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Extension > Yard and Garden News > Exploring the Potential for Large Leaved Rhododendrons for Minnesota

Exploring the Potential for Large Leaved Rhododendrons for Minnesota

Laci High, University of Minnesota Graduate Student

PJM Rhododendron is well-adapted in Minnesota. David Zlesak

After a long, dreary Minnesotan winter, gardeners anticipate and appreciate the beauty of spring flowering shrubs such as rhododendrons and azaleas (genus Rhododendron) which can be the first sign of color in many landscapes.Due to its great soil adaptability and ease of cultivation compared to other members of the genus, PJM hybrids can be found along most residential streets in Minnesota.  These plants are loaded with lavender-pink flowers nestled among inconspicuous, scaly leaves.  Even though PJM hybrids have proven to be reliable performers for home gardeners, they lack the color range and glamorous trusses of broad-leafed forms typically found in more moderate climates.

Although distinctions between subgroups based on most observable traits hold many exceptions, the main difference between azaleas and rhododendrons is that azaleas generally have five stamens while rhododendrons develop ten. The subgroup rhododendron is further classified into lepidote (with scales on leaf surface) or elepidote (without scales); the latter often possesses larger leaves and flower trusses than the former as well.

Elepidote rhododendron 'St. Michel'. Laci High

Several deciduous azaleas and lepidote rhododendrons are well-suited for northern climates, but very few elepidote rhododendrons thrive in our region.  For those who have hiked the Appalachian Trail in the eastern U.S. or visited the Pacific Northwest in the springtime, you have likely encountered hillsides cloaked with elegant specimens of these broad-leafed rhododendrons.  The large, evergreen leaves may be valued by some avid gardeners, but these plants are mostly identifiable by their immense, terminal flower trusses and a felt-like covering on leaf surfaces termed indumentum.

The use of these elepidotes in Minnesota is primarily limited by their cold hardiness.  Traditionally, 'ironclad' cultivars such as 'Roseum Elegans', 'Nova Zembla', and 'Catawbiense Album' have continually survived our harsh winters.  The name 'ironclad' was given to these cultivars derived from the North American species R. catawbiense and have been shown to survive -25°F.  Although these plants possess satisfactory cold hardiness, their leggy habit leaves much to be desired in formal landscape plantings. 

In the 1970s, plant breeders at the University of Helsinki in Finland sought to develop extremely cold-hardy rhododendrons suitable for the Scandinavian climate.  They chose R. brachycarpum ssp. tigerstedtii, native to the Kongo-San mountain range in North Korea, as the primary seed parent.  This subspecies has been cited as the most cold-hardy rhododendron in existence (Tigerstedt and Uosukainen, 1996).  Crosses with 23 species and 48 hybrids yielded approximately 22,000 seedlings that were subjected to cold hardiness trials and evaluated for other valuable traits.  Finland experienced two bottleneck winters in the 1980s in which temperatures plummeted to -37°F and resulted in 70% of seedlings dying (Tigerstedt and Uosukainen, 1996).  However, adequate variation existed within this extensive population such that surviving genotypes were key candidates for further selection. Seventeen Finnish cultivars have been introduced and collectively are considered the most cold-hardy, broad-leaved rhododendrons available. 

It didn't take long for these hybrids to enter the North American distribution chain and are currently sold by many nurseries and garden centers in the northern states.  Growers and landscapers not only value cold-hardiness, but the Finnish hybrid collection offers a wide range of flower colors, glossy foliage, and several such as 'Haaga' have a compact habit.  However, when grown at latitudes south of Finland, such as those in Minnesota, vegetative growth below the terminal flower bud often occurs simultaneously with flowering such that the floral display becomes obscured; this vegetative bypassing effectively reduces the plant's ornamental value.  A few of the more popular Finnish hybrids can be found in local nurseries and several are on display at the Minnesota Landscape Arboretum's Rhododendron Display Garden.  For those of you anxious to establish broad-leafed rhododendrons in your garden that will succeed in our harsh climate regardless if flowers may become hidden, I recommend giving these a try (Table 1).

Table 1.  Finnish elepidote rhododendrons frequently found in the U.S. market.  Cultivar information provided by Kristian Theqvist, an amateur rhododendron breeder who has continued to build on the initial work performed at the University of Helsinki.


Cultivar

Cold Hardiness

Flower Color

Habit (height in 10 years)

Comments

'Haaga'

-33°F

Magenta pink with dark red spots

Compact (4-5 ft.)

Reliable bloomer

'Hellikki'

-29°F

Purple-red

Upright (4 ft.)

Prefers full shade

'Helsinki University'

-38°F

Light pink with dark red spots

Upright (5 ft.)

Reliable bloomer

'Mikkeli'

('St. Michel')

-35°F

Very light pink with green spots

Upright (6 ft.)

New growth covered with white wooly indumentum

'P.M.A. Tigerstedt' ('Peter Tigerstedt')

-33°F

White with dark red spots

Upright and spreading (4-5 ft.)

Slightly gangly in full shade

Bypass shoot development has been previously noted in R. brachycarpum ssp. tigerstedtii-derived cultivars grown in North America (Cox and Cox, 1997), and this adverse condition is worse at latitudes south of Finland.  This occurrence is likely due to differences in environmental factors such as photoperiod and temperature between Finland and Minnesota.  Variable responses to factors influencing flowering and dormancy in evergreen azaleas have been noted previously between American and European cultivars (Ballantyne, 1960; Jorgensen, 1970; Criley, 1985).  Since many other species and related cultivars don't exhibit a latitudinal gradient with respect to simultaneous vegetative growth and flowering, genetic variation must also exist in response to these factors.

Our research will manipulate photoperiod, temperature, and length of cooling period and then analyze how these factors affect timing of reproductive and vegetative budbreak as well as occurrence of bypassing.  Not only will this work aid our rhododendron breeding program, but conclusions may also facilitate research with other crops pertaining to apical dominance (inhibition of lateral bud growth by the terminal bud of a shoot).

The ultimate aim of our research will be to introduce extremely cold-hardy, broad-leafed rhododendrons that break vegetative growth only after the floral display is complete.  Starting our breeding efforts with select Finnish cultivars that already possess many redeeming qualities, we will perform crosses with 'ironclad' cultivars that typically do not show vegetative bypassing.  We anticipate that our breeding efforts will lead to several new hybrids that will not only thrive in cold, temperate climates, but also increase the diversity of broad-leafed rhododendrons on the market.  Minnesotan gardeners will then be able to enjoy the beauty of these ornamental shrubs that have previously been hallmarks in only the most moderate climates.

References

Ballantyne, D.J.  1960.  Growth regulators and the flowering of evergreen azaleas (Rhododendron cv.). (Doctoral dissertation, University of Maryland, College Park).

Cox, P.A. and K.N.E. Cox.  1997. The encyclopedia of Rhododendron species. Glendoick Publishing, Perth.

Criley, R.A. 1985.  Rhododendrons and azaleas, p. 180-197.  In:  A.H. Halevy, (ed.).  CRC handbook of flowering, (v. 4).  CRC Press, Boca Raton, Florida.

Jorgensen, S.  1970.  Azalea production.  Pennsylvania flower grower's bulletin, 232:  5-7.

Tigerstedt, P., and M. Uosukainen.  1996.  Breeding cold-hardy rhododendrons. American Rhododendron Society 50 (4):  185-189.

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