Thanks to patch 5.3 it is no longer necessary to level your Herablism by going through old world zones.  In patch 5.3 they removed the minimum skull required to gather herbs.  You can now gather any Herb in game at skill level 1, however, if you are below the appropriate level you will now get stems instead of the actual herb.  Ten of these stems will combine into the Herb itself.

The good news is that you can level Herbalism in Mists of Pandaria 1-600 without ever having to leave.  So grab your level 90, train Herbalism, and get to gathering.

These routes will get you from 1-600 in no time.

You’ll be collecting Green Tea from Jade Forest, Silkweed from Valley of the Four Winds, and Snow Lily from Kun-lai summit.

Where you start or end up is completely up to you.  I recommend that you stay in Jade Forest 1-600 but I wanted to provide you with other options.

Enjoy.