In all C1, C2 and C4-1 Districts in the Borough of Staten Island, all developments or enlargements containing non-residential uses shall be screened from adjoining zoning lots containing only residential uses by a planting strip at least five feet wide along the common side lot line, densely planted with evergreen shrubs at least four feet high at time of planting and of a variety expected to reach a height of six feet within three years. No chain link fences shall be permitted. However, no such screening shall be required where both such buildings front upon a street line that forms the boundary of a block front mapped entirely as a Commercial District.