I seem to have this problem where when I place a lot of trees and grass textures down, and I put a first person controller into my game; the camera can only see these trees and grass textures withing a 20ft radius. Does anyone know what this is or how to fix it?
Yes. It can slow down your game significantly if all trees etc would be rendered, so you will only render them within a certain range of your camera.
To change this range first select terrain in your hierarchy panel. Notice the buttons that allow you to raise terrain, place trees, etc.. Click the right-most button (terrain settings) Under Tree & Detail Objects increase detail distance.
Note that high detail distance combined with high detail density combined with many details will slow you're game down to a halt.