woad

Woad (Isatis tinctoria) is a flowering plant in the mustard family (Brassicaceae) that has long been cultivated as a source of blue dye. Native to Eurasia, it became a staple dye plant across Europe before the rise of synthetic indigo.

Historically, woad was the principal source of blue textile dye in medieval and early modern Europe. The leaves contain indican, a glucoside. In fermentation vats, often with reducing agents such as urine, indican is hydrolyzed to indoxyl, which oxidizes and dimerizes to indigo. The blue pigment is then absorbed by fibers in a vat dyeing process, producing a pale to medium blue known as woad blue.

Woad is a tall, herbaceous biennial, with grey-green, lanceolate leaves and yellow flowers arranged in loose racemes. It thrives in temperate climates on well-drained soils and is commonly included in crop rotations. Leaves are harvested in the first year for dye production, before the plant bolts in the second year.

In modern times, the use of woad as a dye source declined with the introduction of synthetic indigo in the 19th century, but the plant remains of interest to natural-dye practitioners and in historical or ornamental contexts. It is cultivated in some regions for dye production, seed, or as a decorative plant, and can contribute to pest resistance and soil health in rotation.

Woad is not a food crop and, like many members of the Brassicaceae, contains glucosinolates that can irritate skin for some individuals. Handling precautions are therefore advised when cultivating or processing the plant.