We have some trees and flowers like the ones growing on the side of the mountain - the seeds blow in often enough. There were also a few plants native to the underground already! They mostly grow by the river; there are a few different types of glowing mushroom, and the echo flowers.

[He seems to enjoy this topic more than "the underground in general"]

The echo flowers glow blue, but they have also become imbued with magic over the years. They can repeat the last sound they heard.