Woad leaves

Leaves of woad, Isatis tinctoria, the European source of blue dye before indigo. The leaves were crushed to a pulp, rolled into balls and dried, then later broken up, moistened and fermented before use in a dye vat. Woad was grown in England, with later medieval centres in Lincolnshire and Somerset, and remains of woad have been found at 10th-century York; much woad was also imported.

Sources: en.wikipedia.org