Symbolism, Imagery, Allegory
Billie Jo makes it pretty clear that she's an apple addict, and the apple trees Ma raises in their yard are responsible. We learn that she planted the trees before Billie Jo was born, so:
that she and they might bring forth fruit / into our home / together. (23.2)
In just this brief passage, we can recognize the apple trees as symbolic of a few things: Ma in her own right (she planted them, after all); Ma welcoming the experience of becoming a mother (she can be a little harsh at times, but in planting the trees she shows a clear desire to provide and take care of her family); and Ma as lonely (after all, she plants trees as a sort of comrade in homemaking). How do you like them apples? Pretty juicy, right?
Whenever Billie Jo references the trees, it is always in reference to her mother and parallels Ma's love of family and the land. She describes Ma, for example, as having the same "dazzling" natural beauty as the blossoms, particularly at the beginning of Part 3, when Ma stands in the rain, naked and pregnant, feeling it run down her body.
Billie Jo's use of similes in this passage, describing Ma as "bare as a pear" and "striped like a melon" (29.3), also reference fruit. While on the one hand, this is symbolic of the fruit—a.k.a. baby—Ma is growing inside her at this point, the consistent connection of Ma with fruit—which is sweet—represents the softness and tenderness Ma possesses, even if she doesn't always show it.
In case you weren't convinced yet that Ma and the apple trees go hand in hand in this book, consider this: A cloud of grasshoppers makes a symbolic attack on the trees, "devouring" (37.1) nearly all the apples—and when Billie Jo runs to tell Ma, she learns that Ma has died giving birth to Franklin. Just as the grasshoppers destroy the fruit on Ma's trees, the fruit of her womb, Franklin, is destined to die as well. Fortunately the trees live on, a testament to Ma's presence and vitality long after she's gone.