Rhododendrons and azaleas are spectacular flowering shrubs. Unfortunately, most rhododendrons and azaleas (there are over 900 species and innumerable varieties) cannot be successfully grown in Iowa because of a lack of cold hardiness. A small number, however, can tolerate our harsh winter weather. Selection of cold hardy varieties, correct placement in the landscape, and proper planting are the keys to successfully growing rhododendrons and azaleas in Iowa.
Botanically, all azaleas are actually rhododendrons. However, the deciduous types are commonly called "azaleas", while the evergreen species and varieties are referred to as "rhododendrons".
The following rhododendrons and azaleas possess excellent cold hardiness and are the best choices for Iowa landscapes.
The 'PJM' rhododendron grows well over much of the upper midwest. The small, evergreen foliage is dark green in summer, but turns to a maroon-purple in the fall. The flower buds (hardy to -35 F) produce bright lavender pink blossoms in mid to late April. The mature height and spread of the 'PJM' rhododendron is approximately 4 to 6 feet tall. Several selections of 'PJM' are available and include 'Black Satin' (winter foliage is shiny, coal black), 'Elite' (blossoms are slightly more pink), and 'Victor' (a compact, slow growing type). 'PJM' was introduced by the Weston Nursery in Hopkinton, Massachusetts and was named after its owner Peter J. Mezitt.
Two other Weston Nursery hybrids, 'Aglo' and 'Olga Mezitt,' also do well in Iowa. 'Aglo' has light pink flowers with dark pink throats. It blooms about 7 to 10 days after 'PJM.' The small green leaves turn to a bronze color in fall. 'Olga Mezitt' produces bright pink flowers on 4- to 5-foot-tall shrubs.
Introduced by the Briggs Nursery in Olympia, Washington in 1998, 'Northern Starburst' is the first release from the Genesis series of tetraploid rhododendrons. (A tetraploid has twice the usual number of chromosomes.) The new variety is considered an improved 'PJM'- type rhododendron. 'Northern Starburst' has larger flowers which are more pink than 'PJM.' In the fall, the dark green, glossy leaves turn to cinnamon, eventually darkening to purple-black.
Several varieties of Rhododendron catawbiense are other possibilities. These varieties have large, dark green leaves and 5- to 6-inch-diameter flower clusters. 'Album' (white flowers), 'English Roseum' (light rose), 'Nova Zembla' (red), and 'Roseum Elegans' (lavender-pink) are hardy to -25 F. Catawba rhododendrons require a protected site and do best in southeast Iowa.
An exciting new group of large-leaf rhododendrons are the Marjatta hybrids. Developed at the University of Helsinki in Finland, the Marjatta hybrid rhododendrons have broad evergreen leaves and large showy flowers. Their flower buds are hardy to -30 to -35 F. Several varieties have begun to appear in garden centers and mailorder nurseries. 'Haaga' has an upright growth habit and will grow up to 6 feet in height. It has dark green foliage and rosy pink flowers. 'Elviira' is a low growing, spreading plant with a mature height of 2 feet. Its flowers are bright red. Possessing a dense spreading habit, 'Hellikki' ultimately reaches a height of 5 to 6 feet. Flowers are dark violet-red. 'Mikkeli' grows 5 to 6 feet tall and produces white flowers with green flecks in the upper portion of the corolla. The flowers of 'Peter Tigerstedt' are white with violet flecks in the upper part of the corolla. It's an upright grower which reaches up to 6 feet in height. The Marjatta hybrid rhododendrons perform best in sites protected from winter sun and wind.
The Northern Lights series of azaleas is a group of hybrid azaleas developed and released by the University of Minnesota. All the varieties in this series have flower bud hardiness of -30 to -45 F and the word "lights" in their name. 'Northern Lights' was the initial introduction. Flowers range from light to dark pink. Two varieties which were selected from 'Northern Lights' are 'Pink Lights' and 'Rosy Lights.' Flower colors are light pink and deep rose pink, respectively. Other varieties in this series are 'White Lights,' 'Orchid Lights,' 'Spicy Lights,' and 'Golden Lights' with white, orchid, soft orange, and gold flowers. The newest members of this series are 'Northern Hi-lights,' 'Mandarin Lights,' and 'Lemon Lights.' 'Northern Hi-lights' has creamy white flowers. Each flower is accented with a bright yellow upper petal. 'Mandarin Lights' and 'Lemon Lights' have mandarin orange and light yellow flowers, respectively.
When planting rhododendrons and azaleas, proper site selection is extremely important. Azaleas prefer partial to full sun. Rhododendrons perform best in protected sites in partial shade. An area that receives morning sun and afternoon shade would be a good site for rhododendrons. Avoid windy, exposed sites. During the winter months, the cold, dry winds and sun can dry out the rhododendron foliage and cause extensive leaf burn.
Rhododendrons and azaleas require well-drained soils with a pH of 4.0 to 5.5. The pH of most garden soils in Iowa range from 6.0 to 8.0. An excellent way for home gardeners to lower their soil pH is to incorporate Canadian sphagnum peat into the soil. (The pH of Canadian sphagnum peat generally ranges from 3.0 to 4.5.) Placing rhododendrons and azaleas in a wet, poorly drained soil is a death sentence. They will invariably die. Gardeners with poorly drained sites should build berms or raised beds to insure good drainage.
Rhododendrons and azaleas are beautiful additions to the home landscape. The keys to successfully growing these attractive shrubs in Iowa are selection of cold hardy varieties, correct placement in the landscape, and proper planting.
This article originally appeared in the March 24, 2000 issue, pp. 23-25.