JARS v40n3 - Breeding For Cold Hardy Azaleas In The Land Of The Northern Lights

Breeding For Cold Hardy Azaleas In The Land Of The Northern Lights
Susan Moe and Harold Pellett
University of Minnesota
Chanhassen, Minnesota

At the University of Minnesota Landscape Arboretum we have a successful breeding program devoted to developing cold hardy deciduous azaleas for Minnesota and other areas with cold winter climates. Previous to the initiation of our project, which was started by the late Al Johnson in 1957, there were few cultivars available which bloomed reliably in our climate. To fill this need we are developing a series of azaleas, containing a wide flower color selection, suitable for our climate.

Cold Hardiness
Cold hardiness is not a simple characteristic for which to breed. Genetic and environmental factors interact to determine the cold hardiness of a plant at any particular time. As natural daylength decreases in late summer and autumn, a particular daylength (or photoperiod) is reached to which a plant responds by cessation of growth and development of increased ability to tolerate temperatures slightly below freezing. Thus begins the first stage of cold acclimation. The critical photo-period is genetically determined for each plant and is related to the latitude of the plant's native area of origin. In general, woody plants from northern latitudes or high elevations cease growth at longer day-lengths, which means they start acclimating earlier in the summer than woody plants from more southern latitudes which respond to shorter daylengths and therefore cease growth later in the year.

The second stage of cold acclimation is induced by decreasing temperatures and often triggered by frost. This second stage is characterized by rapid acclimation which enables plants to tolerate low winter temperatures. Again, genetic control is evident as we see that the temperature tolerated is specific for each plant. Generally, plants native to cold climates are capable of tolerating lower temperatures than plants native to warmer areas.

Cold deacclimation may be another factor influencing the cold hardiness of azaleas. Deacclimation occurs if plants are exposed to warm temperatures. If deacclimation occurs before the danger of low temperature is past, injury to the plant may result if the plant experiences low temperatures.

Results from cold hardiness research done in our laboratory, indicate that azalea buds deacclimate rapidly at room temperature and can lose several degrees of hardiness in as short a time as one hour. At present it is unknown how long azalea buds can be exposed to warm temperatures and to what extent they can deacclimate before they lose the ability to reacclimate when exposed to progressively lower temperatures. Such ability to reacclimate may be important in order to survive extreme low temperatures in late winter or early spring.

Cold hardiness also varies in different tissues within an individual plant. In most woody plants, root tissue is the least cold hardy. However, since root tissue is protected by the latent heat of the earth, it is not often exposed to killing temperatures. Of most concern in deciduous azaleas is the cold hardiness of the florets, since they are the least cold hardy exposed plant tissue and because floral display is the primary reason for growing azaleas in the landscape.

Breeding for cold hardiness entails attention to cold acclimation, maximum cold temperature tolerance once acclimation has occurred, and cold deacclination particularly as these influence survival of flower buds, which is the limiting plant tissue. All of these factors combine to determine the characteristic we call "cold hardiness".

Breeding Methods
The first step for our breeding program is to identify desirable parental material. At least some of the material has to possess the ability to contribute cold hardiness capability to its offspring. Most of the cold hardy material was selected from the more northern part of the species range or from hardier strains. Some of the plants identified as cold hardy include individuals from the following:

Rhododendron prinophyllum
Rhododendron vaseyi
Rhododendron canadense
Rhododendron viscosum
Rhododendron arborescens
Rhododendron x kosterianum

There are other plants which are seedling azaleas of unknown parentage that we use as hardy parents (not listed here) in the breeding program.

Other materials included in the breeding program exhibit some cold hardiness along with other desirable characteristics such as flower color, flower size, or floriferousness. These included:

Knaphill hybrids
Exbury hybrids
Rhododendron calendulaceum
Rhododendron japonicum
Rhododendron luteum

Some of the progeny from the breeding program are F ₁ 's from crosses made between and within these two groups. Others are a further mix of many hardy species combined with one or more Exbury-Knaphill cultivars.

We have screened F ₂ populations for segregation that may include hardiness with desirable flower characteristics. By computerization of our plant breeding records we are now able to examine all crosses made with individual plants over the last eight years. As a result, we can identify compatible crosses and the progeny characteristics of individuals used as parents in the breeding program. Organized in this fashion we are able to draw conclusions as to the desirability of particular plants as parents, the inheritance of specific traits, and incompatibility problems.

Seed from the breeding program is collected in October, and sown in the greenhouse in January or February. The seeds are sown on the surface of moist sphagnum peat moss and covered with glass to keep humidity high. The seedlings are transplanted to 2¼ inch peat pots and grown in the greenhouse until June when they are planted outside into a shade house. Small seedlings grow here for two years, but only in the first year do they have shade, as the shade cloth is removed at the end of the first year. The plants are fertilized with ammonium sulfate twice a year and watered as needed. Two years later the plants are 12-30 inches tall and ready to go into the field.

Our natural field soil is a Hayden Clay Loam, has a pH of approximately 7.0 and is heavy. The azalea beds have been modified with 10+ inches of manure, 0.5 lb. P ₂ O ₅ per 100 sq. ft., and 15 lbs. sulfur per 100 sq. ft. The manure adds organic matter and the sulfur lowers the pH to around 5.0. The azaleas are heavily mulched with woodchips and/or stable bedding. There is overhead irrigation. As the azalea beds are very labor intensive to weed, we are presently testing DuPont Landscape Fabric on some of our azalea beds for weed prevention. The other beds receive a fall application of Devrinol which works very well on annual weeds. Roundup is used to spot treat thistles and perennial grassy weeds.

