From the CO2Science archive: Woad, or Isatis indigotica, or its closely-related (and possibly identical) cousin Isatis tinctoria, was an important source of blue dye from ancient times onward (the same valuable dye as in “true indigo” but at a lower concentration). Its name is wonderfully medieval-sounding and indeed it played an important role in medieval artworks including tapestries and the Lindisfarne gospels. (As well as being used to decorate the maniacal Picts or Celts or whatever emerged from the mist and attempted to swarm over Hadrian’s Wall, perhaps a less appealing use. And not to spoil the fun, but it’s not true that William Wallace wore woad, nor that he engaged in swordfights without shirt or armour; as another Wikipedia article notes laconically, the film Braveheart “was criticized for many historical inaccuracies”.) Alas its favour as a coveted source of pigment gave way first to indifference once synthetic dyes took over, and now to outright hostility in some places where it is viewed as a noxious weed. Woe to woad. But Wikipedia reports some fans are cultivating it again in Germany and the UK to make specialty inks. They’re in luck because it benefits from extra CO2. In 2013 and 2017 there were five experiments showing an extra 300 ppm boosted its growth by 43.8 percent. We want woad! We want woad!