Pipe-weed (also known as Halflings' Leaf or simply Leaf) was a plant developed by the Hobbits of the Shire. The cultivation and exportation of this herb was a key industry in the Shire, especially in the south. Among the Dúnedain it was known as sweet galenas, and in Gondor it grew as a wild herb, appreciated primarily for its fragrant blossoms.
Among the Gondorians it was known popularly as westmansweed, a reference to its origin: it was apparently brought to Middle-earth by Númenóreans during the Second Age. There it was first domesticated by Tobold Hornblower of Longbottom in Southfarthing around S.R. 1070 (T.A. 2670). Despite its foreign origins, the Hobbits (possibly those in Bree) were the first to use it for smoking.
The Wizard Gandalf learned to smoke pipe-weed from the Hobbits and was often seen blowing smoke-rings. Saruman initially derided him for this, but at some point he took up smoking himself. After the destruction of Isengard, pipe-weed was found among its stores, but the Hobbits Merry and Pippin failed to realize the sinister implications of the discovery that Saruman had had commerce with the Shire.