tree billboard are just pixels with terrain billboard

Hello!

I was wondering since I am so lazy to make my own billboard textures, if it was possible to automatically create billboard textures placed at the place of my trees when my player is far enough from them. So I searched a bit on my terrain settings and I've seen that there was actually a billboard program already implemented in Unity's Terrain functionalities. But here is the problem, it's not working well. What I see is not a billboard, but some random blurred pixels. It seems to be because my trees where made with something else than the nature trees, but, I want to find a way to optimize and get a good billboard rendered in my game.