The Vul Nut is one of a handful of reannual plants on the Disc, grown in areas such as the Octarine Grass Country. These plants are called such because they travel back in time- they are planted after they're harvested.
They are mentioned in The Colour of Magic as being grown in the Rehigreed Province of the Agatean Empire, and are processed into a drink called Ghlen Livid. Since Ghlen Livid is made from a reannual plant, it enables anyone who drinks it to see the future. This makes it highly sought after by fortune-tellers.