Hardy Dexter Rhododendrons
Jon M. Valigorsky, M.D.
This article reports cold climate success with many of the adaptable varieties of rhododendron developed by Mr. Charles O. Dexter. The inspiration for this article was a request made by Jonathan Leonard last fall for a list of Dexter hybrid bud and plant hardiness levels for his nursery catalogue. Jonathan is proprietor of Briarwood Gardens, a nursery devoted to the propagation and sale of Dexter hybrids located in Sandwich, Massachusetts. His nursery is located two miles from the former Charles O. Dexter estate, which is now the Heritage Plantation.
In the 1970's Heyman Howard brought home to Heritage the great majority of recognized Dexter hybrids. The Dexter display garden has flourished. As in 1980, the estate, its grounds, and gardens will be the focus of the 1990 ARS convention hosted by the Massachusetts Chapter.
In a Rosebay article in 1983, I reported my five year experience in growing rhododendrons in the Berkshires of western Massachusetts. I described ten hybrids suitable for cultivation in this area. Surprisingly, the list included a number of the Dexters. This article adds an additional six years' update as more Dexters were acquired and plants reached blooming age.
The maximum low temperatures for the past eleven years for this USDA Zone 5a/4b garden are as follows: 1978-79 -12°F, 1979-80 -18°F, 1980-81 -25°F, 1981-82 -17°F, 1982-83 -13°F, 1983-84 -31°F, 1984-85 -12°F, 1985-86 -12°F, 1986-87 -12°F, 1987-88 -25°F, 1988-89 -16°F. In eleven winters, none were warmer than -12°F with the coldest winter at -31°F in 1983-84. The lowest temperatures were reached in January and February, with the following two notable exceptions. On Christmas Day 1980 the temperature plunged to -25°F warming up later that day only to -12°F with high winds and a -60°F wind chill factor. The cold came comparatively early in the season which resulted in blasted flower buds in even the hardiest rhododendron. For example, 'Boule de Neige' completely blasted buds down to the snow line. This past year, 1988-89, the lowest temperature of -16°F was reached even earlier, on December 12th. In most years the Berkshires enjoy 80 to 110 inches of snow in a winter and rhododendrons, like all plants, benefit from this insulating snow cover. However, the other aspect of this past winter was the almost total absence of snow, the least amount of snow in more than 50 years in the Berkshires. The ground froze to a depth of five feet.
The effect of a snowless winter was remarkable. Most Dexters showed little or no desiccation injury in the form of leaf burn or brown leaves. In contrast, so called "iron clads" sustained extraordinary damage. 'Boule de Neige' exhibited some leaf burn, and again had approximately 20% blasted buds with most buds loosing pips. The A. Waterer hybrid, 'Catawbiense Album', sited in an ideal winter shaded northeastern exposure, betrayed its R. ponticum blood by sustaining leaf burn. A neighboring 'Nova Zembla' gave up the ghost, and 'Roseum Pink' almost completely defoliated. Other iron clads such as 'Mrs. C.S. Sargent' fared well, but really none of the plants wintered as well as the Dexters.
The Garden Site
The garden is on a northwest facing slope 200 to 500 feet from the shore of Lake Onota. Air drainage is good and diminishes the effect of both early and late frosts. In December the lake freezes to a depth sufficient to support cars and pickup trucks. A short stretch of deciduous woods is all that stands in the way of the northwest wind as it howls and careens across the lake and slams into the garden. Strategically placed evergreens provide wind breaks.
The glacial activity that formed the lake wrought havoc to the subsoil of the property. The topsoil is heavy with clay, and is supported by even heavier rocky clay subsoil which is crisscrossed with veins of dolomite. The soil pH is neutral. Even the pH of the forest soil is 6.0 to 6.5. Surprisingly, high bush blueberries and mountain laurel thrive in plantings 15-30 years old. The same could have been said of Pieris japonica plantings of similar age; however, these have steadily deteriorated in the past 10 and especially the past 6 years.
At this point I have probably strained the reader's credulity to the limit. How can Dexter rhododendrons survive winter temperatures such as these in a windy site, growing over limestone? And, by the way, some are exposed to full winter sun! The answer is and has to be proper site location, soil preparation, and mulching. There is nothing prescient here - as the plants bear witness to my many mistakes.
Some rhododendron beds have been excavated to depths of twelve to eighteen inches. Other sites have been elevated one to three feet above grade. Soil amendments have included sand, but that was a mistake since sand obtained locally is mostly crushed limestone. Peat moss, compost, leaf mulch, pine needles, and bark mulch have been mixed one to one with the natural soil and subsoil to prepare planting beds with an initial pH of 5.5 to 6.0. After one or two seasons pH usually increases to 6.5, a level apparently optimal for rhododendrons. Fertilization in the past was with Milorganite, no longer available in the area and now the new Scott's rhododendron fertilizer is used. Application is once a year in late fall or early spring. Supplemental foliage fertilization with liquid ½ strength Miracid is continued as needed to July first.
