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, possess excellent cold hardiness and grow well in Iowa. Selection of cold tolerant 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 classified as "rhododendrons".
The following rhododendrons and azaleas possess good 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 4 to 6 feet. 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, Hopkinton, Massachusetts, and was named after its owner, Peter J. Mezitt.
Two other Weston hybrids, 'Aglo' and 'Olga Mezitt,' also do well in Iowa. 'Aglo' has light pink flowers with red throats. It blooms about 1 week 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.
'Ramapo' is a good dwarf rhododendron. It grows approximately 2 feet tall and 4 feet wide. Flowers are bright violet-pink. 'Ramapo' is hardy to -25 F.
Several varieties of Rhododendron catawbiense are other possibilities. These varieties have 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 southern and southeast Iowa.
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 member of this series is 'Northern Hi- lights.' It has creamy white flowers that are accented by a golden yellow fifth petal.
The mollis azaleas also do well in much of Iowa. Flower colors are variable, mainly shades of yellow, orange, and red. Flower buds are hardy to -20 to -25 F. The mature height and spread of the mollis azaleas is approximately 6 to 8 feet.
When planting rhododendrons and azaleas, proper site selection is extremely important. Azaleas prefer partial shade to full sun. Densely shaded areas are poor sites for azaleas. Rhododendrons grow best in partial shade. Areas that receive filtered sunlight throughout the day or morning sun and afternoon shade are good sites for rhododendrons. The evergreen rhododendrons often suffer extensive leaf burn during the winter months when grown in sunny areas and windy, exposed sites. The winter sun and northwest winds dry out the foliage, causing it to curl and turn brown.
Rhododendrons and azaleas, members of the Heath or Ericaceae family, require well-drained, acid soils with a pH of 4.0 to 5.5 for best growth. The pH of most Iowa soils ranges from 6.0 to 8.0. Home gardeners can lower their soil pH by adding Canadian sphagnum peat to the soil. (The pH of Canadian sphagnum peat generally ranges from 3.0 to 4.5.) When planting azaleas and rhododendrons, backfill with a mixture of 1/2 (moist) peat and 1/2 soil. Gardeners with wet, poorly drained soils should build raised beds or berms to insure good drainage.
Rhododendrons and azaleas have shallow, fibrous root systems. As a result, plants should be watered during periods of hot, dry weather. To conserve soil moisture, place 2 to 4 inches of shredded leaves, pine needles, or wood chips around each plant. The mulch also helps to control weeds and lower soil temperatures during hot weather.
If hardy varieties are selected and planted in good sites, no winter protection should be needed. Rhododendrons planted in exposed southern or western locations can be protected from sun and wind by erecting a burlap screen on their exposed sides in the fall. Rhododendron leaves will curl on extremely cold winter days. The leaves, however, return to normal as temperatures moderate. If rabbits are a problem in the area, place chicken wire fencing around the plants in the fall to prevent damage.
Rhododendrons and azaleas can be a challenge for Iowa gardeners. The results, however, can be spectacular. During bloom few shrubs can match the color and beauty of rhododendrons and azaleas.
This article originally appeared in the May 12, 1995 issue, pp. 1995 issue, pp. 63-64.