Symbolism, Imagery, Allegory
As if flowers aren't precious enough, there are even more special flowers that only bloom once a decade or so…and in the middle of the night. You can't make this kind of stuff up, folks.
Of course Shang Su Yi has some of these flowers, and they serve as an interesting symbol in the novel.
Oliver T'sien tells Rachel, "You know, it's considered to be very auspicious to witness tan huas blooming in the night […] It's a once-in-a-lifetime event for most people, so I'd say you're very lucky to be here tonight" (2.4.14).
We don't think Rachel is just lucky to see the tan huas bloom, but also to be standing in Shang Su Yi's home at all. It is a rare occurrence for a young woman of little social status to be entertained in the great home of Shang Su Yi. So does that make Rachel the tan hua or Shang Su Yi? Hold on, let's keep figuring this out.
Ultimately, the tan huas are temporary, like much in life, unfurling themselves over a few hours and shriveling "into spent, lifeless petals" (2.4.92). This image juxtaposes with Rachel asking herself "if life with Nicholas would always be" this magical (2.4.92). No, sweetheart, it surely won't.
The wilting of the tan huas represents an eventual waning of the magic that both Rachel and Astrid experience in their worlds: Rachel will soon be confronted with the cruel women who don't want her to marry Nick, and Astrid is setting upon a path to discover that her marriage has been shriveling in front of her.