Required planting screens. Except in Districts R-1, R-2 and R-3, wherever any parking lot, trash collection, outdoor storage, merchandising or service area lies adjacent to a common line with any R-1, R-2 or R-3 Zoning District, a planting screen of sufficient length to obstruct or partially obstruct the view thereof from the adjoining district shall be required, except where the view is blocked by change in grade or other natural or man-made features. Where, because of intense shade or soil conditions, the planting screen cannot be expected to thrive, a wooden fence or masonry wall may be substituted.