Minecraft wiki entry for Azalea_tree, associated text: Azalea trees generate on any empty space above a lush cave, including in biomes that are not grassy or underground. They are the only plants in Minecraft that have subterranean roots. The roots consist of rooted dirt and hanging roots that reach all the way down to the lush cave at cavern systems such as ravines and abandoned mineshafts. The trees appear most commonly at the surface, but they can also generate inside caves given that there is enough room.