Indigo

Indian indigo, also known as true indigo (Indigofera tinctoria) is a species of shrub in the pea (Fabaceae) family, of unknown origin.  It has become widely naturalized to tropical and temperate Asia, as well as parts of Africa.  It has a long history of cultivation and was the original source of the blue dye called indigo. Other species within the Indigofera genus have also become widely naturalized outside of their native regions, and are cultivated to produce both tannin and indigo.  Notable examples are anil indigo (I. suffruticosa, native to the Americas), and Natal indigo (I. arrecta).  As a legume (a nitrogen-fixing plant), it is often used in crop rotations to improve the soil.