Oregon Ash

Oregon ash is the native ash of Western Washington's riparian zones, a deciduous tree you find along stream banks and in wet bottomlands throughout the lowlands. The compound leaves (pinnate with 5-9 leaflets) turn brown and yellow in fall. Male and female flowers are borne on separate trees (the species is dioecious), with female trees producing abundant winged seeds (samaras) that persist through fall and winter. The bark is gray and furrowed on mature trees. The tree reaches 50-70 feet in favorable sites and is moderately fast-growing. In managed landscapes, Oregon ash is less commonly seen than in its native wet-forest context.

In Western Washington, Oregon ash is becoming a difficult management problem due to the imminent threat of emerald ash borer (Agrilus planipennis), a devastating exotic beetle that has killed hundreds of millions of ash trees across North America. The beetle has not yet established in Washington, but its arrival is inevitable, and all ash species are highly susceptible. Landscape foresters are now cautioned against planting or recommending any native ash species, including Oregon ash. If you have existing Oregon ash on your property, monitor its health closely and be prepared to remove it should emerald ash borer establish. The tree's native role in riparian zones is significant, but its replacement with disease-resistant species is the prudent management strategy. Consider alternatives like black cottonwood or bigleaf maple for riparian settings. This is a species in ecological transition; its management today is about monitoring and preparation for a difficult future rather than active promotion.