Rhododendrons are the most impressive flowering shrub in the Northeast and the Northwest, while the most stunning flowering shrub of the Southeast, Gulf Coast and Southern California is the evergreen azalea (neither survive well in the central U.S.). Both are members of the genus Rhododendron, and have very similar blossoms. The basic difference is that azalea flowers have five pollen-bearing stamens while rhododendrons have 10 or more.
Most of the rhododendrons and azaleas grown in gardens are hybrids, and their ability to resist cold differs remarkably from one variety to another. For this reason it is best to purchase rhododendrons and azaleas from a local nurseryman who grows his own plants in open fields. The descriptions below cover rhododendrons that have proved satisfactory in fairly broad regions.
The hybrid rhododendrons are immensely varied in color, size, shape and winter hardiness as a result of their complex genetic background. Generally they are classified as large-leaved, with foliage 3 inches or more in length, and small-leaved, with foliage under 3 inches. The varieties with larger leaves usually have larger flowers, 5 inches or more across, composed of blossoms 2 inches or more in diameter. The individual blossoms of small-leaved types vary from tiny flowers to ones 1 1/2 to 2 inches across, borne in clusters up to 4 inches across.
The cold resistance of a hybrid rhododendron is indicated by a code that indicates the lowest temperature the flower buds can tolerate during the winter and still open perfectly in the spring. Plants bearing the code designation H-1 survive to -25°F, H-2 to -15°F, H-3 to -5°F, H-4 to 5°F, H-5 to 15°F, H-6 to 25°F and H-7 to 32°F. Most varieties grown in the USA range between H-1 and H-4 in hardiness.
The Royal Horticultural Society is the official rhododendron registrar. As of the end of December, 2002, the number of registered rhododendron hybrids was:
Of these, several hundred are readily available in the nursery trade and perhaps another hundred are in the greenhouse azalea trade. The most popular plants vary by locale. Deciduous azaleas are popular in both hot and cold climates. Evergreen azaleas are more popular in warm climates and rhododendrons do well in cooler climates. Vireyas only do well in tropical climates or in greenhouses. [Return to Top]
R. catawbiense was collected in the wild in 1803 in the Eastern US and was introduced to Britain in 1809. It was hybridized with other rhododendrons introduced cold and heat hardiness in the rhododendron hybrids. The first group of rhododendrons to become popular in gardens was the hybrids called the "iron-clads." There were a group of early British hybrids involving crosses of R. caucasicum, R. catawbiense, R. ponticum and R. maximum that survived the coldest winters at Arnold Arboretum in Boston. In 1917, Ernest Henry Wilson (1876-1930) published a list of what he termed were the "Iron Clad" rhododendrons, meaning those which had been planted in the Arnold Arboretum and were able to survive many winters successfully. This group included:
Ten years later he dropped ´H. W. Sargent´ and ´Caractacus´ from this list and merely talked about the "Dozen Iron Clads". Today, this group of a dozen R. catawbiense hybrids is as good as it ever was, even though there must be several thousand evergreen rhododendrons being grown, many of them in the United States. Some have performed well for half a century, others are new and little is known about the limits of their hardiness. Some can be grown only in the South, others in the Pacific Northwest but it is interesting to note that some of the species and varieties have been growing in New England at the Arnold Arboretum since 1891 and the original plants are still alive! [Return to Top]
Iron-clads are by definition plants that have been popular for a long time and are easy to grow, even in cold climates. Some of the original "Wilson's Dozen" plants are no longer easily found and have be replaced by the modern list of "iron-clads":
* indicates that it is among the most cold hardy rhododendrons. [Return to Top]
In the hardiest (H-1) group, which is hardy to Zone 4, with flower buds that tolerate -25°F in winter, recommended varieties are 'Boule de Neige', white; 'Pink Cameo', pink with yellow blotch; 'Prize', pink with brown blotch; and 'Tony', cherry red (all 2 to 3 feet tall after 10 years); 'America', bright red; 'King Tut', light and deep pink; 'Pinnacle', pink with citron blotch; and 'The General', crimson with dark blotch (all 3 to 5 feet tall); 'Lavender Queen', pale lavender, and 'Spring Dawn', rosy pink with yellow blotch (both 5 or more feet tall).
