Aesculus hippocastanum is a large deciduous, synoecious tree, commonly known as horse-chestnut or conker tree. The genus Aesculus comprises 13–19 species of trees and shrubs native to the temperate Northern Hemisphere, with 6 species native to North America and 7–13 species native to Eurasia; there are also several hybrids.
Aesculus hippocastanum is native to a small area in the Pindus Mountains mixed forests and Balkan mixed forests of South East Europe. Cultivation for its spectacular spring flowers is successful in a wide range of temperate climatic conditions provided summers are not too hot, with trees being grown as far north as Edmonton, Alberta, Canada, the Faroe Islands, Reykjavík, Iceland and Harstad, Norway.
It is widely cultivated in streets and parks throughout the temperate world, and has been particularly successful in places like the United Kingdom, where they are commonly found along river banks.
Horse-chestnuts have been threatened by the leaf-mining moth Cameraria ohridella, whose larvae feed on horse chestnut leaves.
Aesculus hippocastanum grows to 36 metres tall, with a domed crown of stout branches; on old trees the outer branches often pendulous with curled-up tips. The leaves are opposite and palmately compound, with 5–7 leaflets; each leaflet is 13–30 cm long, making the whole leaf up to 60 cm across, with a 7–20 cm petiole. The leaf scars left on twigs after the leaves have fallen have a distinctive horseshoe shape, complete with seven “nails”. The flowers are usually white with a small red spot; they are produced in spring in erect panicles tall with about 20–50 flowers on each panicle. Usually only 1–5 fruit develop on each panicle; the shell is a green, spiky capsule containing one (rarely two or three) nut-like seeds called conkers or horse-chestnuts. Each conker is 2–4 cm diameter, glossy nut-brown with a whitish scar at the base. In Britain and Ireland, the seeds are used for the popular children’s game conkers.
The common name “horse-chestnut” (often unhyphenated) is reported as having originated from the erroneous belief that the tree was a kind of chestnut (though in fact only distantly related), together with the observation that eating the fruit cured horses of chest complaints despite this plant being poisonous to horses.
COMPARE to Castanea sativa, the sweet chestnut:
The seeds, especially those that are young and fresh, are slightly poisonous, containing alkaloid saponins and glucosides. Although not dangerous to touch, they cause sickness when eaten; consumed by horses, they can cause tremors and lack of coordination. Some mammals, notably deer, are able to break down the toxins and eat them safely.
Raw Horse Chestnut seed, leaf, bark and flower are toxic due to the presence of esculin and should not be ingested. Horse chestnut seed is classified by the FDA as an unsafe herb. The glycoside and saponin constituents are considered toxic.
Quercetin 3,4′-diglucoside, a flavonol glycoside can also be found in horse chestnut seeds. Leucocyanidin, leucodelphinidin and procyanidin A2 can also be found in horse chestnut.
The genus Aesculus exhibits a classical arcto-Tertiary distribution. The genus has traditionally been treated in the ditypic family Hippocastanaceae along with Billia,but recent phylogenetic analysis of morphological and molecular data has caused this family, along with the Aceraceae (Maples and Dipteronia), to be included in the soapberry family (Sapindaceae).