Mentioned by seattle.gov
Museums & Attractions - Parks
"The Washington Park Arboretum and Botanical Gardens is a 230-acre park full of interesting gardens and rare plant species. The arboretum is co-managed by Seattle Parks and Recreation and the Arboretum Foundation, as well as the University of Washington. The Arboretum is free (as it is a city park) and offers a small visitors center with restrooms, miles of trails, classes, tours, events, and venue rental."