Not everyone's a green thumb. And not all sites are good for growing plants. On hillsides, in desert climates, and in the bottoms of light wells where light is, well, scarce. That's where rock gardens can fill a niche and still create an outdoor escape when traditional plantings aren't an option. We even saw a lovely rock garden at the base of a tree on an UES sidewalk last week. It was filled with pure and simple black river rocks. Some rock gardening inspiration: • A how-to with easy-to-grow perennials from the Brooklyn botanic Garden • Miami Beach Pebbles is a great source for smooth pebbles of all kinds. See their gallery for inspiration then purchase pebbles directly from Miami Beach Pebbles: $20 for a 35-pound bag delivered. • A History of Rock Gardening in North America at the North American Rock Garden Society.