I am trying to understand the actions of Saruman in the Shire. I will refer to the chronology of Appendix B of the Return of the King. All dates are relative to the Third Age.
Frodo leaves Bag End on September 23rd of 3018. Isengard is conquered on the 3rd of March 3019. At this time Saruman already had some interactions with the Shire, since he was "dearly buying" pipe weed and so on. But when had he begun? Gandalf had been imprisoned in Orthanc in July 3018: at that point if I'm not mistaken (but surely you'll know better) Saruman understands that the One Ring was hidden in the Shire and accuses Gandalf of having kept the news for himself.
Treebeard lets Saruman and Gríma free and the Hobbits meet the two, apparently wandering without much intent, on August 28th 3019. Now "Saruman turns toward the Shire" and will arrive there on Bilbo's 129 birthday, 22 September 3019.
On October 28th they arrive in Bree and it is already a very deeply changed Shire, with terror enforced by the Shirrifs, felled trees, broken mills and so on. Saruman has been scourging the Shire. Then the revolution begins and all's well that ends well (ok, almost...).
Saruman is surely powerful enough, given time enough, to arrange such a plot. But without his staff only his "poisonous speech" is left, he is weakened (so much that he is vulnerable to Gríma, something which is really strange given that to kill Gandalf a Balrog was needed, and Saruman at his full power was surely more powerful than Gandalf the Grey): clearly the speech is enough to corrupt men and hobbits alike, if it had been able to persuade an Ent, but still it's not much.
So, aren't the timings a bit too short? If Saruman had not know with proper advance about the real importance of the Shire, how could he so quickly set up his webs of treason and corruption, and then in less than a month completely "change" the mood of the Shire itself?
It almost seems that Saruman had been plotting against the Shire for a long time. Have I missed/confused passages and dates?
UPDATE: see this discussion on meta.