You need predictable and changing seasons to grow grapes and the Game of Thrones world features long unpredictable seasons…so where does all that wine come from?
The seasons in George RR Martin’s medieval fantasy are a random, unpredictable mess. They could last anywhere from a few months to a decade and there’s no way to forecast them. As the story opens, the characters are near the end of a long, ten-year summer. They also worry about the coming winter, which will cause mass starvation if it also lasts years on end. This wonky climate is an irreplaceable part of Game of Thrones. Westeros would not be remotely the same without it.
But grapevines have a life cycle that depends on regular seasons. In winter, grapevines are dormant. Come spring they sprout leaves. As summer begins, they flower and tiny little grapes appear. Throughout the summer the grapes fill up with water, sugar and acid. The grapes are finally ready for picking in early autumn, then go back to sleep in winter. This cycle is why wineries can rely on a yearly grape yield. Obviously, in Westeros, something must be different about how grapes work.