Grace is a town of orchards. In some of the first scenes of the novel, we see Codi walking through a pecan orchard on her way to the Domingos house. Later, as she walks back toward downtown with John Tucker, they see that the ground is covered with tiny, immature nuts. Codi asks John Tucker what's going on, and he tells her it's "Fruit drop" (7.21), partly because he speaks in monosyllables, and partly because that's just what it's called.
Fruit drop is caused by watering the trees with water from a river tainted with the sulfuric acid that the mine uses to wring a few more bits of copper out of the tailing piles. Since the mines no longer employ anyone, the orchards are the mainstay of Grace's economy, and without them, the whole town will die.
That's their plot importance, but symbolically, the fruit trees represent children. Remember that it's Emelina's oldest son who introduces Codi to the idea of fruit drop, and we learn about these immaturely dropped fruits right after we learn in detail about Codi's lost child in Chapter Six. The immature green nuts on the ground are basically so many miscarried children, and like babies, they're the future of Grace.
Loyd's aunt Sonia understands this baby-fruit relationship, too. She gives Loyd his own peach orchard, but he can only have it if he has children of his own. That's because "when you have a family, you need trees" (18.146). The implication here is interesting. It's not just that trees stand in symbolically for babies in this text, but rather that both trees and children symbolize the same thing: hope for the future.