Pacific Rhododendron

An erect evergreen shrub native from British Columbia to northern California, from sea level to 6,000 feet along coastal foothills and the Cascade Range, reaching 15 feet. Five-lobed spreading bell-shaped purple flowers about 1 inch across. The state flower of Washington. The only rhododendron species native to the Pacific Northwest west of the Cascades.

Hardy in Zone 7b-8b. Full sun to part shade on acidic, well-drained soil. Moderate growth rate. In the wild, forms understory groves beneath Douglas-fir and western hemlock. Twenty-one diseases and 15 pests documented at the genus level. All parts contain grayanotoxins and are toxic if ingested.

Pacific rhododendron is the native Ericaceae that anchors the regional plant palette. It needs acidic soil (pH 5.5-7.0), consistent moisture, and shade from afternoon sun. The disease that kills rhododendrons in our region is Phytophthora root rot, not the cosmetic foliar issues. Sharp drainage is non-negotiable; rhododendrons planted in poorly drained soil without amendment are on a timeline, whether that is alluvial clay in the valleys or glacial till with a shallow hardpan. Petal blight (Ovulinia/Botrytis) ruins the flower display in wet springs but does not threaten the plant. The species blooms on old wood, so any pruning happens immediately after flowering. Thirty-five diseases and 21 pests are documented, which sounds alarming but most are cosmetic. The one combination that kills is wet roots plus Phytophthora. Fix the drainage and most of the disease list becomes irrelevant.

— Chris Welch, ISA Certified Arborist