Johnny is falsely convicted of robbing a bank and sentenced to 25 years, though the stolen money was not yet recovered.
As he's sitting in prison, bitter at the world and ruminating on his bad luck and the injustice, he receives a letter from his father:
Dear Johnny,
I miss you so much & I was thinking of this past winter when we agreed to start a new garden. It's almost time! Sadly, I'm getting older & I'm not sure how I'm gonna do it without you. How I wish you were here to help. 
I love you so much,
Dad
Johnny has always loved gardening with his father and starts feeling bad that the can't be there to help him. But then he remembers that all letters are monitored by the prison and a big smile grows on his face as he replies:
Dear Dad,
I feel so terrible that I can't be there to help out with the new garden. But don't fret, I'm absolutely certain some kind soul will lend a helping hand in my absence. 
I love you, too, Dad,
Johnny
PS: Dad, whatever you do, DO NOT dig up the far right corner of the back yard.