Aesculus turbinata : Japanese Horse Chestnut
Common English Name: Japanese Horse Chestnut
Japanese Name (Katakana): トチノキ
Japanese Name (Kanji): 栃の木
Height: 20 - 30 meters
Endemic to Japan, Aesculus turbinata is known for its large compound leaves and edible nuts that were an important food source in ancient Japan. It can be found in Hokkaido (below Sapporo), Honshu, Shikoku, and Kyushu. The origin of the Japanese name トチノキ (Tochinoki) is disputed. Some theories propose it comes from the language of the Ainu, the indigenous people of northern Japan. Others suggest the name derives from the plentiful amount of nuts the trees provide, as the Japanese katakana for トチ (tochi) is very similar to the kanji ‘十千’ which means ‘ten thousand.’ In wild habitats, these trees can often be found in the mountains alongside a creek, though it is also a commonly planted species of street and park tree.
The seeds of Aesculus turbinata were an important food staple in the ancient Jomon era (縄文時代) of Japan (roughly 14,000 BCE - 300 BCE). The seeds contain large amounts of saponins, which are bitter-tasting compounds produced by plants to deter other organisms from eating it. Though more toxic to organisms like fish than humans (because saponins are poorly digested by mammals), the seeds of Aesculus turbinata must be thoroughly processed to make consumption safe for people and less bitter. The processing of Aesculus turbinata takes years of experience to perfect and involves a multi-day procedure of softening, peeling, and dipping the nut into running water and ash to leach out the saponins.
There is evidence that ancient people had figured out this process by the mid-Jomon era (approximately 2500 BCE - 1500 BCE). Aesculus turbinata seeds have been found at more than 80 ancient archaeological sites across Japan. At many of these sites, areas designed to cultivate Aesculus turbinata trees and evidence of communal seed processing spots have been found.
Even after the end of the Jomon era, Aesculus turbinata seeds remained an important food source at various points of Japanese history (especially during times of famine) because of its reliable and consistent production of seeds. It has also served as an alternative food source in mountainous areas where rice was scarce. Traditionally, Aesculus turbinata seeds have been eaten as “Tochi-mochi” (栃餅), a horse chestnut rice cake that is made by boiling seeds and sticky rice together. Since this mochi doesn’t harden as quickly as rice mochi, Tochi-mochi has also been used as a wintertime food for hunters.
Leaves: Leaves are compound and comprised of 5 -7 dark green, serrated leaflets (for the difference between compound and simple leaves, please visit this page by the Cofrin Center for Biodiversity). Each leaflet can grow quite large, at up to 20-36 cm long. Red Horse Chestnut (Aesculus x carnea) leaves are similar, but its leaflets have doubly-serrated edges. The Japanese Bigleaf Magnolia (Magnolia obovata) is also similar, but has leaves that are simple, not compound.
Flowers and Fruit: Aesculus turbinata is in flower from May - June. The flowers are white and appear in upright, cone-shaped, branching clusters of flowers (or panicles). Each panicle can grow 15 - 25 cm in height. Producing a sweet, subtle scent, the flowers attract pollinators, such as bees, and produce nectar for three days after blooming. Almost all flowers on the panicle are male, with some hermaphroditic flowers at the bottom of the panicle.
The fruit appears from September - October, appears pear-like in shape and coloration, and is about 5 cm wide. Unlike some other horse chestnut species, Aesculus turbinata fruits do not have spines. Inside the fruit is a large seed, about 2.5 - 3 cm in width. The seed is notable because half of it is covered by its hilum - a light brown scar that denotes where the seed was attached to the inside of the fruit. The other half of the seed is dark brown, smooth, and shiny.
A note about edible plants: Tokyo Naturalist does not recommend attempting to consume or prepare gathered nuts, leaves, or other materials without the direct guidance of an expert. The information provided here is meant to provide cultural and historical background on the uses of this species in Japan, and should not serve as a guide for gathering or preparation. We can not assume responsibility for the potential adverse effects of consuming improperly identified or prepared plants. Please exercise great caution and consult with an expert for more information.
Dirr, Michael A. Dirr's Encyclopedia of Trees and Shrubs. Timber Press, 2017.
“Herbarium.” Cofrin Center for Biodiversity, University of Wisconsin-Green Bay, 2004, www.uwgb.edu/biodiversity/herbarium/trees/simple_compound_leaves01.htm.
Kitagawa, Junko. “食料の安定供給源としてのトチノキ.” https://www.chikyu.ac.jp/sato-project/news_letter/news_letter_14.pdf.
“Saponin - An Overview.” ScienceDirect , Elsevier, www.sciencedirect.com/topics/medicine-and-dentistry/saponin.
“Tochi Mochi (Horse Chestnut Rice Cake).” The Best of Gifu, Department of Agriculture, Gifu Prefectural Government , gifu-kiwami.jp/en/products/1231/.
“樹木シリーズ⑳ トチノキ.” 森と水の郷あきた あきた森づくり活動サポートセンター 総合情報サイト, 13 Aug. 2017, www.forest-akita.jp/data/2017-jumoku/20-tochi/tochi.html.
Field Guides Consulted:
Title: 葉っぱで見分け五感で楽しむ樹木図鑑 (Happa de miwake gokan de tanoshimu jumokuzukan)
Author: 林将之 (Hayashi Masayuki)
Publishing Date and Publisher: 2014, ナツメ社
Title: 樹皮・葉でわかる樹木図鑑 (Juhi・Hadewakaru jumokuzukan)
Author: 菱山忠三郎 (Hishiyama Chuzaburo)
Publishing Date and Publisher: 2011, 成美堂出版
Photography: Siri McGuire