Why do you think he (Walt Whitman) called his life's work Leaves of Grass?

There are the obvious pastoral elements to Leaves of Grass. There is a strong nature element that runs through Whitman's work. The title is also a pun on the word grass. Many artists, in Whitman's day, used the word "grass" to represent more minor works of literature while the "leaves" denoted the pages in his book. Whitman did not write his poems to be made into popular ballads of the day. He knew they would be controversial in nature and hence remain minor works in the canon of popular works. I think, the opposite happened as Leaves of grass remains classic reading for anyone studying English literature.