Pipe-weed (also known as Halflings' Leaf) is a plant developed by the Hobbits of the Shire. The cultivation and exportation of this herb is a key industry in the Shire, especially in the south. Among the Dunedain it is known as sweet galenas, and in Gondor it grows as a wild herb, appreciated primarily for its fragrant blossoms.
Among the Gondorians it is 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.
Popular varieties of pipe-weed include Longbottom Leaf, Old Toby, and Southern Star.
In the prologue to The Lord of the Rings, Tolkien states specifically that "it is a strain of the herb nicotiana", i.e. tobacco.
The Wizard Gandalf learned to smoke pipe-weed from the Hobbits and is often seen blowing smoke-rings. Saruman initially derides him for this, but at some point he takes up smoking himself. After the destruction of Isengard, pipe-weed is found among its stores, but the Hobbits Merry and Pippin fail to realize the sinister implications of the discovery that Saruman has had commerce with the Shire.