Yoko Ono’s participatory artwork “Wish Tree for Washington, DC” (2007) is open for the summer at the Smithsonian’s Hirshhorn Museum and Sculpture Garden. For most of the year, visitors are invited to whisper their wishes to the tree, a white flowering dogwood in the museum’s collection. But during the summer through Labor Day, they may write their wishes on paper tags and tie them to the tree’s branches.