Nothing says spring like azaleas! One of the National Arboretum’s most popular plantings, the 

Glenn Dale Azaleas draw thousands for annual spring viewing. Horticulturist Benjamin Y. Morrison worked for over 25 years to create this superior group of winter-hardy azaleas with large, 

colorful flowers suitable for the Washington, DC region. We present a small vignette of the 454 

named introductions to entice you to grow these lovely spring treasures. Check our on-line 

photo gallery and be sure to make an April visit to the Arboretum part of your tradition too!

U.S. National Arboretum Plant Introduction

U.S. Department of Agriculture, Agricultural Research Service, 3501 New York Ave., N.E., Washington, DC 20002

Rhododendron Glenn Dale Hybrid Azaleas

U.S. National Arboretum Plant Introduction

The south face of Mt. Hamilton at the National Arboretum in Washington, DC was planted with approximately 

15,000 azaleas from Glenn Dale in 1946-47. In 1949, the Arboretum opened to the public for the first time 

during the azalea bloom.
Botanical Name: Rhododendron Glenn Dale hybrids

Plants in the Glenn Dale azalea breeding program were initially 

assigned Bell or "B" numbers an old name for the Plant Introduction 

Station at Glenn Dale, MD. Plant Introduction (PI) numbers were 

assigned later. The first “B” number was assigned to ‘Dimity’ in 1937. 

and ‘Satrap’ received the last “ B” number in 1951 and PI number in 

1952.

Family Ericaceae

Hardiness: U.S.D.A. Zones 6b-8 (some hardy to Zone 5) 

Development: In the late 1920’s B.Y. Morrison began an azalea hybridizing program 

at the USDA Plant Introduction Station, Glenn Dale, MD, with the 

goal of developing large-flowered landscape azaleas, similar to those 

which grew in southern gardens that would be winter hardy in the 

Washington, DC region and provide overlapping periods of bloom from 

April through June. Morrison used a wide variety of parent material, 

crossing cold hardy selections with early and late flowering colorful, 

large-flowered evergreen forms, that were not hardy in the mid-atlantic region. Among these were Rhododendron kaempferi (Kaempfer’s 

azalea), R. poukhanense (Korean azalea), R. indicum and parent in 

more than 100 Glenn Dale hybrids, R. mucronatum, R. phoeniceum

and Kurume hybrids, clones of the “Southern Indian” azaleas now 

known as “Belgian Azaleas”, R. simsii, and the mysterious ‘Vittatum’, 

parent in over 100 Glenn Dale hybrids. The program lasted 25 years, 

involved hundreds of crosses and resulted in 454 new evergreen azalea 

cultivars. The hybrids were distributed from 1942 through 1954 to 

nursery growers, gardens and individuals across the United States.

Significance: Prior to the introduction of the Glenn Dale Hybrids, commonly available azaleas were small flowered, bloomed over a shorter period of 

time, offered few color choices and were not always attractive year 

round. The Glenn Dale Hybrids represent the accomplishment of an 

ambitious mission: landscape azaleas hardy in the Mid-Atlantic (Zone 

7) region, with colorful flowers of up to 4 inches wide, covering the mid 

April - mid June bloom gap, and for the sheer volume of cultivars produced. Almost all of the cultivars and many of the parents still exist. 

The Glenn Dales also reflect Morrison’s interest in flower character 

and quality: clean, pure colors; definite contrasts in patterned flowers; reflective or transmissive character; flower stalks (pedicels) strong 

enough to bear the weight of the flowers and the ability of the flower to 

fall off, rather than die and persist on the plant.

Morrison grasped the opportunity to incorporate a wide range of germplasm in the breeding program, using not only azaleas from important 

U.S. private and commercial sources but also plants or seed collected 

by plant explorers in Asia, especially China in the 1920's and later. 

 

Glenn Dale Hybrid Azaleas

U.S. National Arboretum Plant Introduction

U.S. Department of Agriculture, Agricultural Research Service, 3501 New York Ave., N.E., Washington, DC 20002
Description: Height and Width: Range from 3 to 8 feet in height and width or 

larger after 10 years.

Habit: Varied; rounded shrubs may be broad spreading, low growing, 

upright or erect.

Foliage: Green in summer with a variety of foliage shapes and 

degrees of glossiness, sizes and downiness. Relatively evergreen in 

winter in the mid-Atlantic region, some with fall color. 

Flowers: Colors range from white to red and orange-reds, pink to 

purples, may include chartreuse markings or be solid or patterned. 

Flowers are 2-4 inches in diameter and occur as singles, semi-doubles, 

doubles and hose-in-hose. The hybrids are classified as early, early 

and late midseason, or late blooming groups and generally bloom from 

early April through early June; earlier in warmer zones. 

