James F. Dawson and Frederick Law Olmsted Jr. designed the first plantings. The work was done during the Great Depression: Works Progress Administration (WPA) workers built many features such as the Stone Cottage and Azalea Way.
Many replantings, redesigns, and acquisitions occurred after World War II under the direction of Brian O. Mulligan. He designed the Winter Garden and the Woodland Garden among many others.
In 1978, a Master Plan Update resolved problems and disagreements between the city and university. In 1980, the University reaffirmed its managerial role by establishing the Center for Urban Horticulture (CUH) at Union Bay, at a campus site 1.5 miles from the Arboretum. The Center provides physical facilities, programs, and staff, and manages the Arboretum.
The mission statement of Washington Park Arboretum is:
"The Washington Park Arboretum is a living plant museum emphasizing trees and shrubs hardy in the maritime Pacific Northwest. Collections are selected and arranged to display their beauty and function in urban landscapes, to demonstrate their natural ecology and diversity, and to conserve important species and cultivated varieties for the future. The Arboretum serves the public, students at all levels, naturalists, gardeners, and nursery and landscape professionals with its collections, educational programs, interpretations, and recreational opportunities."
As of 2005, the Washington Park Arboretum contains more than 4,400 different taxa. It comprises the largest collection of temporate woody plants in the Northern Hemisphere. According to a recent University of Washington report, "Plants are selected and arranged to display their beauty and function in an urban landscape, to demonstrate their natural ecology and diversity, and to conserve important species and cultivated varieties for the future" ("Expertise and Resources ..."). The park includes wetlands along Union Bay as well as landscaped areas.
Over the years, there has been some confusion about the arboretum's name, with many calling it the University Arboretum or the University of Washington Arboretum. Included in the Arboretum property are the university's Botanic Gardens and the Japanese Garden, established under the supervision of world-renowned Japanese garden designer Juki Iida in 1960.