Fallopia japonica (Japanese Knotweed) in Japan: Why Is It Not a Pest for Japanese People?

Fallopia japonica is native to Japan, Taiwan and Korea. It is now widely naturalized in Europe and North America, and is regarded as one of the worst invasive alien species. For Japanese people, F. japonica has been one of the most familiar and useful wild plants for centuries. It is rare to regard the plant as a harmful weed in Japan. Fallopia japonica has many ethnobotanical uses as an edible and medicinal plant. Therefore it was recorded in many historical documents, by different names. A dictionary of local plant names recorded 689 words for F. japonica, and it was the plant with the most names among all plants listed. As a case study of F. japonica, we conducted surveys on its habitats and growth from the foot to the alpine belt of Mt. Fuji, the highest mountain in Japan. Fallopia japonica was found widely in man-made and natural habitats but was restricted to open, sunny sites. The plant was smaller in the habitats where the vegetation is cut repeatedly. Japanese phytosociological studies regarded F. japonica as the character species of an Artemisietea principis and as a constituent species of diverse herbaceous, shrub and open woodland communities. In Japan, vegetation is controlled by cutting or mowing near habitations and other infrastructure. In meadows, tall grasses dominate the vegetation. In mountainous areas, trees are dominant and open areas are limited. It is difficult for F. japonica to overgrow a wide area and to form a large stand. In addition, the plant has been regarded as useful plant. These matters seem to make F. japonica a common and familiar plant, not a pest, for Japanese people.