This is located in the further widdershins part of the Central Continent1 and is one of the few places where reannual plants, such as the Vul Nut, grow and are cultivated. The cultivated nuts are used in making a rare, expensive and sought-after drink called Ghlen Livid.
(1) Considerable doubt as to its location and political allegience is raised by Twoflower's mention of it as if it were in his homeland, Agatea. It may be a colony or there may be an error somewhere. Sufficient data are not available.
In Discworld Noir the archaeologist Ilsa Vargberg discusses the fact that the Agatean Empire once extended beyond it's current continent, including settlements in what is now Tsort. It is possible that Rehigreed Province was a similar colony. This is supported by snippets in The Compleat Discworld Atlas, which says Rehigreed Province was part of a larger area called, in Agatean, Fah-Flung.