Which plant traditionally provides the natural indigo used to dye blue jeans?

Indigofera tinctoria traditionally provides the natural indigo used to dye blue jeans.

Indigofera tinctoria is a shrub in the bean family cultivated for centuries as a source of indigo dye. Its leaves contain indican, a compound that can be processed and oxidized to produce the deep blue pigment historically used on cotton textiles.

Traditional indigo dyeing does not simply paint blue color onto fabric. Yarn or cloth is dipped into a reduced dye bath, then exposed to air, where the dye oxidizes and turns blue. Repeated dips can build a darker shade.

Most modern denim uses synthetic indigo rather than plant-derived dye. The plant remains important in the history of textile dyeing, but it is not the cotton source: cotton comes from species of Gossypium.