a.k.a. Ritha tree (scientific name: Sapindus mukorossi)
The Soapnut-tree or Ritha tree belongs to Sapindaceae (Soapberry family). In Japanese: むくろじ (無患子) pronounced mukuroji
It is a deciduous tall tree that can reach 15 m. It is found westward from Chubu district of Honshu to Shikoku, Kyushu of Japan and all over Asia.
The bark is pale yellowish-brown, smooth but fissured when old. The tree has narrow lancelike pinnate compound leaves occurring in 4-6 pairs. It bears panicles of small yellowish-green flowers in June. The fruits are drupes and ripen in fall.
As I was walking through the Institute for Nature Study forest this autumn, a fellow hiker told me that if you dropped the fruit into a pet bottle of water, you’d have a foamy soap solution. This is because the endocarps contain saponin so it was used as substitute for soap in olden days in Japan and apparently, by other cultures such as the Native Americans as well as in India where they are a popular ingredient in Ayurvedic shampoos and cleansers. They are used in Ayurvedic medicine as a treatment for eczema, psoriasis, and for removing freckles. Soap nuts have gentle insecticidal and antimicrobial properties and are traditionally also used for removing lice from the scalp.