Horizon Monarch Rhododendron
Rhododendron 'Horizon Monarch'
Ericaceae · broadleaf evergreen shrub · introduced
Last updated
A spreading evergreen shrub reaching 6 feet in 10 years and 8 to 10 feet at maturity. Exceptionally large openly funnel-shaped flowers, 4 inches wide, pale greenish-yellow to light yellow with a small vivid red flare, carried in ball-shaped trusses of 15 blooms. Midseason flowering. A hybrid of 'Nancy Evans' crossed with 'Point Defiance', bred by Brockenbrough (ARS 902).
Among the largest-flowered yellow rhododendron hybrids, with individual blooms reaching 4 inches across. The vivid red flare in the throat adds contrast that most yellow hybrids lack. Hardy to 5°F (Zone 7b-9a). Part shade to full sun on acidic, well-drained soil (pH 4.5-6.0). Substantial mature size (8 to 10 feet) requires generous spacing. Not for cold-pocket sites. Twenty-one diseases and 15 pests documented at the genus level. All parts contain grayanotoxins and are toxic if ingested.
Quick Facts
Root Colonization (Spring–Fall)
Infection at Bud Break
Apothecia Formation & Spore Release
Aecial Stage (Alternate Host)
+ 13 more — see full disease and pest lists below
Phenological Calendar
| Stage | Typical Window |
|---|---|
| Bud swell BBCH 01-03 | Mar 15-Apr 15 |
| Bloom start BBCH 61 | Apr 20-May 5 |
| Full bloom BBCH 65 | Apr 28-May 12 |
| Bloom end BBCH 69 | May 8-May 25 |
| Bud set BBCH 55-59 | Jul 15-Sep 15 |