Overlooked Glenn Dale Azaleas
Barbara L. Bullock
Washington, DC
Reprinted with permission, American Nurseryman, July 1, 2008

        Azaleas (Rhododendron) have been a successful crop for the domestic ry industry for more than a century. Hundreds of azalea species and cultivars have been collected from Europe and Asia, bred and crossbred by everyone from nursery industry professionals to college professors, from medical doctors to amateur gardeners. Thousands of azalea varieties, selections and cultivars now exist due to the efforts of breeders searching for improvements such as lustrous, evergreen foliage; longer bloom retention; wider range of colors; shorter stature of growth; and blossoms with an unusual or enhanced size and shape. There are cultivars that bloom during several seasons in the same year. There are azaleas with unique colors, such as green, summer blooms, and increased number of chromosomes or tetraploidy. A nursery today might sell only 10 percent of the named cultivars that were available 20 to 30 years ago. Contemporary azalea cultivars have their merits, but it is this author's intent to make the reader aware of some outstanding cultivars of the past that have been overlooked.
        The Glenn Dale azaleas at the US National Arboretum, Washington, DC, are popular plantings that draw thousands for annual spring viewing. The well-known, 20th-century horticulturist Benjamin Y. Morrison worked for more than 25 years to create this superior group of winter-hardy azaleas with large, colorful flowers. Morrison, former director of the arboretum, named and introduced 454 Glenn Dale cultivars between 1941 and 1952, and described them in his monograph on the Glenn Dale azaleas (USDA Monograph 20; issued October 1953).

Worthy Cultivars
There are some Glenn Dale azalea cultivars not common in the trade today that deserve recognition. Many people may be familiar with some of them, but most likely not all of them. Most are unaware of their long-term performance in the garden and their mature habit. While the Glenn Dale azaleas grow best in zones 6 to 8, they can survive with protection from strong winds in Zone 5. The following are a few cultivars that warrant a closer look.
        One of the first Glenn Dale azaleas to bloom each spring is R. 'Dayspring' (Fig. 1). Like some magnolias, its early blossoms can be damaged by a late-spring frost; but like other early spring-flowering plants, it is worth trying for the years when the blooms are full and bright. The 2-inch flowers have a white center, shading to a light purplish pink at the margins. The plant has a shrubby habit with dense branching, eventually growing up to 6 feet in 20 years.

        Another of the earliest spring Glenn Dale azaleas to bloom, R. 'Allure' (Fig. 2) has the clearest, brightest and most perfect flower color of the pinks. Growing up to 6 feet in 20 years, it has an upright, rounded form with arching branches. The single flowers are generally 2 1/2 to 3 inches across and bloom around mid-April.

        R. 'Jubilant' (Fig. 3) takes on a beautiful, arching habit as it matures. It has brilliant, single, red, 2-inch flowers, which appear in the first week of May (early midseason). After approximately 20 years, 'Jubilant' grows up to 8 feet, becoming more and more attractive as its branches become increasingly arched. Morrison described the shrub to be robust, dense and erect.

        R. 'Fawn' (Fig. 4) is another striking Glenn Dale azalea that is not well-known in the trade. It has 2-inch, white flowers that have distinct, purplish pink, wavy margins, opening so wide they are almost flat. When fully open, the flowers are hypnotic. Its color is similar to the popular Glenn Dale, 'Martha Hitchcock', but the shrub only grows up to 4 feet in 20 years. 'Fawn' retains a denser and more compact form than 'Martha Hitchcock'. It blooms early midseason, typically around the first week of May.

        R. 'Ambrosia' (Fig. 5) is a rather unusually colored Glenn Dale azalea. Azalea collectors often refer to the color as yellowish pink. Morrison noted its color had the landscape effect of pale apricot. Very few early-blooming azaleas exhibit this color except for some of the Kurume azaleas. 'Ambrosia' is unique because its large, single flowers are greater than 2 inches in diameter, and its mature height is taller than the Kurume azaleas. Its early-season flowers bloom near the end of April. Reaching up to 5 feet in 10 years, it has an upright, fairly loose, open habit at maturity.

        One of the unique characteristics of certain Glenn Dale azaleas is their two-toned coloration. R. 'Red Hussar' (Fig. 6) has deep purplish pink to vivid purplish red, single flowers in late April, but the base of each flower tube is salmon red, which contrasts wonderfully with the rest of the flower color. Growing up to 6 feet in 20 years, it maintains a desirable form with densely arching branches.

