Rhododendron

Ericaceae · broadleaf evergreen shrub · native and introduced (species-dependent)

Last updated May 23, 2026

Quick Facts

Height

2-20 ft (species-dependent: dwarf alpines 1-2 ft, large species 15-20 ft)

Spread

3-15 ft

Growth Rate

Slow to moderate

Light

Part shade to filtered sun (protection from afternoon sun critical)

Soil

Well Drained

Water

Moderate

Hardiness

Zones 4–9 (species-dependent; most PNW cultivars 6-8)

Fall Color

Yellow to bronze (deciduous azaleas); evergreen types hold foliage

Origin

Northern Hemisphere; greatest diversity in Himalayas and SE

Azalea Lace BugHigh

First-Generation Nymph Emergence

Botryosphaeria DiebackModerate

Infection Through Wounds

Phytophthora Root RotModerate

Growing Season Stress Expression

Powdery MildewModerate

Spring Emergence / Primary Infection

+ 3 more — see full disease and pest lists below

Diseases: Regionally Documented (6)

Phytophthora spp. (P Phytophthora Root Rot Petal Blight Multiple obligate biotrophic fungi (Erysiphales: Erysipha... Powdery Mildew Symptom category — not a single disease Leaf Spot Botryosphaeria dothidea Botryosphaeria Dieback Phytophthora ramorum (Sudden Oak Death)

Pests: Regionally Documented (7)

Stephanitis rhododendri Rhododendron Lace Bug Stephanitis pyrioides Azalea Lace Bug Otiorhynchus spp. Root Weevil Otiorhynchus sulcatus Black Vine Weevil Spider Mites Scale Insects Aphids

Phenological Calendar

View full calendar (6 stages)

Stage	Typical Window	What to Look For
Bud swell BBCH 01	Feb 15-Mar 31	Flower and vegetative buds visibly swelling; bud scales separating slightly
Bud break / leaf emergence BBCH 11	Mar 15-Apr 30	Vegetative buds opening; new leaves emerging from terminal buds; light green new growth visible
Bloom BBCH 61	Apr 1-Jun 30	First trusses opening; color visible; varies widely by cultivar (early types Feb, late types Jun)
New growth flush BBCH 31	May 1-Jul 31	Active shoot extension after bloom; new leaves expanding; lighter green color distinguishes new growth
Flower bud set BBCH 55	Jul 1-Aug 31	Next year's flower buds forming at shoot tips; plump rounded buds distinguishable from narrower vegetative buds
Dormancy BBCH 00	Nov 1-Feb 15	Growth ceased; evergreen foliage persists but may curl in cold; deciduous azaleas drop leaves

Cultivars (6)

'R. macrophyllum (Pacific Rhododendron)'

WA state flower; native throughout Western WA forests; pink-purple flowers; 6-15 ft; shade-tolerant

Do not transplant from wild. Difficult in cultivation; needs acidic, well-drained forest soil. Best left in place in native landscapes.

''PJM' group'

Extremely cold-hardy (-25F); small lavender flowers in early spring; compact 3-5 ft; semi-evergreen

Very reliable; tolerates more sun and poorer soil than most rhododendrons; good for difficult sites

''Nova Zembla''

Red flowers; cold-hardy; 5-8 ft; vigorous grower

The standard red rhododendron in PNW nurseries; reliable and widely available

''Unique''

Yellow-cream trusses fading to pale yellow; compact rounded habit; 4-5 ft

Puget Sound favorite; compact size suits foundation plantings

'R. occidentale (Western Azalea)'

Native deciduous azalea; fragrant white to pink flowers with yellow blotch; 6-10 ft; excellent fall color

Found along streams in the wild; needs consistent moisture. Native to SW Oregon and WA. Underused in PNW gardens.

''Cunningham's White''

Very tough; white flowers with green-yellow flare; 4-6 ft; dense habit

Good for challenging sites where soil pH is borderline; commonly used as rootstock for grafted cultivars

Data Maturity

Baseline Extension data. Expert review underway.