The garden helps people overcome their prejudices by bringing them together. This particular neighborhood of Cleveland is deeply divided along racial lines. People tend to stick to their own, rarely (if ever) socializing with those of different races. The community garden helps to overcome this fragmentation by giving everyone a rare opportunity to meet up and work together with folks of whose existence they'd previously been unaware.
A particularly vivid example of the transformative power of the garden comes in the case of Amir, who originally hails from India. One day, he's subjected to some racial abuse by an old Italian lady. Yet when he meets up with her at a harvest festival he's helped organize, the lady apologizes, saying that back then she didn't know him.
This indicates how racism ultimately tends to arise from ignorance of various kinds. The old lady didn't know Amir; she just saw a guy with a brown face. And so she felt entitled to call him a "dirty foreigner." But now that she actually knows him, she can finally see the real man beneath the skin.
Such a remarkable transformation in attitudes has only come about due to the establishment of the community garden. It really has brought people together.