Hello, in my game I place some trees or rocks and I can just walk throught them.
You need to manually add a collider (Physics Menu)...
To do this, do something along the lines of, find the tree in the original menu (The WHOLE tree).
Drag and drop to the scene.
Add Your own collider on the tree.
Make it a prefab.
Put the prefab in the original folder (Make sure that you make it a different name).
Then use that.
=)
Good luck!