I also vaguely recall from the early chapters of The Sil that flowers were said to spring up at the coming of certain Ainur (Vana, possibly someone else as well -Melian perhaps?) This, rather than reflect the character of the place, reflects the character of the character.
It's not an original "device" in this case; it has been used since the times of ye olde myths and legends.
I completely agree with
Pitch. Flowers are an essential part of their place, and also they just feel natural. They add to the mood. You know how Merry describes the mountains in Dunherg? You know how the hobbits, and later the "hunters", describe Fangorn forest? It just gives you a feel of the place, and makes that place alive for you, makes you feel like you're there as well. Flowers do that too. Any nature does.
Other than that, I can't really add anything to
BB and
Pitch. Pretty much what they said.