Setup foliage after modeling, using unity's shuriken particle system. Usually tree are setup as series of planes with textures mapped to them. The benefits of a particle system is how easy it is to adjust in engine, rather than going back and adjusting on the model level. Particle will always rotate to face the direction of our camera. It makes our foliage feel much more
dynamic. Rather than flat planes attached to a model. However depending on how many trees are in your game, the particle method can quickly get out of control. It's a good quick trick for trees, but it can get heavy if left on its own.