Reannual plants are an unusual sort of flora native to the Disc. It works like this: when a farmer plants a reannual this year, the plant is harvested last year. There are several challenges involved in doing so: for one, if you forget to sow the seeds after you harvest them, you risk disturbing the entire fabric of causality (although, as mentioned in Mort, causality is stronger than most people think). Thus, it takes farmers who are much given to introspection and close examination of the calendar (like Mort's father Lezek, and unlike Mort), to pull it off correctly.
The Octarine Grass Country is one of the few places on the Disc where such plants are grown - the Rehigreed Province in the Agatean Empire is another. The reannual Vul Nuts are mentioned in The Colour of Magic as being grown in the latter place, and when harvested they make a drink called Ghlen Livid. Mort's family specializes in making wine from reannual grapes, which cause their own problems, as you have the hangover the morning before, and have to drink quite a lot to get over it.
They are highly sought after by fortune-tellers, as they enable people to see the future.
In Pyramids Pteppic also celebrates his final exam with Counterwise Wine made from a reannual grape variety bought by Chidder, and causing Pteppic to have nausea and a sickening headache throughout his final Run.