Which plant species is the principal natural source of the blue dye historically called woad?

Isatis tinctoria is the plant species that provides the historic blue dye called woad.

Woad is a biennial plant native to parts of Eurasia. Its leaves contain precursors of indigotin, the blue compound responsible for the dye. After the leaves are harvested and processed through fermentation or related preparation, the precursors are converted into the blue pigment.

Woad was important in Europe before imported indigo became widely available. The dye from woad is chemically related to indigo obtained from Indigofera species, but the plants, cultivation histories, and processing traditions differ. This is a common source of confusion because both can produce indigotin.

The plant has also spread beyond its original range and can behave as an invasive species in some regions. Its yellow flowers and rosette of leaves make it recognizable before dye processing begins.