I've been googling the name of the plant for a week and finally found it. This is Chinese knotweed (Persicaria chinensis). The tiny flowers are so cute! Chinese knotweed is supposed to flower from May to November in Japan, which is why it took so long to find out its name. I see Chinese knotweed only there and have no idea where it came from.
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| Chinese knotweed (Persicaria chinensis) ツルソバ（蔓蕎麦） |
