Persicaria is the botanical name for a group of often invasive perennials, annuals and climbers that are sometimes grown for their handsome flowers which are often in spikes. From the family of the Polygonaceae.
The plant in the photo could be Persicaria lapithifolia or it could be Persicaria Ooinutade (see photos at this link). Japanese names: タヌタデ(Tanutade) イヌタデ(Inutade)
Persicaria can be mistaken for the tamarisk and knotweed.
Source: botany.com
NATURE IN JAPAN 