balkans. The largest native populations are in mainland Greece, in the central Thessaly Mountains. Seeds were sent to Vienna around 1576 and from there to several European countries. Plants were first raised in Northern Europe by Charles de l’Écluse (also called Carolus Clusius) from seeds arriving via Constantinople. Gerard knew the plant and provided a good description in the revised and corrected 1633 edition of his herbal. He says “The Horse Chestnut groweth in ltaly, and in sundry places of the East countries. It is now growing with Mr.Tradescant at South Lambeth” (I think that will be John Tradescant the Elder, naturalist, gardener and traveller). In Britain it was known from the wild by 1870 (Preston et al. 2002) and now can be found over much of the country. Its seeds germinate readily and it colonises both grassy and wooded places.
Horse Chestnut in an urban setting, at the bus stop, Morningside Road, Edinburgh, 18th May 2024. Photo: John Grace.
The flowers are in conical panicles with male flowers at the top and hermaphrodite flowers below. The lower flowers open first, and are visited by many large insects including honeybees, bumblebees, hoverflies and solitary bees. Insects are attracted by scent and by the yellow pigment on the petals. The reward is pollen and rich nectar. Once the flower has been pollinated the pigment changes to red, which according to early researchers is unattractive to the pollinators who see it as black, and so they go elsewhere (a novel sort of ‘traffic light’ system). Pollen has been detected in urban air (around 1% of the total air-borne pollen), and so wind pollination probably has some role to play.
The average date of bud-break was reported by Sparks et al (2000) as April 10th and the first-flowering date was May 8th. These dates are based on long runs of past observational data, and they are valuable as baselines to assess impacts of climate change. In the case of flowering, the date was found to be highly dependent on the air temperatures from March to May. From their Figure 3 (in Sparks et al 2000) we estimate that 1 degree of warming is likely to bring forward the flowers by ten days.
Flowers, May 15th 2024, Blackford Hill, Edinburgh
The main features of the tree are well known. It is a tall tree up to 30 metres, with conspicuously upturned lower branches. It has resinous overwintering buds that we called ‘stickybuds’ when we were children. Its leaves are palmate (usually five leaflets, sometimes more). The old leaf scars look like horse-shoes. Its most famous feature is the seed, which everyone calls a ‘conker’.
Lesser-known features include its mycorrhizal system. Most temperature trees have ectotrophic mycorrhizas (the sort that form a sheath around the roots and produce the fruiting bodies we call toadstools), but Horse Chestnut is an exception. It has arbuscular mycorrhizal fungi and there is no evidence of ectomycorrhizas (Thomas et al 2019). Neither the reason nor significance of this is known.
Its wood is soft and yellowish with a pink tinge; weak, and therefore no good for structural purposes but useful for making kitchen utensils and children’s toys. Also, good for fruit racks and storage trays.
Lesions on the bark, ‘stag-headed’ crown and topmost leaves yellowing already in mid-May. All symptoms of Bleeding Canker disease. This is one of several trees remaining in Morningside Drive, Edinburgh. Infected trees are removed and replaced with other species. Photos: John Grace.
The tree is susceptible to attack by Leaf Miners, the caterpillars of the moth Cameraria ohridella (C. ohridella). Caterpillars enter the mesophyll and chew away, leaving brown necrotic patches on the leaf. The condition is rather common in continental Europe and England/Wales, but less common in Scotland. It does not usually kill the trees. More serious is the Bleeding Canker disease, now known to be caused by the bacterium Pseudomonas syringae pv. aesculi (Webber et al 2008). Lesions appear on the bark, from which a dark fluid oozes. The disease is especially frequent in urban trees according to a survey by the UK’s Forestry Commission in 2007, with over 50% of Horse Chestnut trees infected. The external symptoms include the death of top-most branches.
