We don't have a specific date for the composition of the Herblore of the Shire, but in its introduction (quoted in The Lord of the Rings Prologue, 2 Concerning Pipe-weed) Merry displays knowledge that he could not have had before the War of the Ring (for instance, a familiarity with the Prancing Pony). He also mentions that he has made 'many journeys south', which suggests that the book was written some years, and possibly decades, after the War. At the very latest the book was produced by about IV 70, when Merry and Pippin set out for their final journey from the Shire.

It is curious to note that, during the War of the Ring itself, Merry is recorded as describing the history of pipe-weed to King Théoden, and he uses almost exactly the same wording as would appear in the Herblore of the Shire years later. So, while the book itself must have been produced long after the War, it seems clear that its author was pondering some of its contents at least as early as the year III 3019.

We know that there was at least one copy held in the library at Brandy Hall (and since this was its author's home, that copy would presumably have been the original). We're told that the book was well known by the Shire-hobbits, implying that multiple copies were made. By the end of the first century of the Fourth Age there were several large collections of books in the Shire (as well as the library at Brandy Hall, those at Undertowers and Great Smials are also noted) and it seems fair to assume that these would also have held their own copies of the Herblore of the Shire.