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Rhododendron x hybridum Glenn Dale

Common Name(s):

Previously known as:

  • Rhododendron hybrida 'Glenn Dale'
Phonetic Spelling
roh-do-DEN-dron hy-BRID-um
This plant has low severity poison characteristics.
See below
Description

Glenn Dale azaleas are hybrid, woody, evergreen shrubs in the Ericaceae (blueberry) family that were developed for their large showy flowers and winter hardiness. The genus name derives from the Greek words rhodo, which means rose, and dendron, meaning tree. The common name is a reference to the place name where the azalea hybridizing program began in Glenn Dale, MD. 

These plants prefer moist, well-drained, acidic soils in dappled shade and good air circulation. Protect them from afternoon sun. They do not tolerate dense clay, so if you have clay soil, amend with organic material. The roots may rot if soil does not drain well. Plant the root ball high in the soil to help with drainage issues. Mulch with pine bark or oak leaves. Pruning, if desired, should be done before mid-July. 

These are large-flowered landscape azaleas with a varied rounded habit, meaning some grow more erect and others are more spreading. Their size at maturity is 4 to 6 feet tall and 3 to 4 feet wide. There are many cultivars and colors, and some cultivars are winter hardy to zone 5. They bloom from April to early June. 

Plant Glenn Dale azaleas as understory shrubs in groups in woodland or naturalized areas, or as specimens in pollinator gardens where the showy flowers will attract bees.  They worlk well as foundation plantings in shaded locations.

Insects, Diseases, and Other Plant Problems:  Rhododendrons are susceptible to insect and disease problems.  Insect problems include aphids, borers, lace bugs, leafhoppers, mealybugs, mites, nematodes, scale, thrips and whitefly.  Diseases include canker, crown rot, root rot, leaf spot, rust, and powdery mildew. This plant is frequently damaged by deer.

While these shrubs remain very popular for landscape use, many cultivars are susceptible to Phytophthora root rot—this leads to leaf loss, reduced vigor, branch dieback, and wilting. Implement good cultural practices first, such as improving drainage with organic matter or berms and avoiding overwatering or overfertilization. However, if you have a site with a history of this disease, consider planting a root rot-resistant alternative species. 

For suitable alternatives, see this video created by Charlotte Glen as part of the Plants, Pests, and Pathogens series.

 

 VIDEO Created by Elizabeth Meyer for "Trees, Shrubs and Conifers" a plant identification course offered in partnership with Longwood Gardens.

More information on Rhododendron.

Profile Video:
See this plant in the following landscape:
Cultivars / Varieties:
  • 'Glacier'
    Attractive year-round, large white flowers 5 feet tall
  • 'Martha Hitchcock'
    Magenta with white center and dark blotches, spreading, fast growing
'Glacier', 'Martha Hitchcock'
Tags:
#evergreen#hardy#showy flowers#poisonous#specimen#wildlife plant#nectar plant#fall interest#specialized bees#foundation planting#large flowers#pollinator plant#naturalized area#flowers late spring#evergreen shrub#flowers early summer#understory shrub#problem for cats#problem for dogs#bee friendly#problem for horses#woodland garden#flowers mid-spring
 
Cultivars / Varieties:
  • 'Glacier'
    Attractive year-round, large white flowers 5 feet tall
  • 'Martha Hitchcock'
    Magenta with white center and dark blotches, spreading, fast growing
'Glacier', 'Martha Hitchcock'
Tags:
#evergreen#hardy#showy flowers#poisonous#specimen#wildlife plant#nectar plant#fall interest#specialized bees#foundation planting#large flowers#pollinator plant#naturalized area#flowers late spring#evergreen shrub#flowers early summer#understory shrub#problem for cats#problem for dogs#bee friendly#problem for horses#woodland garden#flowers mid-spring
  • Attributes:
    Genus:
    Rhododendron
    Family:
    Ericaceae
    Life Cycle:
    Woody
    Wildlife Value:
    Members of the genus Rhododendron support the following specialized bee: Andrena (Andrena) cornelli.
    Dimensions:
    Height: 4 ft. 0 in. - 6 ft. 0 in.
    Width: 3 ft. 0 in. - 4 ft. 0 in.
  • Whole Plant Traits:
    Plant Type:
    Poisonous
    Shrub
    Woody Plant Leaf Characteristics:
    Broadleaf Evergreen
    Habit/Form:
    Erect
    Spreading
    Maintenance:
    Low
    Texture:
    Medium
  • Cultural Conditions:
    Light:
    Dappled Sunlight (Shade through upper canopy all day)
    Partial Shade (Direct sunlight only part of the day, 2-6 hours)
    Soil Texture:
    High Organic Matter
    Loam (Silt)
    Sand
    Soil pH:
    Acid (<6.0)
    Soil Drainage:
    Good Drainage
    Moist
    Available Space To Plant:
    3 feet-6 feet
    NC Region:
    Coastal
    Mountains
    Piedmont
    USDA Plant Hardiness Zone:
    6b, 6a, 7b, 7a, 8b, 8a
  • Flowers:
    Flower Color:
    Gold/Yellow
    Orange
    Pink
    Purple/Lavender
    Red/Burgundy
    White
    Flower Inflorescence:
    Raceme
    Flower Value To Gardener:
    Long Bloom Season
    Showy
    Flower Bloom Time:
    Spring
    Summer
    Flower Shape:
    Trumpet
    Flower Petals:
    4-5 petals/rays
    Flower Size:
    1-3 inches
    Flower Description:
    Many colors, white, red, orange-red, pink to purple trumpet shaped 2 to 4 inches; very showy, vary from single, semi-double to double, singles have 5 petals. Appear from mid-spring to early summer which is longer than other azaleas.
  • Leaves:
    Woody Plant Leaf Characteristics:
    Broadleaf Evergreen
    Leaf Color:
    Green
    Leaf Feel:
    Glossy
    Leaf Type:
    Simple
    Leaf Arrangement:
    Alternate
    Hairs Present:
    Yes
    Leaf Description:
    Green in summer, varying degree of glossiness, sizes, downiness; some with fall color.
  • Stem:
    Stem Is Aromatic:
    No
  • Landscape:
    Landscape Location:
    Naturalized Area
    Woodland
    Landscape Theme:
    Pollinator Garden
    Design Feature:
    Foundation Planting
    Specimen
    Attracts:
    Pollinators
    Specialized Bees
    Problems:
    Poisonous to Humans
    Problem for Cats
    Problem for Dogs
    Problem for Horses
  • Poisonous to Humans:
    Poison Severity:
    Low
    Poison Symptoms:
    Vomiting (not in horses), diarrhea, weakness, cardiac failure
    Poison Toxic Principle:
    Grayantoxin
    Causes Contact Dermatitis:
    No