This achievement is unlocked after collecting no less than 10,000 herbs. These can be found throughout the landscape, particularly in flourishing meadows. As you collect herbs your skill will increase, eventually allowing you to collect more than one herb with each pick. In addition, if there are multiple herbs gathered together in a particular spot, a multiplier will be added to each pick. This means that it should take 1 - 2 hours to collect the full amount.
You can monitor the progress of herbs collected by navigating to Player > Statistics > Craft > Herbs Collected.