Rosebay Rhododendron
Rhododendron maximum
Ericaceae · [VERIFY] · native
Last updated
Rosebay rhododendron (Rhododendron maximum) is a large evergreen shrub in the Ericaceae family native to the eastern United States, reaching 5-15 ft tall. It is one of the largest native rhododendrons and produces showy flower clusters in late spring to early summer.
Rosebay rhododendron prefers acidic, humus-rich, well-drained soil in part shade. GDD bloom threshold is documented at 1010 (OSU data). All parts are poisonous to humans, cats, dogs, and horses. Detailed cultivation data requires further enrichment.
Quick Facts
Phenological Calendar
As of April 19, 2026, Puget Sound stations range from 1342.9 to 1500.8 GDD₃₂. Rosebay Rhododendron typically reaches beginning of flowering at 2828 GDD₃₂, predicted around Jun 14.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 19, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,501 | Pre-season | Beginning of flowering | 1,327 |
| Seattle / UW | 1,465 | Pre-season | Beginning of flowering | 1,363 |
| Kent / Auburn | 1,441 | Pre-season | Beginning of flowering | 1,387 |
| Olympia / Tumwater | 1,415 | Pre-season | Beginning of flowering | 1,413 |
| Bellingham / Whatcom | 1,390 | Pre-season | Beginning of flowering | 1,438 |
| Tacoma / Puyallup | 1,381 | Pre-season | Beginning of flowering | 1,448 |
| Sequim / Rain Shadow | 1,343 | Pre-season | Beginning of flowering | 1,485 |
| Stage | GDD32 | Typical Window |
|---|---|---|
| ○ Beginning of flowering BBCH 61 NEXT | 2828 | — est. Jun 14 (avg) |
GDD = Growing Degree Days (base 32°F, Jan 1 start). Why base 32? Source GDD₅₀ thresholds from Herms 2004 (OSU, Secrest Arboretum, Ohio) and UMD IPMnet (Gill & Klick, mid-Atlantic), converted to GDD₃₂ via Kent bloom-date mapping. Season tracker for Kent / Auburn as of Apr 19, 2026. Predicted dates use 16-day weather forecast through May 5, 2026, then climate normals.