That an ornamental sight-obscuring fence, or wall, having a height of at least six feet, or a compact evergreen hedge not less than three feet in height when planted and capable of reaching at least six feet within three years be placed at the front yard setback line and at the setback line of any other adjacent area to a street, and along all other property lines; provided, however, that the Planning Commission may require additional screening and landscaping where topography or other special conditions indicate such to be necessary to adequately screen the area.