During the Roman conquest of Britain, Julius Caesar wrote, "All Britons dye themselves with woad which makes them blue, so that in battle their appearance is more terrible." Actually, the Romans imported the dye from the East to die their own shields, so I'm not too sure why Caesar was so excited to see it...
Woad is extracted from the European and Asian plant Isatis tinctoria. The blue part of the colorant is genetically identical with that of indigo (Indigofera), which is cultivated entirely in Asia. The plant will suck all the nutrients from any soil in which it is grown, causing those who cultivate it to move frequently. The plant is a member of the mustard family; it can be a winter annual, a biennial or a short-lived perennial. Aboveground the plant is about one to five feet in height, but the tap root can be up to five feet in length, and the plant tends to grow like a weed in disturbed areas (destroying the soil further). The spread of the plant in the Western states, especially Utah, is seriously disruptive to dormant farmland. Leaves are a bluish green with white veins; they also sport yellow flowers and bear fruit in the form of a purplish single-seed pod.
The ancient process of manufacturing woad was almost identical to that of indigo. The plants were dried out in the sun, then ground up and fermented in a giant pot. Inside the pot, they were beaten out so that a blue-colored froth would rise to the surface. Next, the plants were soaked in urine, which gives off ammonia gas as it evaporates. Because this process smelled absolutely disgusting, the dye-maker was forced to live on the outskirts of a village.
Thanks to the March 2002 edition of Natural History and my local nursery, Jarod's.