The Arboretum has more than a dozen taxa (kinds) of buckeyes and horsechestnuts (Aesculus spp.) in the collections. Species within this genus are native to temperate regions in North America, Europe, and Asia. Aesculus traditionally has been placed in its own family, Hippocastanaceae, but some taxonomic references now place it in the much larger soapberry family (Sapindaceae).
Aesculus species range in habit from large trees like horsechestnut (A. hippocastanum, up to 100 feet tall; read about its association with beer gardens [pdf]) to small trees like red buckeye (A. pavia, typically just 10 to 20 feet tall) or large, spreading shrubs like bottlebrush buckeye (A. parviflora, 8 to 10 feet tall and up to 15 or more feet wide). All have palmately compound leaves composed of five or more leaflets.
Buckeyes and horsechestnuts bear upright panicles packed with small white, yellow, pink, or red flowers, depending on species. Most species bloom between mid spring and early summer, but the spectacular floral display of the aptly named bottlebrush buckeye occurs in midsummer–it’s well worth a July trip to the Arboretum to see specimens in bloom. Aesculus species are prone to natural hybridization; for example, there’s a broad zone of hybridization in the southeastern U.S. where the ranges of red buckeye, yellow buckeye (A. flava), and painted buckeye (A. sylvatica) intergrade. Aesculus hybrids have also occurred thanks to intercontinental plant importation by humans. The most notable example of this may be red horsechestnut (A. x carnea), which arose in the early 1800s in Europe as the result of European-native horsechestnut crossing with red buckeye that had been imported from its native range in the U.S. The hybrid, and especially cultivars like ‘Briotii’, is noted for its showy pink flowers.
Buckeyes and horsechestnuts bear large, nutlike seeds encased in thick husks that are either smooth or prickly, depending on species. Tradition holds that carrying around a glossy brown seed of Ohio buckeye (A. glabra) will bring good luck. The large seeds of horsechestnut have long been used for the game of “conkers,” in which they are strung on short strings and contestants swing them at each other, attempting to crack the opponent’s seed.