Mainau is a small island in Lake Constance, about twenty minutes by ferry from Konstanz, owned by the Bernadotte family (the Swedish royal house) and given over almost entirely to gardens. The microclimate of the lake allows plants that have no business growing this far north: palm trees, dahlias, rhododendrons, Italian rose gardens, and a butterfly house. In spring the tulip and narcissus beds are the draw; in summer it's the roses and dahlias.
The island is reached by ferry from Konstanz harbour or, if you prefer, by bike along the lakeside path to the causeway. It draws large crowds in high summer — arriving early or on a weekday is noticeably more pleasant.