David G. Leach Research Station

Rhododendron by pond
David G. Leach photo
David G. Leach


The Holden Arboretum’s David G. Leach Research Station is a satellite facility maintained for horticultural research on a 30-acre property, 40 minutes east of Holden’s Kirtland campus. The land was a gift to Holden from David Leach, PhD, who developed it as a rhododendron breeding and evaluation garden. Normally closed to the general public, the research staff hosts a series of open houses each spring when the gardens are in bloom. Times and dates are published in the the spring class schedule.


Leach was an eminent American horticulturist who became a world authority on genus Rhododendron, and is renown for his development and introduction of hardy (zone 5) hybrid rhododendrons for the gardening public. The Leach Station is staffed and equipped to support extensive and diverse display gardens, in addition to large breeding populations and research plots.



The Leach cultivars R.‘Bikini Island’ and R. ‘Capistrano’ are good examples of interspecific rhododendron hybrids that combine the saturated flower colors of Asian species with cold hardiness from North American species.

Red Rhododendron Flower
Yellow Rododendron flower
R. 'Bikini Island'
R. 'Capistrano'

Under the direction of plant breeder and geneticist Stephen Krebs, PhD, the station maintains its commitment to developing superior rhododendrons for continental climates (i.e. cold winters and hot summers). The traditional breeding program is now complemented by a vigorous research component focused on adaptations of rhododendrons to biotic and abiotic stresses, such as winter freezing injury to leaves and buds, summertime ‘bleaching’ of leaves (photoinhibition), and diseases caused by fungal pathogens (Phytophthora root rot and powdery mildew). The basic questions being addressed are:

  • How much natural variation exists among Rhododendron species for tolerance to a particular stress?

  • What are the genetic and physiological determinants of this tolerance?

  • Can stress tolerance be transferred via conventional or modern techniques to a new generation of hybrids that have improved landscape performance under stressful conditions?

  • How is expression of these traits influenced by other key features of woody plant biology, such as juvenility and dormancy?


Research on these questions is being conducted on- and off-site, frequently in collaboration with faculty and graduate students at participating universities.


While much of the research focus is on a single genus (Rhododendron) at the whole plant level, our findings are broadly relevant to other woody plant species or to other levels of investigation (molecular, cellular, or community).


Rhododendrons share many biological features in common with other members of Ericaceae, the heath family, which includes 120 genera (2,500+ species) that are valued throughout the world as native plants, fruit crops, and ornamental garden plants.



  Rhododendron root rot is caused primarily by the soil fungal pathogen Phytophthora cinnamomi. The fungus colonizes the root vascular system of susceptible hosts, cuts off water supply to the shoot, and causes a characteristic leaf wilting and rolling prior to plant mortality. Resistance to the disease is rare (< 3 percent of screened cultivars and species), and we are using breeding and screening methods to transfer resistance to a broader array of rhododendron cultivars.  

Photo of diseased Rhododendron


R. ‘Hindustan’        

no inoculation      

R. ‘Hindustan’        

+ P. cinnamomi


Winter freezing damage can be observed in a variety of tissues. Floral buds are less cold hardy than leaves or stems. Field plants showing damage (browning) to both leaves and buds (A) are less hardy than plants with buds only affected (B). Breeding selections are made for plants that have undamaged tissues (C) following winter lows of –15 to –20F.

Rhododendron Photo
Flowering Rhododendron Photo


Contact: Stephen Krebs, skrebs@holdenarb.org


Selected Publications

Peng, Y, W Lin, H Wei, SL Krebs, R Arora (2008) Phylogenetic analysis and seasonal cold acclimation-associated expression of early light-induced protein genes of Rhododendron catawbiense. Physiologia Plantarum132: 44-52. PDF


Jones, JR, TG Ranney, NP Lynch, and SL Krebs (2007) Ploidy levels and relative genome sizes of diverse species, hybrids, and cultivars of rhododendron. Journal of the American Rhododendron Society 61: 220-227. PDF


Kalberer, SR., R Aurora, N Leyva-Estrada, and SL  Krebs (2007) Cold hardiness of floral buds of deciduous azaleas: dehardening, re-hardening, and endodormancy in late winter. Journal of the American Society for Horticultural Science 132: 73-79. PDF


Wei, H, AL Dhanaraj, LJ Rowland, Y Fu, SL Krebs, and R. Arora (2005). Comparative analysis of  expressed sequence tags from cold-acclimated and non-acclimated leaves of Rhododendron catawbiense Michx. Planta 221: 406-416. PDF


Krebs, SL (2005). Loss of winter hardiness in R. ‘Supernova’, an artificial polyploid. J. American Rhododendron Society 59: 74-75.


Marian, CO, SL Krebs and R Arora (2004).  Dehydrin variability among Rhododendron species: a 25- kDa  dehydrin is highly conserved and associated with cold acclimation across diverse species. New Phytologist 161: 773-780. PDF


Krebs, SL and M Wilson (2002) Resistance to Phytophthora root rot among contemporary  rhododendron cultivars. HortScience 37: 790-792. PDF


Lim, CC, SL Krebs, R Arora (1999) A 25 kD dehydrin associated with genotype- and age-dependent leaf freezing tolerance in Rhododendron: a genetic marker for cold hardiness? Theoretical and Applied Genetics 99: 912-920. PDF


Krebs, SL (1996) Normal segregation of  allozyme markers in complex rhododendron hybrids. Journal of Heredity 87:131-135.