Washington designated the western hemlock (Tsuga heterophylla) as the official state tree in 1947. A large evergreen coniferous tree native to the west coast of North America, the western hemlock tree is the largest species of hemlock (growing an average of 50 - 70 meters tall, and sometimes to 78 meters) with a trunk diameter of up to 2.7 meters. The western hemlock lives a long life (trees over 1200 years old are known). Washington's abundant evergreen forests are the basis for its unofficial nickname: The Evergreen State.

Closely associated with temperate rain forests, most western hemlock trees live less than 100 km from the Pacific Ocean (with the exception of an inland population in the Rocky Mountains of southeast British Columbia, northern Idaho and western Montana).

A very shade-tolerant tree, young trees usually start growing under the canopy of other conifers such as Sitka Spruce and Douglas-fir, eventually replacing them in climax forest as those species cannot grow in the dense shade cast by Western Hemlocks.

Source:

State Symbols: Washington State Legislature
Tsuga heterophylla: Wikipedia.org

Links:

State Trees - 50 States List