What is good symbolism from or deep meaning to what Nancy Farmer writes in The House of the Scorpion?
Flowers are symbolic in the story. Specifically the white poppy flower. It's a pretty flower. Poppy flowers (opium flowers) are not always white, but in this novel their color is described as white.
He'd looked out the window where fields of white poppies stretched all the way to the shadowy hills. The whiteness hurt his eyes, and so he turned from them with relief to the cool darkness inside.
White generally has the connotation of peaceful, clean, and pure. And while opiates do have medicinal purposes, that is not their purpose in the book. El Patron is using them to produce opium for his drug trade. So the poppies are symbolic of something sinister that at first appears innocent and good. That's very much how El Patron himself develops through the story. At first no threat, then very much a threat.
A second symbolic thread in the novel is material possessions. A reader can more or less track a character's goodness in this novel by the amount of possessions he/she owns. It's an inverse relationship though.
El Patron has everything. All kinds of stuff. More stuff than he knows what to do with. He believes that the more things he owns, the more it reflects on the deep meaning of his power and influence. But he's an evil man.
Celia on the other hand is a good, loving, and honorable character; she has very few material possessions. Those that she does have hold significant deep meaning for her. She doesn't have them just to have them.
And when Matt decides to run away to Aztlan, it's symbolic that he leaves just about everything behind. He is throwing off/away the person that El Patron is trying to create and becoming more like Celia and like the man he finally becomes by the end of the book. He went from having a lot of possessions with deep meaning to having few, and during that transition the reader sees his character develop much more and with much deeper meaning.