A prisoner in jail receives a letter

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 came with shovels to the house, and dug up all the back garden.”

The prisoner wrote another letter back:

“Dear wife, now is the best time to plant the lettuce.”