Recommended in the H-2 group, which is hardy to Zone 5, with flower buds that tolerate -15°F in winter, are 'Cheer', shell pink, and 'Spring Glory', light pink with crimson blotch (both 2 to 3 feet tall); 'Blue Peter', violet with dark blotch, and 'Rocket', coral pink with scarlet blotch (both 3 to 5 feet tall), and 'Belle Heller', white with golden blotch (5 or more feet tall).
Choice varieties in the H-3 group, hardy to Zone 6, with flower buds that tolerate -5°F in winter, are 'A. Bedford', pale mauve with dark blotch, 5 or more feet tall; 'Anna Rose Whitney', deep pink; 'Britannia', crimson; 'Crest', primrose yellow; 'Gomer Waterer', pink buds open white; 'Jean Marie de Montague', bright scarlet; 'Mrs. Furnival', pink with maroon blotch; 'Purple Splendor', royal purple with black blotch; and 'Scintillation', light pink (all 3 to 5 feet tall).
Recommended varieties in the H-4 group, hardy to Zone 7, with flower buds that tolerate +5°F in winter, are 'Loder's White', white, 3 to 5 feet tall; 'King George', pale pink flower clusters that turn white as they mature, grows 6 feet tall but may reach a height of 12 feet. [Return to Top]
Best Small Leaved (Lepidote) RhododendronsThe small-leaved, small-flowered hybrid rhododendrons rarely exceed 3 feet in height, and most grow 2 feet tall. They are useful next to foundations, in rock gardens and in front of taller rhododendrons.
Standing out in the H-1 group, which is hardy to Zone 4, with flower buds that tolerate -25°F in winter, is 'P.J.M.' P.J.M. is reported to be the hardiest rhododendron in cultivation. It is cold hardy and tolerant of heat and sun. It has one drawback. Its early bright lavender-pink blooms are so early that sometimes the blooms are killed by an early frost. The glossy green foliage of summer becomes mahogany-colored in winter. It is an excellent landscape plant, will grow under most conditions, and the flowers are long lasting. The flowers are somewhat smaller than most other rhododendrons. There are newer varieties of P.J.M.:
P.J.M. 'Elite' with improved flower, P.J.M. 'Regal' with improved foliage, and P.J.M. 'White' which starts out pinkish and fades to white.
Recommended varieties in the H-2 category, hardy to Zone 5, with flower buds that tolerate -15°F in winter, are 'Blue Diamond', lavender blue; 'Conestoga', pink; 'Conewago', rose magenta; 'Laetevirens', also known as Wilson rhododendron, rose pink; 'Purple Gem', deep purple; 'Ramapo', lavender purple; and 'Wyanokie', white.
Choice H-3 varieties, hardy to Zone 6, with flower buds that tolerate -5°F in winter, are 'Bow Bells', bright pink; 'Carmen', dark red; and 'Moonstone', yellow.
A recommended variety in the H-4 group, hardy to Zone 7, with flower buds that tolerate -+5°F in winter, is 'Snowlady', grows to a height of only 30 inches, produces an abundance of snowy white flowers and has fuzzy green leaves. [Return to Top]
If you understand the basic ideas of genus, subgenus, section and species you will have a better understanding of rhododendron naming conventions. A rhododendron species can be uniquely identified with the genus and species. Hence Rhododendron albiflorum defines a unique species. There may be another genus that uses the name albiflorum since it is Latin for "white flowering".
Species are naturally occurring plants that will reproduce true to their parents from seeds. A hybrid is the product of a cross with a pollen parent and seed parent which are two different plants, either species or hybrids or a combination of a hybrid and a species. This is confusing since there is some variation within each species. There are also naturally occurring hybrids. Hybrids all have one trait in common; they will not reproduce true to the parent from seed.
Hybrids are called varieties and identified by cultivar, hybrid and clone.
