Woad (isatis tinctoria) blooms early, tall stalks with lots of tiny yellow flowers.
It’s a biennial plant. The best colour comes from first year leaves.
Woad was used for centuries in Europe and the Middle East to get a light-fast blue. The introduction of more efficient indigo plants from Eastern Asia spurred bans in 16th century France and Germany, to protect the lucrative woad industry.
The leaves are soaked and strained, the water made alkaline and aerated.
The pigment is reduced, giving a bath with hints of blue in the foam.
The blue magically appears after the cotton is taken out of the vat and exposed to the air.