Mentha japonica

Mentha japonica is a perennial plant which grows to a height of 20-40 centimeters. It spreads and reproduces clonally via thin rhizomes. Its leaves are oblong with entire margins, and its flowers vary from white to pale violet. It has an odor similar to pennyroyal (m. pulegium). The essential oil components of M. japonica appear to be variable. One 1970 survey found the primary constituents to be menthone (50.8%), isomenthone (18.6%) and pulegone (12.6%). Other chemical constituents identified in smaller amounts include 3-Octanone, 3-Octonal, d-Limonene, alpha-Pinene, beta-Pinene, Menthyl acetate and Piperitone. These numbers were broadly confirmed by a follow-up survey, but an analysis shortly after (1978) found a significantly higher pulegone content (50%) and lower menthone content (27.9%). A study of wild plants in Hokkaido recorded pulegone content as high as 64.3 to 68%, while experimentally-grown plants in Fukushima Prefecture yielded only between 24.8 and 33.9% depending on soil conditions. At least one named variation exists, a long stem variety, form. prostrata Sugimoto. This variant grows its stem more than one meter and tends to fall to the ground.

Mentha japonica is endemic to the islands of Hokkaido and Honshu within its native range. It occupies lowland & mountain wetlands, and blooms from August to October in Japan. It is considered a rare plant in its native distribution. According to the Threatened Wildlife of Japan Red Data Book, published by Japan's Ministry of Environment, Mentha japonica is classified as Near Threatened. This is an upgrade from its 2004 classification, which had listed it as Vulnerable. Documented there in the 1940s and 1950s, it is now considered extinct within Aichi Prefecture, due to loss of its preferred habitat.