Symbolism, Imagery, Allegory
Mmm, onions. Or not. Even though they are one of the stinkier foods out there, we kind of love what they do for Holes. After all, it's the wild onions that keep Stanley and Zero alive on the mountain and then allow them to survive the swarm of yellow-spotted lizards.
These bulbs also have some important symbolic meanings, too. They are most closely associated with Sam, who uses them to make his wonderful healing powders, creams, and ointments. When you think about it, onions are pretty much the lowliest of foods. Along with carrots and other yummy things, they grow in the dirt, and the edible part actually grows underground. Like Sam himself, onions are humble and unassuming: there's nothing fancy about them.
But wait: what is it they always say about onions? "It's like peeling an onion," right? Well, that's kind of the moral of the story here in Holes, isn't it? You have to look further – dig deeper, we might say – to get to what really matters.