Seeds (called ‘conkers’ ) and their seed case. Attribution: Andrew Dunn, Solipsist, CC BY-SA 2.0 https://creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons
On a lighter note, what of the game of conkers? It began in the mid-19th Century. A hole is drilled in the conker and a string (perhaps a shoe-lace) is threaded through. The conker is then suspended by the defending player for the attacking player to strike with his or her own stringed conker. Each year on the second Sunday of October, the World Conker Championships are held in the village of Ashton in Northamptonshire. The word ‘conker’ may originate from ‘conquer’ or from the word for a shell, ‘conch’, as a similar game was formerly played with snail shells. ‘Conkers’ is a highly competitive game, but susceptible to cheating (eg the use of araldite and conker-hardening agents). It isn’t likely to become an Olympic sport any time soon.
The game and several other pieces of folk-lore associated with the Horse Chestnut are described by Mabey (1996) and Vickery (1995).
Global distribution of Horse Chestnut. From GBIF.
We were always told by our parents that conkers are poisonous (unlike the completely unrelated Sweet Chestnut) and indeed they are (child deaths have been recorded). The toxic component is a saponin called aesculin. Despite this, some animals do eat conkers (wild boar, deer), and in Victorian England a type of flour was made after leaching the milled material with hot water (Mabey (1996). A ‘coffee’ has been made from roasted conkers.
A red-flowered variety growing at Culzean Castle, Ayrshire. Photo: John Grace.
There have been several other uses for Horse Chestnuts over the years. In World War I the seeds were used as a source of starch to turn into the solvent acetone, a vital ingredient in the manufacture of the explosive, chordite. The process was developed by Russian-born Chaim Weizmann who was then a lecturer at Manchester University. In wartime, starch was in short supply but readily available from Horse Chestnuts. He used the bacterium Clostridium acetobutylicum for the fermentation. Shortly after that he became the director of the British Admiralty Laboratories, and later he was the first President of Israel.
The plant has abundant medicinal properties, described at the website of Plants for a Future:
I saw many offers of Horse Chestnut medicines on Amazon.com, but I cannot vouch for medicinal preparations as this is not my field of expertise! I did however learn something about the use of the plant in equine medicine from the website of the Oxford University herbarium: “This plant’s common name may come from its use as a horse medicine, which may explain its rate of spread in the UK. Blacksmiths used a mixture of horse chestnut bark and seed to treat horses with stomach complaints. Ground seeds were also used to treat horse’s coughs. When its leaves drop in autumn the scar left behind even looks like a horseshoe!”
Acknowledgement
The review by Peter Thomas, published in the Journal of Ecology 2019 has aided my task of writing this blog, and is highly recommended for further reading.
Notes
1. Aesculus belongs to the Soapberry family, the Sapindaceae.
2. Over the years, many varieties of this species have been developed by selection. There is a striking red-flowered form, and another which has no conkers. The conkerless variety is for planting in built-up areas where the practice of throwing sticks to bring down conkers might cause injury.
References
Mabey R (1996) Flora Britannica. Chatto and Windus.
Preston CD, Pearman DA & Dines TD (2002). New atlas of the British and Irish flora. Oxford, UK: Oxford University Press.
Pocock MJ O & Evans D M (2014). The success of the horse-chestnut leaf-miner, Cameraria ohridella, in the UK revealed with hypothesis-led citizen science. PLoS ONE, 9, e86226. https://doi.org/10.1371/journal.pone.0086226
Sparks T, Jeffree E & Jeffree C (2000) An examination of the relationship between flowering times and temperature at the national scale using long-term phenological records from the UK. International Journal of Biometeorol 44, 82–87 (2000). https://doi.org/10.1007/s004840000049
Thomas PA et al (2019) Biological Flora of the British Isles: Aesculus hippocastanum. Journal of Ecology 107, 992-1030.
Vickery R (1995) Dictionary of Plant-Lore. Oxford University Press.
Webber J F et al (2008). Isolation and identification of Pseudomonas syringae pv. aesculi causing bleeding canker of horse-chestnut in the UK. Plant Pathology, 57, 368. https://doi.org/10.1111/j.1365-3059.2007.01754.x
©John Grace
