Landscaping shall be provided as part of the site plan and subdivision design for planned unit developments and for commercial, industrial, and medium and high density residential subdivisions. Careful thought shall be given as to how best to preserve existing plant material at the site. Landscaping may include plant materials such as trees, shrubs, ground covers, perennials, and annuals, and other materials such as rocks, water, sculpture, art, walls, fences, paving materials and street furniture.