What is the symbolic role of fences in this play?

As Renelane suggests, Rose has a perspective that is different from Troy's sense of "fences."  At the opening of Act I, Scene II, Rose sings, "Jesus, be a fence around me every day / Jesus, I want you to protect me as I travel on my way."  Wilson himself commented, in the Savron interview about this play, that by the conclusion, most of the characters are closely associated with one institution or another--the military, a hospital, prison / workhouse, or (in Rose's case) the church--and the connections with these institutions do not bode well for the possibility of their exercising control over their own lives.  Despite her own incredible strength, Rose has retreated into a space she sees as protected by divine power; she relies on the comforts of her religious faith to keep those she loves safe from the forces that threaten her family.  At the center of the play is a problem of control for each character over his or her destiny, as well as a problem of control for all of them over the threats to their family unity.  Rose represents the desire for a unified, safe family group, but her Jesus-fence is not an effective way of exercising the needed control.  (Wilson did not, however, reject the role played by the chuch in African-American history.  The African-American church has had obvious liberating and empowering influences.)