The United States Botanic Garden is a favorite summer destination, and the list of reasons to visit has gotten longer. In a recently opened exhibit, Celebrating America’s Public Gardens, the nation’s most important public gardens have sponsored mini-displays in two sections called Green Today, Growing Tomorrows (in the National Garden, at the Mall end of the grounds) and A Sense of Place (on the Conservatory Terrace, facing Capitol Hill). In the latter exhibit, each garden’s…