Mainau is an island in Lake Constance, Germany. It is a "flowering island" notable for its parks and gardens. Frederick I, Grand Duke of Baden, created the island's arboretum, which now contains 500 species of deciduous and coniferous trees, many exotic and valuable, including fine specimens of Sequoiadendron giganteum and Metasequoia glyptostroboides. The island also contains about 200 rhodod rhododendron and azalea varieties.