For decades, much has depended on his red wheelbarrow, streaked with rain, next to some white chickens, even if no one has known — or perhaps even wondered — exactly who he was.

But now, the owner of the humble garden tool that inspired William Carlos Williams’s classic poem “The Red Wheelbarrow” will finally get his due.

On July 18, in a moment of belated poetic justice, a stone will be laid on the otherwise unmarked grave of Thaddeus Marshall, an African-American street vendor from Rutherford, N.J., noting his unsung contribution to American literature.

“When we read this poem in an anthology, we tend not to think of the chickens as real chickens, but as platonic chickens, some ideal thing,” William Logan, the scholar who recently discovered Mr. Marshall’s identity, said in an interview.