All garden plants are commonly called varieties, but the botanical definition is more precise.
When a species is selected from the wild and cultivated it is called a variety. Such a variety is a naturally occurring variation of a species, expressed as the abbreviation var, as in Rhododendron forrestii var tumescens. When cultivated it may also be known as a selected form.
Cultivar or cultivated variety refers to plants, occurring either naturally or in cultivation, that are not capable of reproducing true to the original plant naturally and which must be perpetuated by vegetative propagation.
Clones are vegetative replicas of the original cultivar or hybrid.
A rhododendron like 'Jingle Bells' is a cultivar that was produced by crossing two hybrids. It can be perpetuated by vegetative reproduction only. When written, cultivar names are contained within single quotation marks and are not italicized. Also, in order to avoid confusion with species, such names should not be in Latin. Until the naming rules were standardized in 1958 there were many cultivars with Latin names; these still stand but new cultivars must have modern language names.
By following our cultivar example, 'Jingle Bells', back to its species parents we can learn a lot about its behavior and characteristics. This is especially important when hybridizing because it provides valuable clues about the potential results of crosses.
| ----R. dichroanthum ssp dichroanthum | ||
| ------'Fabia'------ | | | |
| | | ----R. griersonianum | |
| 'Jingle Bells'----| | ||
| |
----R. campylocarpum ssp campylocarpum | |
| --'Ole Olson'---- | | | |
| ----R. fortunei ssp discolor |
This parentage provides numerous hints about the nature of the plant. A 'Fabia' background suggests orange flowers (by way of R. dichroanthum) and narrow pointed leaves (from R. griersonianum), The other parent, 'Ole Olson', may provide the touch of yellow (from R, campylocarpum) and may have led to more rounded foliage (through R. fortunei). But Halfdan Lem, the hybridizer, also posed interesting questions with this parentage: how would the large round leaves of R. fortunei combine with the long narrow leaves of R. griersonianum; and how would the dominant apricot-orange color of R. dichroanthum be affected by the pink R. fortunei?
The end result is a plant that shows little R. fortunei influence, but which clearly states its R. griersonianum background. 'Jingle Bells' is very much an improved 'Fabia', with brighter colors, bigger flower trusses and healthier foliage. So although the influence of the 'Ole Olsen' parentage is not immediately apparent, a look into the background of 'Jingle Bells' soon reveals where the improvements over 'Fabia' came from. [Return to Top]
All rhododendrons and azaleas will grow well in light shade; the Carolina rhododendrons will bloom more abundantly in full sun if the soil is kept moist, but in hot climates or in windy places shade is usually mandatory. Balled-and-burlaped plants may be transplanted in blossom but it is better to transplant them early in spring in areas where their hardiness is questionable, and in spring or fall where there is no likelihood of winter damage.
The most important factor in achieving vigorous growth is an acid soil mixture high in organic content. Many commercial growers set rhododendrons and azaleas in pure peat moss, or in a 50-50 mixture of peat moss and coarse sand or perlite. A favorite mixture on the West Coast is 1/2 peat moss and 1/2 ground redwood, but in such mixtures, plants must be fed regularly.
Because the roots grow near the surface, a bed prepared especially for rhododendrons and azaleas need not be more than 12 inches deep; deep planting keeps the roots from getting the air they need. In fact, it is a good idea to set them about 1 inch higher than they grew at the nursery.
Cultivating the soil around rhododendrons and azaleas would damage their roots. Instead, keep the roots cool and moist with a permanent 2- to 3-inch mulch of wood chips, oak leaves, chunky peat moss or other light organic material. Plants that have been given a soil mixture rich in organic matter probably will not need feeding for several years. Do not stimulate fast growth because it produces long weak stems and few flowers. But if a plant seems weak or sickly, use cottonseed meal or a special rhododendron-azalea-camellia fertilizer, dusted on the soil early in the spring. For maximum flower production, pinch off faded flowers or the seed capsules that follow.
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Best Large Leaf (Elepidote) Rhododendrons