Persicaria tinctoria

Persicaria tinctoria is a species of flowering plant in the buckwheat family. Common names include Chinese indigo and Japanese indigo. It is native to Eastern Europe and Asia.
The leaves were a source of indigo dye. It was already in use in the Western Zhou period (c. 1045–771 B.C.), and was the most important blue dye in East Asia until the arrival of "Indigofera" from the south.