Pacific Rhododendron
Rhododendron macrophyllum
Ericaceae · broadleaf evergreen shrub · native
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
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
Pacific rhododendron (Rhododendron macrophyllum) is an evergreen shrub in the Ericaceae family native to western North America from British Columbia to California. It reaches 15 ft tall at maturity with an erect, multiple-stem form and coarse, dark green foliage. Purple bell-shaped flowers approximately 3 inches across bloom on previous season's wood.
Pacific rhododendron is hardy in zones 7b-8b and prefers sun to part shade with moist soil (pH 5.5-7.0) and high moisture. It requires a minimum 12-inch root depth and has no drought tolerance. Thirty-five diseases and 21 pests are documented in managed landscapes.
Quick Facts
Phenological Calendar
As of April 23, 2026, Puget Sound stations range from 1434.4 to 1592.7 GDD₃₂. Pacific Rhododendron has reached 'flower buds visible' (1025 GDD₃₂) and is approaching 'bud break', predicted around May 3.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 23, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,593 | 'Flower buds visible' | 'Bud break' | 180 |
| Seattle / UW | 1,554 | 'Flower buds visible' | 'Bud break' | 219 |
| Kent / Auburn | 1,537 | 'Flower buds visible' | 'Bud break' | 236 |
| Olympia / Tumwater | 1,505 | 'Flower buds visible' | 'Bud break' | 269 |
| Bellingham / Whatcom | 1,483 | 'Flower buds visible' | 'Bud break' | 290 |
| Tacoma / Puyallup | 1,472 | 'Flower buds visible' | 'Bud break' | 301 |
| Sequim / Rain Shadow | 1,434 | 'Flower buds visible' | 'Bud break' | 339 |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ● 'Flower buds visible' BBCH '51' NOW | 1025 | '' |
| ○ 'Bud break' BBCH '07' NEXT | 1773 | '' est. May 3 (forecast) |
| 'First bloom' BBCH '61' | 1906 | '' est. May 9 (forecast) |
GDD = Growing Degree Days (base 32°F, Jan 1 start). Why base 32? GDD₃₂ thresholds from USA National Phenology Network citizen science observations (WA+OR). Season tracker for Kent / Auburn as of Apr 23, 2026. Predicted dates use 16-day weather forecast through May 9, 2026, then climate normals.