One of the oldest botanical gardens in North America, the US Botanic Garden, sitting on 3 acres, is a 30,000 SF oasis in the middle of Washington, DC.  Along with it's permanent exhibits featuring rare and exotic plants from around the world, the botanic garden features rotating exhibits that interpret the role of plants in supporting the earth and its ecosystem and how plants enrich human life.  With it's 3 story tall greenhouse, the botanic garden is easy to spot at the corner of Independence Ave. and First Street, across from the United States Capitol.