The area between the building and the street shall be landscaped in accordance with the requirements of Section 70A, in a way to provide visual interest, with a mixture of different species of shrubs, trees, ground cover, annuals and perennials. Large unbroken expanses of lawn shall be avoided. A minimum of one shrub of a minimum height of 24 inches shall be planted for each three feet of building frontage. The Commission, at its discretion, may permit the substitution of perennial gardens.