Landscaped areas situated outside of the parking lot, such as required landscaped strips, peripheral areas and areas surrounding buildings, shall not constitute interior landscaping. For the purpose of computing the total area of any parking lot, all areas within the perimeter of the parking lot shall be counted, including all parking spaces and access drives, aisles, islands and curbed areas. Ground cover alone is not sufficient to meet this requirement. Trees, shrubs or other approved material shall be provided. At least one shade tree shall be provided for each 300 square feet (or fraction thereof) of required interior landscaping area. These trees shall have a clear trunk at least five feet above finished-grade level. Parked vehicles may not overhang interior landscaped area more than 2 1/2 feet; where necessary, wheel stops or curbing shall be provided to ensure no greater overhang. If a parking lot of fewer than 20 spaces is built without interior landscaping and later additional spaces are added so that the total is 20 or more, the interior landscaping shall be provided for the entire parking lot.