Culture: These acid loving plants perform best under high or dappled shade 

with well drained soils enriched with organic compost or humus and a 

soil pH of 4.5-6.0 and good air circulation. Mulch with shredded pine 

bark, oak leaves, or other mulch that resists compaction and permits 

air and water to pass through. Protect from winter sunshine in colder 

climates to avoid desiccation. Summer pruning before mid-July will 

increase the number of shoots and the number of blooming branches 

the following spring, later pruning will reduce bloom. Control petal 

blight fungus promptly by removing blighted flowers and/or using a 

recommended fungicide.

Propagation: Root from softwood to semi-hardwood cuttings (late spring through 

early fall) under mist or plastic, low rates (0 - 3000 ppm) of IBA or 

other rooting hormone, in 4-6 weeks. Also propagated by grafting, 

layering, and tissue culture. Protect newly rooted cuttings in winter. 

Landscape Use: Naturalized, formal or informal gardens, along the edges of woods; 

foundation plantings with shade. Depending on desired effect, allow 

four to six feet for the spread of most azaleas, or plant more closely for 

initial impact and transplant later to avoid crowding. 

Availability: Many are readily available from mail-order firms, retail and wholesale nurseries. Some cultivars are rare or available only from specialty nurseries. Several outstanding, but lesser known, cultivars were 

selected for redistribution beginning in 2007.

References: Since the 1953 publication of Morrison's official history and 

description, The Glenn Dale Azaleas, Agriculture Monograph No. 20, 

USDA (no longer in print), parentage, cultivar data, modern color 

designations and analysis of the USNA Glenn Dale azalea collection have been published in The Azalean, the Journal of the Azalea 

Society of America, Azaleas, by Fred Galle, Timber Press, 1987, and 

both The Bell Book, A Companion to Monograph 20, 1994 and The 

Glenn Dale Azaleas, Revised, 1996 of Azalea Works, Bethesda, MD, 

among others.

U.S. National Arboretum Plant Introduction

U.S. Department of Agriculture, Agricultural Research Service, 3501 New York Ave., N.E., Washington, DC 20002

Glenn Dale Hybrid Azaleas
'Glacier' Glenn Dale Hybrid Azalea

Rhododendron 'Glacier' is considered one of the most attractive, year-round, of the Glenn Dale 

hybrid azaleas. Blooming in the spring, it is distinguished by extremely attractive lustrous 

dark green foliage, and large shining white 2 1⁄2 inch flowers with a greenish throat. Grows 

vigorously up to five feet in height in 20 years. Blooms midseason.

Parentage: Rhododendron 'Malvatica' [Malvaticum] × 'Yozakura' 'Glacier'

(NA 66868; PI 160073; Bell # 32537)

Description: Evergreen; vigorous, erect to spreading growth, up to 5 feet. Very 

handsome lustrous, dark green leaves. Flowers 2 1⁄2 to 3 inches 

across, usually borne 2 to 3 in a head, shining white with a faint 

green tone. Late April Introduced 1947.

Culture: Click here.

U.S. National Arboretum Plant Introduction

Floral and Nursery Plants Research Unit

Visit us on the web: www.usna.usda.gov

September 2008 The U.S. Department of Agriculture is an Equal Opportunity Employer.
U.S. National Arboretum Plant Introduction

Floral and Nursery Plants Research Unit

Visit us on the web: www.usna.usda.gov

September 2008 The U.S. Department of Agriculture is an Equal Opportunity Employer.

'Martha Hitchcock' Glenn Dale Hybrid Azalea

Rhododendron 'Martha Hitchcock' is an outstanding and extremely popular Glenn Dale hybrid 

azalea. In spring this vigorous shrub is covered with large 2 1⁄2 to 3 inch diameter flowers of 

brilliant magenta crimson with a white center and darker blotch. The garden effect is striking. Broad-spreading and fast-growing, it must be controlled by pruning unless given plenty of 

space. Blooms midseason.

Parentage: Rhododendron mucronatum [Indica alba] × 'Shinnyo-no-tsuki'

(NA 70556; PI 163955; Bell # 39513)

Description: Broad spreading evergreen shrub; will eventually reach 5 feet in 

height in 20 years. Leaves medium green, hairy. Flowers 1 to 3 

per head, 3 inches across, white margined with magenta [RHS 72A, 

strong reddish Purple]. Strongly growing shoots usually produce selfcolored flowers. Do not remove, as laterals give flowers with correct 

pattern thereafter. Early to mid-May. Introduced 1948.

Culture: Click here.