In order to obtain nourishment, plants need the sun. Without sunlight, the process of photosynthesis cannot occur, and plants will die.

Therefore, where there is no sunlight, there is no plant life.

I say this because there is a cave in Coerthas that leads to Owl's Nest, and despite the fact that there is zero sunlight in this cave, the ground is covered with lush green grass.

All I'm sayin' is it's weird. That's all.