When I use the landscape grass tool, it automatically disables the collisions for some reason, so when I want to use the tool to generate trees on my map, you can just simply walk through the trees. Is there a way to keep the tree’s collisions while still using the grass tool?
Actually, I may have to manually insert all the trees since the grass tool isn’t letting me place less than 1 density for the trees per 10 metres squared (I’d prefer 0.5 or something) and it’s too much. 
Landscape grass does not support collisions. You should probably consider relying on foliage painting tool to place trees.