During the next few years these seedling azaleas are screened in the field for flower bud cold hardiness, foliage characteristics, form, mildew resistance, etc. Every year many are discarded because of inadequacies in any one characteristic trait. Superior plants are selected and numbered. These numbered plants are propagated and subjected to a laboratory cold hardiness test. This laboratory test confirms the flower bud hardiness of the selection and determines the lowest temperature the flower buds can withstand.

For our laboratory cold hardiness testing, azalea flower buds are collected in mid-winter so as to be at their maximum cold hardiness capability. The flower buds are exposed to slowly decreasing temperatures in a freezer which is programmed for specific rate of temperature drop. At selected temperatures, samples are removed and evaluated for cold injury. The temperature at which 50% of the florets are killed is considered the lethal temperature. The lowest temperature which results in less than 50% dead florets within the sample is considered the cold hardiness capability of that azalea selection.

The numbered selections are further observed and evaluated under field conditions, until a decision to discard or introduce can be made. Field evaluation to supplement our lab testing takes many years because Minnesota winters can be variable. We will only introduce azaleas that have proven to bloom reliably in our climate.

Azalea Introductions
To date, we have been able to develop azaleas in pink, gold, orange and white colors which are of adequate hardiness for Minnesota. This series of hardy azaleas is called the Northern Lights Azalea Series.

1.  Our first introduction in 1978 was the result of controlled crosses of selected strains of R. prinophyllum and R. x kosterianum from the University of Minnesota Landscape Arboretum. The resulting seedlings are sold under the name Northern Lights F ₁ hybrids. These seedlings are quite uniform in size and form. Flower bud hardiness is consistent at -45° F. and flower color ranges from light pink to dark pink. Most of the F ₁ hybrids are sterile.

Since this first introduction we have introduced other selections of differing parentage, all of which are clonally propagated. The name Northern Lights is also used to refer to the entire series of azaleas introduced by the University of Minnesota Landscape Arboretum. Named clonal selections from this series are described below:

2.  'Pink Lights' azalea is a clonal selection from Rhododendron x kosterianum x R. prinophyllum . It is an extremely floriferous plant with mature height and spread of 6 to 8 ft. Flowers are pale pink with a darker pink blotch. The flower buds are hardy in mid-winter to -45° F. with no injury. 'Pink Lights' is a 1983 introduction.

3.  'Rosy Lights' azalea is a dark pink clonal selection from R. x kosterianum x R. prinophyllum . It also has a mature height and spread of 6 to 8 ft. 'Rosy Lights' is flower bud hardy to -45° F. Flowers are sterile. 'Rosy Lights' is a 1983 introduction.

4.  'White Lights' azalea is a clonal selection from seedlings having R. prinophyllum and some White Exbury in their background. Flower buds of 'White Lights' are pale delicate pink in the balloon stage. Upon initial opening, the flowers have a faint pink tinge which fades at full bloom to give virtually a white appearance in the landscape. Flower buds are hardy to -35° F. 'White Lights' has a mature height and spread of 4 to 5 ft. The flowers of 'White Lights' are sterile. It is a 1983 introduction.

5.  'Spicy Lights' azalea is a salmon-colored clonal selection from open pollinated Rhododendron prinophyllum . It matures at 6 ft. in height and 8 ft. in spread. In late May or early June 'Spicy Lights' produces a floral display of 1½" salmon-colored flowers having a slight fragrance. Flower buds can withstand -35° F. without injury. 'Spicy Lights' is sterile and produces no seed pods. It is a 1985 introduction.

6.  'Orchid Lights' azalea is a 1986 introduction. It is selected from a cross of Rhododendron canadense x R. x kosterianum . It has smaller orchid-colored flowers, blooming mid-May, on a compact dense shrub reaching 3 to 4 ft. in height and spread. Flower shape is unique as it is intermediate between the two parents. 'Orchid Lights' is extremely hardy and flower buds can withstand -45° F. without injury. 'Orchid Lights' is sterile and sets no seed pods.

7.  'Golden Lights' azalea will be a 1987 introduction. It is a clonal selection from seedlings having Rhododendron atlanticum and Exbury azaleas in their background. The plant matures at 5-6 ft. in height and spread and produces a floral display of golden-colored flowers in late May. Flower buds are hardy to -35° F. 'Golden Lights' flowers are fertile.

Breeding azaleas for cold hardiness is a long-term effort, with many years of testing needed to develop cultivars hardy for northern climates. The breeding program at the University of Minnesota Landscape Arboretum has as one of its goals the development of a series of azaleas in the full color range, for our area and other areas with similar climate. The Northern Lights Azalea Series is being developed in response to these goals. We wish to take this opportunity to thank the American Rhododendron Society for making available some of the necessary information, personal contacts and monetary support to help us reach our azalea breeding goals.

Literature Cited
1.  Graham, P.R. and R. Mullin. 1976. "A Study of Flower Bud Hardiness in Azaleas." J. Amer. Soc. Hort. Sci. Vol. (1):7-10.
2.  Weiser, C.J. 1970. "Cold Resistance and Injury in Woody Plants." Science 169:1269-1278.

ARS research foundation grants help support the azalea breeding project.