Washington officially designated the coast rhododendron (Rhododendron macrophyllum) as the state flower in 1959, though it was actually selected as the state flower in 1892 by the women of Washington for the 1893 World's Fair in Chicago.

Also called Pacific rhododendron, or big leaf rhododendron, the coast rhododendron is a broadleaf evergreen rhododendron species native to western North America. The coast rhododendron is found primarily near the Pacific coast, but its range extends to the Cascade Mountains in Oregon, Washington and British Columbia.

Coast rhododendron is less popular for decorative landscaping than many other rhododendron species that are easier to grow and have a more pleasing appearance (Pacific Rhododendron shrubs can be scraggly and the flowers are often much less impressive than those of its relatives).

Archibald Menzies found this shrub in 1792 when he and George Vancouver landed near present day Port Discovery. Coast rhododendron seeds were sent to England in 1850 by William Lobb.

Source:

Washington State Symbols: Washington State Legislature
Western North America Rhododendron Species Project
Rhododendron macrophyllum: Wikipedia.org

Links:

State Flowers - 50 States List
National Flower