Which plant is the main source of the natural blue dye indigo?

Indigofera tinctoria is the main traditional source of natural indigo dye.

Indigofera tinctoria is a shrub in the legume family, Fabaceae, native to tropical regions of Asia. Its leaves contain indican, a precursor that can be processed and oxidized to produce the deep blue pigment indigo. The dye is used for textiles, including historically important forms of blue cloth.

Traditional production involves fermenting or otherwise processing plant material so that indican breaks down into compounds that can form indigo. The blue color appears through oxidation rather than simply being squeezed from fresh leaves as a ready-made pigment.

Indigofera tinctoria was not the only historical indigo source. Woad, Isatis tinctoria, supplied indigo in parts of Europe. These plants are different species, but they produce closely related dye chemistry. Synthetic indigo later transformed large-scale textile manufacturing.