Aesculus hippocastanum is a large tree, growing to about 39 metres (128 ft) tall[9] with a domed crown of stout branches. On old trees, the outer branches are often pendulous with curled-up tips. Theleaves are opposite andpalmately compound, with 5–7 leaflets 13–30 cm (5–12 in) long, making the whole leaf up to 60 cm (24 in) across, with a 7–20 cm (3–8 in) petiole. The leaf scars left on twigs after the leaves have fallen have a distinctivehorseshoe shape, complete with seven "nails". Theflowers are usually white with a yellow to pink blotch at the base of the petals;[9] they are produced in spring in erect panicles 10–30 cm (4–12 in) tall with about 20–50 flowers on each panicle. Its pollen is not poisonous for honey bees.[10] Usually only 1–5fruits develop on each panicle. The shell is a green, spikycapsule containing one (rarely two or three)nut-likeseeds called conkers or horse-chestnuts. Each conker is 2–4 cm (3⁄4–1+1⁄2 in) in diameter, glossy nut-brown with a whitish scar at the base.[11]
InflorescenceFloral diagram ofA. hippocastanum. The light green structure denotes nectary.Floral formula: ↘ K5 C2:2 A5+2G(3).
The common name horse chestnut originates from the similarity of the leaves and fruits to sweet chestnuts,Castanea sativa (a tree in a different family, theFagaceae),[9] together with the alleged observation that the fruit or seeds could help panting or coughing horses.[12][13]
Although it is sometimes known asbuckeye,[7] for the resemblance of the seed to adeer's eye, the termbuckeye is more commonly used forNew World members of the genusAesculus,[14] such asAesculus glabra.
The compact native population of horse chestnut in Bulgaria is distinct from the horse chestnut forests of northern Greece, western North Macedonia and Albania. It is limited to an area of 9 ha (22 acres) in thePreslav Mountain north of theBalkan Mountains, in the valleys of the Dervishka and Lazarska rivers. Bulgaria'srelict horse chestnut forests are critically endangered at the national level and protected as part of the Dervisha Managed Nature Reserve.[18]
It is widely cultivated in streets and parks throughout thetemperate world, and has been particularly successful in places likeIreland, Great Britain and New Zealand, where it is commonly found in parks, streets and avenues. Cultivation for its spectacular spring flowers is successful in a wide range oftemperate climatic conditions provided summers are not too hot, with trees being grown as far north asEdmonton, Alberta, Canada,[19] theFaroe Islands,[20]Reykjavík, Iceland andHarstad, Norway.
In Britain and Ireland, the seeds are used for the popular children's gameconkers. During theFirst World War, there was a campaign to ask for everyone (including children) to collect the seeds and donate them to the government. The conkers were used as a source of starch for fermentation using theClostridium acetobutylicum method devised byChaim Weizmann to produceacetone for use as a solvent for the production ofcordite, which was then used in military armaments. Weizmann's process could use any source of starch, but the government chose to ask for conkers to avoid causing starvation by depleting food sources. But conkers were found to be a poor source, and the factory only produced acetone for three months; however, they were collected again in theSecond World War for the same reason.[21]
A selection of fresh conkers
The seeds, especially those that are young and fresh, are slightly poisonous, containingalkaloidsaponins andglucosides. Although not dangerous to touch, they cause sickness when eaten; consumed by horses, they can cause tremors and lack of coordination.[22]
The horse-chestnut is a favorite subject forbonsai.[23]
Though the seeds are said to repel spiders, there is little evidence to support these claims. The presence of saponin may repel insects, but it is not clear whether this is effective on spiders.[24]
Aesculus hippocastanum is affected by the leaf-mining mothCameraria ohridella, whose larvae feed on horse chestnut leaves. The moth was described fromNorth Macedonia where the species was discovered in 1984 but took 18 years to reach Britain.[25]
In Germany, they are commonly planted inbeer gardens, particularly inBavaria. Prior to the advent of mechanical refrigeration, brewers would dig cellars forlagering. To further protect the cellars from the summer heat, they would plant horse chestnut trees, which have spreading, dense canopies but shallow roots which would not intrude on the caverns. The practice of serving beer at these sites evolved into the modern beer garden.[26]
An inexpensive detergent for washing clothes can be made at home from conkers, and this is said to be an environmentally benign ('eco-friendly') detergent.[27]
There is risk ofacute kidney injury, "when patients, who had undergone cardiac surgery were given high doses of horse chestnut extract i.v. for postoperativeoedema. The phenomenon was dose dependent as no alteration in kidney function was recorded with 340 μg/kg, mild kidney function impairment developed with 360 μg/kg and acute kidney injury with 510 μg/kg".[30]
Raw horse chestnut seed, leaf, bark and flower are toxic due to the presence ofaesculin and should not be ingested. Horse chestnut seed is classified by the FDA as an unsafe herb.[31] Theglycoside andsaponin constituents are considered toxic.[31]
Bleeding canker. Half of all horse-chestnuts in Great Britain are now showing symptoms to some degree of this potentially lethal bacterial infection.[32][33]
Guignardia leaf blotch, caused by the fungusGuignardia aesculi
Horse-chestnut leaf miner,Cameraria ohridella, a leaf mining moth.[34] The larvae of this moth species bore through the leaves of the horse chestnut, causing premature colour changes and leaf loss.[33]
Phytophthora bleeding canker, a fungal infection[35]
A horse-chestnut tree stood outside the window of the"secret annex" in the centre ofAmsterdam, whichAnne Frank mentioned inher diary; the tree survived until August 2010, when a heavy wind blew it over.[36][37] Eleven young specimens, sprouted from seeds from this tree, were transported to the United States. After a long quarantine in Indianapolis, each tree was shipped off to a new home at a notable museum or institution in the US, such as9/11 Memorial Park,Little Rock Central High School, and twoHolocaust Centers. One of them was planted outdoors in March 2013 in front ofThe Children's Museum of Indianapolis, where they were originally quarantined.[38]
^Brouillet L, Desmet P, Coursol F, et al."Aesculus hippocastanum Linnaeus".data.canadensys.net. Database of Vascular Plants of Canada (VASCAN). Archived fromthe original on 2018-02-17. Retrieved26 September 2020.
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