The Garden State (Not New Jersey)

"But let us cultivate our garden."
Candide's reasoning behind this statement is that he comes to realize that we control our fate and our lives. Candide was a character who viewed life with an external locus, believing everything that happened to him was due specifically to fate or luck, but never stopped to consider that he could have prevented it with his own decisions. At the end of this novel, it took Candide basically his whole life to come to the conclusion that he change his actions, take responsibility, and make what is left of his life happy. As of that point, Candide no longer believes in the world as cause and effect. He realizes that in order to be happy, we must make ourselves happy. He realizes that he cannot focus solely on why he was put on earth and his supposed purpose he has, but live out his life the way he wants with the people around him. The garden represents one life and they "must cultivate" it by making their own decisions and mistakes in order to create their very own garden for themselves.  We have to take charge of our situation and make it the best possible rather than simply hoping it will be good (or believing that it's already the best of all possible outcomes). We must care for our garden and bask in the rewards it shall yield through dedicated, hard work.  We get out of it what we put into it.  
I also feel that this is another example of how Adam and Eve and The Garden of Eden fit into the novel. They are ending their adventures in a garden, not beginning them there as Adam and Eve did. Instead of enjoying the free bounty of nature as Adam and Eve did, they must work tirelessly in order to reap any benefits from their garden.  But I think that there's something more in working for your rewards rather than just having them handed to you and I think that's what Voltaire is getting at.