Landscaping. All areas of a garden apartment development not used for the construction of buildings, roads, accessways, parking area or sidewalks shall be fully landscaped. Where a garden apartment development boundary line abuts a lot in a residential zone, which lot is not owned by the garden apartment developer, there shall not be cut, uprooted, destroyed or taken away any existing trees, shrubbery or other planting within the area of 20 feet inside the boundary line of the garden apartment development abutting a residential lot. If no adequate trees, shrubs or planting exists in the twenty-foot area in the natural state of the premises before development, the area shall be provided with an adequate approved planting plan to provide a belt of screening within the twenty-foot area.