There is a 3-4 tour that takes you through all the gardens in world showcase and they tell you about the planning (how to make them fit the countries) and different plants they use. It costs about $60 and is in addition to park admittance.
There is also a walking tour of the Land gardens in Epcot called Behind the Seeds. It is a short tour for about $6. However, this one is not currently available as they do construction on the new Soarin ride.