        Another Glenn Dale known by collectors for having two-toned flowers is R. 'Dauntless' (Fig. 7). Its single flowers are deep purplish red with the base of each flower tube a bright scarlet, similar to the tiny-flowered R. 'Amoenum'. Blooming midseason, it matures into an attractive, broad-spreading form. The new foliage is nearly chartreuse in the spring, but it fades to a medium green the rest of the summer. In perfect contrast with its lime green foliage, its unique color is so beautiful that it seems to glow or vibrate. It appears as a darker purple than almost any other azalea in bloom at that time.

        This brief introduction to the Glenn Dale azaleas demonstrates some of the outstanding characteristics collectors have long noticed about Morrison's creations. Look for 'Fawn' and 'Dauntless' to become available in the near future, as the US National Arboretum distributed these two cultivars in 2007.
        In the late 1920s, Morrison began an azalea-hybridizing program at the USDA Plant Introduction Station, Glenn Dale, MD. The goal was to develop large-flowered landscape azaleas - similar to those that 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's breeding program used a wide variety of parent material, including large-flowered evergreen forms that were not hardy in the Mid-Atlantic region. Among these were R. kaempferi (Kaempfer's torch azalea), R. yedoense var. poukhanense (Korean azalea), R. indicum (sweet indica azalea, a parent plant of more than 100 Glenn Dale hybrids), R. mucronatum (possibly of cultivated origin), Kurume hybrids, clones of the Southern Indian azaleas, R. simsii (Sims azalea) and the mysterious 'Vittatum' (parent of more than 100 Glenn Dale hybrids).
        The breeding program lasted for 25 years, involved hundreds of crosses and resulted in the selection of 454 evergreen azalea cultivars. The Glenn Dale hybrids were distributed between 1941 and 1952 to nursery growers, public gardens (Figs. 8, 9) and individuals across the US.
        In addition to the sheer numbers of cultivars produced, the Glenn Dales also reflect Morrison's interest in flower character and quality. They have clean, pure colors; definite contrasts in patterned flowers; reflective 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 took the opportunity to incorporate a wide range of germplasm in the breeding program, using not only azaleas from important US private and commercial sources, but also from plants or seed collected by plant explorers in Asia, especially China, in the 1920s and later.

Azalea Cultural Practices
The Glenn Dale azaleas can be grown in the same production system as any other azalea. For our azalea cuttings, we use a growing medium high in composted bark under lath. Under this amount of shade, they develop a rather open habit. If you want uniform, tightly branched plants, they may be grown under full sun with adequate irrigation and fertilization.
        In the landscape, Glenn Dale azaleas need the same conditions as most azaleas. They flower most profusely with dappled shade from tall trees with a high heading. Plant them in a location with well-drained soils enriched with organic compost or humus, soil pH of 4.5 to 6.0 and good air circulation. Use mulches of shredded pine bark, oak leaves or other materials that resist compaction and permit 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 blooming branches the following spring; later pruning will reduce bloom. Control petal blight fungus promptly by removing blighted flowers or by using a recommended fungicide.
        Use Glenn Dale azaleas in a naturalized, formal or informal garden setting, along woodland edges (e.g., Fig. 10) or as foundation plantings where there is shade. Depending on the desired effect, allow 4 to 6 feet for the spread of most azaleas or plant more closely for initial impact and transplant later to avoid crowding.

Reintroducing Rare Hybrids
The popularity of the new Glenn Dale azalea cultivars quickly spread within the nursery trade and in a relatively short period of time (1950s and 1960s) after Morrison's retirement. Many nurseries specialized in having all of the Glenn Dales available for dedicated consumers who had become interested in having complete collections of their own. Although they were widely grown, little data were gathered about their performance, ease of propagation and sales appeal.
        Over the past 18 years, the US National Arboretum has been gathering many rare Glenn Dale hybrids for the Azalea Collection. In that time, we've observed several of these Glenn Dales to be outstanding enough to warrant reintroduction. Consumers are becoming more sophisticated in their plant selections, choosing plants primarily for their specific characteristics or where they will fit in their garden. Trends in color have also changed, and the combination of chartreuse green foliage and deep magenta blossoms in 'Dauntless' mirrors recent designer color combinations. The small stature of 'Fawn' might fit better in small gardens than larger cultivars that are more readily available. In the next few years, our goal is to propagate and distribute superior Glenn Dale cultivars that are currently not represented in the trade.

Barbara L. Bullock is curator of the Azalea Collection at the US National Arboretum, Washington, DC.