Nursery Crop Science
Fragrant Rhododendrons for Southern Gardens: Current Breeding Efforts at NCSU
Ryan N. Contreras, Tom Ranney, Tom Eaker,
Joel Mowrey and Nathan Lynch
Department of Horticultural Science, North Carolina State University
The genus Rhododendron is comprised of over 900 species, classified in eight subgenera, encompassing enormous genetic variability in ecologically adaptive and ornamental traits. Species range from creeping alpines growing only one to two inches to single trunk arboreal species reaching heights up to 80 feet. Epiphytic species occur in tropical montane habitats. There is a wide range in flower color. Hues cover a broad spectrum of colors, including bluish-violet, crimson to red, orange to yellow and white with hybrids representing countless combinations of these colors. Since the early 1800’s, hybridizers have been utilizing the amazing genetic diversity within the genus to develop cultivars that combine some of the more desired characters. Past hybridization generally has been limited to crosses within subgenera, such as the cross between R. catawbiense Michx. and R. maximum L. resulting in the cultivar R. ‘Maxicat’. Both species are classified in subgenus Hymenanthes because they share similar morphological traits. Thus, the resulting hybrid is not especially novel, at least in terms of combining divergent traits. However, if more distantly related taxa are used in hybridization, it may be possible to develop new cultivars that have traits not found in a single taxon or subgenus. One example of this technique is R. ‘Fragrans’, a hybrid resulting from a cross between R. catawbiense and R. viscosum (L.) Torrey (subgenus Pentanthera) performed by Chandler of Paxton and Sons Intl. (UK) in 1843. These species are distinct morphologically, resulting in an intermediate hybrid displaying novel traits.
Fragrance is an extremely desirable and marketable trait for landscape plants. Unfortunately, fragrance is limited in temperate evergreen rhododendrons (e.g., R. auriculatum, R. davidsonianum, R. decorum, R. diaprepes, R. fortunei,R. griffithianum), and their hybrids. Fragrance tends to be linked genetically to poor cold hardiness and pale flower color. However, it is possible to introgress excellent fragrance from deciduous azaleas into evergreen rhododendron through inter sub-generic hybridization resulting in novel “azaleodendrons”. Hybrid sterility is an issue, but development of tetraploid azaleodendrons can restore fertility. We have developed tetraploid azaleodendrons with excellent fragrance, semi-evergreen foliage, and moderate fertility. These plants will serve as a foundation for backcrossing to diploid and tetraploid, evergreen rhododendrons.Advantages and disadvantages of such wide hybridization as well as the current status of its use in our breeding program will be reviewed.
