The white tiger orchids are such a big part of this book that it could've been titled "White Tiger Orchids." (In fact, we kind of like the subtlety.) Check out how often they come up:
Okay, so why so many references to these flowers?
The rangers probably put it best, while they're burning the orchids to death. The orchids are "One of the most beautiful plants in the world. But too successful. They turned into the ultimate weed. What we call a monoculture" (22.47). As the rangers explain, when you have a weed like this, it kills everything else off until all you have is orchids; as the ranger says, "That's what monoculture means: Everything the same" (22.49).
But once you have nothing but orchids, then the orchids themselves start to die off because they require a type of hummingbird that nests in trees. In other words, too much of a pretty thing can lead to total destruction. (Although if the flowers led to "total destruction" by exploding, that would be pretty fun.) That's the landscape that Tally and David pass through when they're on their way to the city: "Tally whistled. The sand seemed to go forever" (40.19).
So, if you were reading this book with Shay by your side (and as long as we're going with this fantasy, say you're hoverboarding together, although that might make the reading difficult), Shay would immediately jump on this line and say something like, "See, when everything is the same and pretty, then disaster follows. And that's what's going to happen with the pretty people, too." (That's so Shay.)
And she has a certain point: since all the pretty people with brain lesions think alike, there's a chance for a big disaster, just like too many orchids is a big disaster for the environment. (For more on big disaster, look at the "Rusty Ruins" as symbol.)
Of course, the white tiger orchid wasn't always a desert-making weed. (Proof: Look outside your window. Is there a desert there caused by orchids? Probably not. We hope.)
As a ranger explains, "About three hundred years ago, some Rusty figured a way to engineer the species to adapt to wider conditions. She messed with the genes to make them propagate more easily" (22.43). Which means that the orchid desert is the fault of people messing with nature. Gee whiz, that sure sounds a lot like the pretty surgery, where people are conducting surgery that interferes with nature.
So the white tiger orchids are starting to sound a lot like the city's pretty surgery system: a lot of pretty things, made by people, that may lead to a disaster.
But there's one other thing that the orchids might symbolize: Tally. When Tally is in the Smoke, thinking about how dangerous she is, she compares herself to the orchids: "She thought of the orchids spreading across the plains below, choking the life out of other plants, out of the soil itself, selfish and unstoppable. Tally Youngblood was a weed. And, unlike the orchids, she wasn't even a pretty one" (29.7).
Again, it's a good point: the orchids aren't trying to make trouble, but they are. Similarly, Tally isn't trying to ruin everyone's life, but… she kind of is.
But there the connection falls apart: Tally isn't pretty (well, not in the typical way), Tally wasn't engineered by people, and Tally's not like everyone else. In fact, by comparing herself to a weed, Tally helps us see how she's not so much like the orchid. Even if she is about to turn the Smoke into a ruin by accident.