Required buffer strips in Industrial Districts. Where an M-1 Industrial District abuts a residential district, there shall be provided along any rear, side or front line, coincidental with any industrial-residential boundary, a buffer strip not less than 40 feet in width as measured at right angles to said lot line. Plant materials at least six feet in height, of such variety and growth habits as to provide a year-round, effective visual screen when viewed from the residential district, shall be planted in the exterior 25 feet abutting the residential district. If the required planting screen is set back from the industrial-residential boundary, the portion of the buffer strip facing the residential district shall be attractively maintained. Fencing may be used in lieu of planting materials to provide said screening. The fencing shall be not less than five nor more than eight feet in height and shall be of such materials as to effectively screen the industrial area. The exterior 25 feet of the buffer strip shall not be devoted to the parking of vehicles or storage of any material or accessory uses. The interior 15 feet may be devoted to parking of vehicles.