Discovering the Origin of the Azalea Plant: A Historical Perspective

North American deciduous azaleas have long been adored by horticulturists. These plants, which belong to the genus Rhododendron sect. [section] Pentanthera, comprise 15 species distributed from Texas to Florida, extending northwards to southern Maine, and with one species occurring in mountainous areas of Oregon and California. These species display a wide range of flower color, from pure white to deep orange, pink, and many shades in between, providing tremendous spring and summer interest in the garden. Quickly recognized for their horticultural merit, North American deciduous azaleas piqued the interest of plant collectors upon their discovery and continue to be widely lauded by gardeners today. This has led to a proliferation of cultivars and interspecific hybrids that provide a beautiful floral display every year both in gardens and in the wild (Azalea Society of America 2016). Over 240 unique accessions of Rhododendron sect. Pentanthera exist at the Arnold Arboretum including many interspecific hybrids, various cultivars, and 12 of the 15 deciduous azalea species native to North America (Table 1). The accessions at the Arnold Arboretum have origins in a wide range of environments and could contain many useful adaptations to common abiotic (non-living) stressors. This collection thus represents a valuable asset to woody plant researchers who seek to understand the adaptive potential of these plants to better benefit gardeners and ecosystems alike.

Those who have spent any time researching azalea species or cultivars quickly discover a confusing naming system. Azaleas are actually a now informal designation within the genus Rhododendron, a large genus of 1,024 species in the heath family (Ericaceae) with a distribution spanning across the northern hemisphere (American Rhododendron Society 2015). Horticulturists and plant enthusiasts commonly split the genus Rhododendron into two informal groups, “rhododendrons” and “azaleas,” based on appearance. Rhododendrons are typically considered to be large evergreen shrubs with large, leathery, elepidote (without scales) leaves, or smaller evergreen shrubs with lepidote (with scales) leaves. Rhododendrons are also differentiated from azaleas in flower morphology by the presence of 10 or more stamens (Azalea Society of America 2016). Azaleas, in contrast, are usually considered to be smaller, sometimes deciduous shrubs with pubescent leaves and five to six stamens, though these morphological characteristics can be quite varied within species (Azalea Society of America 2016). The word azalea is derived from the Greek azaleos meaning “growing in dry soil.” This name reflected the cliffside and mountain habitats of Rhododendron luteum (yellow azalea), a European species (Lee et al. 1953). However, this is a misleading name because azaleas are generally intolerant of drought and are most commonly found in moist or mesic forest sites (Hume 1948). Considered a distinct genus (Azalea) until the mid-twentieth century, azaleas were moved into the genus Rhododendron after Sleumer’s taxonomy was published in 1949 (Chamberlain et al. 1996).

Such taxonomies—the relationships of species based on morphological traits and, increasingly, genetic variation—are complex and frequently revised in a large genus like Rhododendron as new species are discovered. Traditional morphology-based taxonomy uses leaf, bud, floral, and other physical traits to determine relationships between species. However, this kind of physical classification can confuse convergent evolution (evolution of similar adaptations independently across varying taxa) with species similarity, leading to incorrect conclusions about species relationships. Modern molecular-based taxonomy uses differences in DNA sequence to group species based on the likelihood of previously shared ancestry. Since this is based on the timespan back to the most recent common ancestor, the classifications will not be confused by convergent evolution of morphological traits. In the genus Rhododendron, subsequent molecular phylogenies that group species based on genetic variation have shown no distinction between species commonly known as azaleas and rhododendrons (Goetsch et al. 2005). This has led to the assimilation of the North American deciduous azaleas as a section within the broader subgenus Hymenanthes, which includes many large-leafed evergreen Rhododendron species. Nonetheless, “azalea” and “rhododendron” remain important horticultural terms for marketing and identification purposes.

Rhododendron in the Garden

The earliest recorded cultivation of plants in the genus Rhododendron began in the fifteenth century in Japan and involved a cultivar group now known as the Satsuki azaleas, derived from the former Tsutsusi, or subgenus of evergreen azaleas (Callaham 2006). At the time, only the rich and privileged classes in Japan were allowed to grow and possess azaleas as they were deemed “too beautiful” for commoners (Callaham 2006). Early botanical explorers such as Robert Fortune in Asia and John Tradescant in colonial North America first brought Rhododendron to prominence through their plant collecting activities in the eighteenth and nineteenth centuries (Cox et al. 1997). They discovered diverse flower color, leaf morphology, and plant habit across the Rhododendron species they encountered. Soon after these plants appeared in European gardens, plantsmen began creating interspecific hybrids. The great diversity in flower colors, fragrance, plant size, leaf shape, and leaf pubescence (indumentum) resulting from these efforts led to expanded horticultural interest in rhododendrons and azaleas in the early twentieth century (Hume 1948). Throughout the twentieth century, hobbyists played a major role in the proliferation of new cultivars (Galle 1974). Rhododendron cultivation today spreads across temperate regions of the world, and there are over 28,000 named cultivars in existence (American Rhododendron Society 2016).

