The Clever Jail Prisoner Joke

Here's a funny little joke that involves a very clever prisoner in jail:
A prisoner in jail receives a letter from his wife: "Dear Husband, I have decided to plant some lettuce in the back garden. When is the best time to plant them?" The prisoner, knowing that the prison guards read all mail, replied in a letter: "Dear Wife, whatever you do, do not touch the back garden. That is where I hid all the money." A week or so later, he received another letter from his wife: "Dear Husband, You wouldn't believe what happened, some men with shovels came to the house, and dug up all of the back garden." The prisoner wrote another letter back: "Dear wife, now is the best time to plant the lettuce."