The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as I. A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia.
Dyer's knotweed (Polygonum tinctorum) was the most important blue dye in East Asia until the arrival of the Indigofera species from the south, which yield more dye.
Oxidation by exposure to air converts indoxyl to indigo.
Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2–0.8% of this compound.
The association of India with indigo is reflected in the Greek word for the dye, indikón (ινδικόν, Indian).
The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.
Indigo plantations also thrived in the Virgin Islands.The oldest known fabric dyed indigo dating to 6,000 years ago was discovered in 2009 at Huaca Prieta, Peru.India was a primary supplier of indigo to Europe as early as the Greco-Roman era.Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England.Many indigo plantations were established by European powers in tropical climates.