Symbolism, Imagery, Allegory
In Georgia, peaches seem so omnipresent (both literally and symbolically), that it's tempting to totally ignore the repeated mentions of the town's peach trees. But McCullers is a careful writer, so we know she's not just mentioning them just because the story's set in Georgia.
[…] not much is there except the cotton mill, the two-room houses where the workers live, a few peach trees [...] (Ballad.1)
These trees keep track of the changing seasons and the changing emotional landscape. Here they are when Lymon first approaches, still a mystery to onlookers:
The moon made dim, twisted shadows of the blossoming peach trees along the side of the road. (Ballad.9)
The next day, love is already in the air:
The air was fresh and the peach trees light as March clouds with their blossoms. (Ballad.42)
Over the winter holidays, as Lymon continues to follow Marvin like a lost little pup: "the peach trees were scrawny and bare." (Ballad.178) Though their leaflessness seems to reasonably correlate with the colder season, their symbolism does some heavy lifting: all is not well in the world.
What were the trees like when Miss Amelia was nineteen, about to be married to Marvin Macy? So glad you asked:
The town then was the same as it is now, except there were two stores instead of three and the peach trees along the street were more crooked and smaller than they are now. (Ballad.84)
Not much is different, the narrator shrugs. The trees grew. Big whoop. But this "meh" attitude tells us a lot, symbolically: this is a sleepy little town that doesn't change much, even throughout the raging internal drama of its citizens.