Azaleas and Their Cold Tolerance
Research Activities at the University of Minnesota Landscape Arboretum
Dr. Harold Pellett
We are increasing our research efforts with deciduous azaleas with the primary goal of developing or identifying cold hardy azaleas with a broad spectrum of flower color, blooming date and plant habit. We have had fairly good success to date using R. prinophyllum (R. roseum) as a parent. Flower buds of the R. prinophyllum plants we have can acclimate to withstand -45° F. without injury. Hybrids with R. kosteranum (Mollis) can also acclimate to the same level and have been reliable for us every year. F-1 seedling grown hybrids of this cross were introduced into the trade this past year and are sold under the name Northern Lights Hybrids. We are currently working with the nursery industry to build up propagation stock for release of two clones from R. kosteranum x R. prinophyllum crosses.
In efforts to identify additional parent plants that possess sufficient cold hardiness to withstand our winters, we find very limited detailed information available concerning hardiness. Although hardiness ratings can be found for most azalea and rhododendron cultivars these ratings are only approximations and are often based on limited observations. Microclimate is often neglected in reports of survival following extremes in temperature and thus the plants perhaps were not exposed to temperatures as low as that recorded at a local weather station. In other cases plants may actually be hardier than rated but merely have not been evaluated under colder climates.
We have laboratory equipment available that permit us to subject plant tissues to closely monitored cold temperatures and determine very accurately at what temperatures plant tissues are actually injured.
We have developed techniques that allow us to acquire plant tissues in midwinter from plants growing in milder climates, acclimate the tissues to their maximum capabilities in the laboratory and then determine their level of cold temperature tolerance. We plan to use these techniques to determine flower bud hardiness potential of a number of native and exotic rhododendron species and also of representative clones from various hybrid groups. This information will be helpful to us in identifying additional parental materials for our breeding program. These laboratory results should also be of value to other breeders and to azalea enthusiasts for selection of plants for their gardens. These results should be of value to anyone no matter where they reside. For example if we find that some flowers of a given species or clone are killed at -20° F. and all are killed by -25° F, we and others with similar climates will know that in most years we wouldn't have much bloom with that plant. However, people living in areas where annual minimum temperature is -20° F. or above would know that usually they would have good bloom. We have had good correlation between our laboratory results and field performance and I don't know of any reason why these results shouldn't correlate as well in other locations.
My primary purpose in writing this article is to solicit aid from azalea enthusiasts in conduct of the cold hardiness testing described above. I need to identify people with easy access to various species and clones of azaleas that would be willing to collect flower buds from the plants in mid-winter and send them to me. I am especially interested in testing species native to North America, and readily available clones of various hybrid groups such as the Ghents, Knaphills, Exburys, etc. For the tests I need approximately 25 flower buds cut with a small piece of stem attached. These should be put into plastic bags, sealed tightly, labeled as to species or clone, and sent to reach me in the shortest possible time. If you would be interested in helping with this effort please let me know what plant materials you could supply. If we can use the materials you have we will contact you with details on collection and shipping. We will test as many types as time permits but will give priority to deciduous azaleas of the groups listed previously. Results of this research will be submitted to the American Rhododendron Society bulletin for publication. A grant received from the American Rhododendron Society's Research Foundation will help us initiate this research.