The Holden Arboretum’s David G. Leach Research Station is a satellite facility maintained for horticultural research on a 30-acre property in Madison, Ohio, about a 40-minute drive east of Holden’s Kirtland campus. The land was a gift to Holden from David Leach, who developed it as a rhododendron breeding and evaluation garden. A collection of 2000 mature rhododendron and azalea hybrids are beautifully displayed in a woodland setting along with over 500 ornamental companion trees and shrubs. These hybrids are the culmination of over 50 years of rhododendron breeding and selection carried out by Dr. Leach, and some of them have been named and introduced into commerce. Although the Station is closed to the public on a daily drive-in basis, the research staff hosts an open-house weekend in May when the gardens are in peak bloom.
David Leach was an eminent American horticulturist who was wild about rhododendrons and it is easy to see why. Genus Rhododendron includes over 800 species worldwide of the plants commonly called rhododendrons and azaleas, and the diversity of flower color, bloom dates, foliage, and plant forms is a gardener’s dream. However, most of these species occur in Asian locations with milder climates than northeast Ohio (USDA hardiness zone 5), and one of Leach’s accomplishments was to adapt highly ornamental plants to our climate by cross-pollinating different species. For example, Leach hybrids with deep red and yellow flowers, such as R. ‘Capistrano’ at the right and R. ‘Red Sea’ below, were achieved by crossing a white flowered, cold hardy species native to the Appalachian Mountain with more colorful but less hardy species from China.
Getting the desired plants often required multiple generations of breeding and evaluation of thousands of progeny in field rows at the Station. In addition to his hybridizing work, Leach helped popularize rhododendrons in the U.S. through his authoritative writings on the subject and was an influential advocate of public gardening in his roles as president of the American Horticultural Society and as a board member the Garden Conservancy.
Under the direction of plant breeder and geneticist Stephen Krebs, the station maintains its founder’s commitment to developing superior rhododendrons for cold climates and has targeted additional performance traits. The traditional breeding program is complemented by a research component focused primarily on adaptations to biotic and abiotic stresses that currently limit where rhododendrons can be successfully grown. For example, temperature extremes, high sunlight conditions, and poorly drained alkaline (high pH) soils are environmental factors that can reduce the performance of most rhododendrons. In addition, challenges from insects and microbes, especially a root rot disease caused by the invasive soil pathogen Phytophthora cinnamomi, threaten the survival of many rhododendrons and azaleas. With globalization and climate change, many of these stresses are expected to intensify and place greater value on breeding and identifying plants that can meet the challenges.
Current Rhododendron Breeding Projects
Root Rot Disease Resistance Breeding: This fatal disease is caused by the fungus-like soil pathogen Phytophthora cinnamomi, a global invasive that affects over 2000 plant species. The pathogen attacks the root system in its quest for a carbohydrate food source, ultimately destroying the vascular system that carries water and nutrients between the roots and shoots. At that point, the disease manifests itself above ground as sudden and irreversible wilting. Although most rhododendrons and azaleas are susceptible, there are a few, important sources of resistance. Beginning in 2005, we have used a species from Taiwan, Rhododendron hyperythrum, as a source of both resistance and heat tolerance in a breeding program to develop better-adapted hybrids. The best hybrids from this program have been field-trialed in Ohio and southern Louisiana, where root rot disease pressure is high due to the warmer and wetter climate. The southern trials have been conducted with a commercial partner, Plant Development Services, Inc. – that is interested in marketing rhododendrons in the South, a region where they heretofore have performed poorly. Two introductions shown are in commercial production, and new selections from the Leach Station are continually being sent to Louisiana for evaluation.
Rootstock Evaluations: Many of the problems that make rhododendrons difficult to grow are soil related – poor drainage, high pH, and soil pathogens. Grafting rootstocks to shoots (scions) is a time-tested method of overcoming many of these challenges and improving consumer success. By this method, an ornamentally superior but poorly adapted rhododendron can be physically joined to a genetically different root system that can better tolerate stressful soil environments. We are currently evaluating a German proprietary rhododendron rootstock called INKARHO™ to test its claim to a higher pH tolerance than most rhododendrons, which prefer acid soils. This could be a very useful product for opening new markets in regions of North America that have higher pH soils but are otherwise suitable for growing rhododendrons. Disease resistant plants from our root rot breeding program are also being tested as potential rootstocks.