Deciduous azaleas in sect. Pentanthera are represented by one species each in Europe, Asia, and western North America and 14 accepted species in eastern North America (Towe 2004; Zhou et al. 2008). Deciduous azaleas constitute the largest group of Rhododendron species in North America. Because it holds such a large number of species (compared to other parts of the world), the southeastern United States is considered a major center of diversity for deciduous azaleas (Hume 1953). The great amount of phenotypic variation for flower color, fragrance, and cold hardiness among deciduous azaleas has long generated interest for using North American species in breeding. John Bannister, an English botanist, recorded the first deciduous azalea in North America, Rhododendron viscosum (swamp azalea), in Virginia in 1690 (Galle 1974). Seeds from the newly discovered deciduous azalea species were sent back to Europe where plants were grown for observation beginning in 1734 by the American botanist John Bartram. It was not until 1825 that the first recorded and popular interspecific hybrids were produced using North American deciduous azaleas (Hume 1953) (see textbox above).

Although they grow well within their native ranges, lack of cold hardiness and/or intolerance of high soil pH are major limiting factors to cultivation of these azaleas across much of North America. Relatively few Rhododendron species are native to continental climates, typified by extreme temperature variations throughout the year and characteristic of many regions within the United States. This is reflected in the cultivation history of Rhododendron species, which, until fairly recently, were only commonly grown and propagated in the mild and humid climates of Japan, Europe, and the eastern and western coasts of North America. For woody plant breeders in the midwestern United States, developing cultivars for their tolerance to continental climates is of paramount importance for the horticultural success of deciduous azaleas.

Fortunately, despite originating from milder climates, individual genotypes from many North American native azalea species have been identified that are exceptionally hardy under extreme temperature variation (Widrlechner 1982). North American deciduous azalea germplasm has undergone relatively little systematic evaluation and had received little breeding focus until the twentieth century, when diversified landscape needs led nursery growers to seek hardier Rhododendron germplasm (Hokanson 2010). For example, suburbanization in the United States following World War II led to an increased need for landscaping plants to beautify new developments (Whitehand and Larkham 1992). During the middle of the twentieth century, few ornamental plants adapted to the upper midwestern United States existed (UMN Fruit Breeding Farm 1954). Later, University Agricultural Experiment Stations in states like Minnesota began funding breeding programs for ornamental plants to develop and introduce promising cultivars for landscape use (Widrlechner 1982). Many ornamental shrubs were trialed at the time on the grounds of the newly founded Minnesota Landscape Arboretum, including several deciduous azalea species and interspecific hybrids developed previously in Europe. After years of parental and seedling testing, plants from crosses made in 1957 between mollis azaleas (R. × kosteranum), which are a group of Asian hybrid deciduous azaleas, and the American species R. prinophyllum proved to be hardy to at least -35°C (-31°F) (Johnson and Snyder 1966). One seedling from this breeding population later became the cultivar ‘Northern Lights’ and was released in 1978 as part of the Northern Lights Series, the first flowering azaleas bred for midwestern climates (Pellett and Vos 1978). The University of Minnesota woody landscape plant breeding program has maintained the longest standing breeding program for deciduous azaleas in North America, and continues to actively release cultivars under the Northern Lights Series today (Widrlechner 1982; Hokanson 2010; Hokanson et al. 2015).

The history of azaleas at Rice is tied to the history of Houston

FAQ

What is the origin of the azaleas?

How did azaleas come to America?

What is the cultural significance of azaleas?

What is the history of the kurume azalea?

What is the history of Azalea?

Both Japanese and Chinese traditional varieties, some of them with a cultivation history of more than 400 years, were at the basis. The history of Belgian pot azalea started about 200 or more years ago; the first Belgium-bred and commercially released cultivar ‘Madame Van der Cruyssen’ was introduced in 1867.

How do azaleas grow?

Modern cultivars of pot azaleas are vegetatively propagated by cuttings. Depending on pot size, one to five cuttings are placed in an acid peat substrate (pH = 4.5). During vegetative growth, plants are pinched several times, resulting in the outgrowth of axillary buds and ultimately the desired diameter and globular plant form.

When did Azalea grow in Belgium?

The history of Belgian pot azalea started about 200 or more years ago; the first Belgium-bred and commercially released cultivar ‘Madame Van der Cruyssen’ was introduced in 1867. Historical research supported by phylogenetic and molecular marker studies revealed a genetic continuum must be accepted which spans many species of the Tsutsusi subgenus.

When do azaleas flower?

Depending on the chilling requirements, azaleas can be classified as very early (from August 15), early (from December 1), medium early (from January 15), and late (from February 15) flowering types. Finally, azalea plants are forced to flower in heated greenhouses, using supplementary light in the winter period.