Woad, Isatis tinctoria, is a member of the brassica family, which also includes cabbages, and is native to Europe, where it is has been cultivated since the Stone Age. Similar to vegetable indigo, but less easy to process, and more delicate in colour, woad must be oxidised, turning the dye from a yellowish-green to blue.
The French Revolution (1789-99) caused the fall of many previously royal or bourgeois industries, but when Napoleon declared himself Emperor in 1804, he took to wearing blue (woad dyed) silk as the Imperial colour, very much as the aristocracy had done before the revolution. Over time indigo was imported with cotton, first into the port of Nîmes, and as cheaper blue fabric became available commoners started to adopt indigo dyed cotton garments, imitating the woad dyed silk of the ruling classes. Serge de Nîmes gradually became the staple that it is today, and the much more expensive and complex art of woad dyeing died out.