The principle objective in soil preparation is to permit maximum winter adaptability. As in other cool climates, the remaining three seasons, spring, winter, and fall, are quite ideal for rhododendron cultivation. The last frost in spring is usually May 10-15th, but occasionally is May 30th. The average growing period is short at 90 days. Summer temperatures rarely reach 90 degrees except for the summer of 1988 when 90 degrees was exceeded on 11 or 12 days. This caused extraordinary bud set. Cool temperatures preclude the appearance of Phytopthora root rot. Rhododendron borer generally does not occur and weevils are rare. Lacebug is a problem. Gardening in the Berkshires is made more comfortable by the absence of poison ivy and deer ticks carrying the dreaded Lyme disease.
The Lists: The Dexter Hybrids Rated By Bud Hardiness
I have divided the Dexter hybrids as rated by flower bud hardiness into five nominally spaced groups. The spacing is narrow since the temperature range span is only from -12°F to -25°F. Criteria for bud hardiness is the survival of the majority of the pips in a truss that appears full or almost completely so even though several pips may have blasted. At -31 °F even the hardiest rhododendron such as 'Catalgla' or 'Russell Harmon' loses pips. Comments after each listing are not meant to be the complete descriptions of these well known hybrids - rather, they are personal observations.
A. One Way Tickets
Bud hardy to -12°F
1. 'Dexter's Champagne'
2. 'Dorothy Russell'
5. 'Dexter's Giant Red'
7. 'Brown Eyes'
8. 'Mrs. W.R. Coe'
Except for 'Dexter's Champagne', 'Dorothy Russell', and 'Mrs. W.R. Coe', these plants all have one way tickets for destinations outside the Berkshires. At -12°F they bloomed four out of eleven years. That's not bad is it? With certain exceptions, this group has plant hardiness problems.
'Dexter's Champagne' may be hardier than -12°F because it bloomed this year with partial and a few full trusses. Champagne's trusses are a complex combination of pink and cream with a yellow throat. The plant attracts my garden's visitors like a magnet.
'Dorothy Russell' is recognized by its enormous, light red, speckled, multi-flower trusses. Despite the plant's gigantic root system, it burns badly in the winter. The growth of the bush also seems to be retarded when compared to specimens grown in milder climates.
| 'Dorothy Russell', 1987, note leaf burn
Photo by Jon M. Valigorsky
This year 'GiGi' bloomed with full trusses so it may be hardier than -12°F. The main problem with 'GiGi' had to do with its lack of adaptability to wide temperature changes. After two weeks of sixty degree weather in February 1984, the temperature plunged to -10°F in March. A five foot specimen which had been growing in the garden for three years was totally killed to the snow line. I know several other growers who have also had the same experience.
'Aronomink' had large rose red flowers and has lost pips at -12°F, although general plant hardiness is good.
'Dexter's Giant Red'
Giant red has huge trusses of red budded, pink and red flowers. Foliage burn is a problem and the plant does not reach its full potential in this climate.
| 'Tripoli' after -16°F in 1989, some leaf burn, partial truss
Photo by Jon M. Valigorsky
This hybrid has formed a perfectly symmetrical mounded habit five feet in height in nine years from a rooted cutting. It has high foliage density due to its great capacity to branch. There is light foliage burn after each winter.
'Brown Eyes' has been an inconsistent performer with only partial trusses at -12°F and almost full trusses at -16°F. The branch structure of the plant is insufficient to support the trusses completely. This year the entire bloom loaded plant flopped over in heavy rain.
'Mrs. W.R. Coe'
'Mrs. W.R. Coe' has one of the largest flowers and some of the largest leaves of any Dexter. In most winters a few branches are lost to bark split.
B. The Solid H-2s
Bud hardy to-15°F
2. 'Glenda Farrell'
3. 'Ross L. Dexter'
4. 'Dexter's Brandygreen'
5. 'Powder Puff
'Accomplishment' possesses one of the most beautiful foliage colors - a glossy deep blue green with lighter blue new growth which is set off by red stem bracts. The blossoms of this plant are a red bi-color and are freckled on all lobes. My specimen, obtained in 1981, had been mislabeled as Accomac. It bloomed almost perfectly this year after -16°F.
I have tried growing most of the Dexter red rhododendrons in the Berkshires, with the exception of 'Avondale', 'Dexter's Red Velvet', and 'Winning Ways'. 'Glenda Farrell' is the most bud hardy of the Dexter reds; to -17°F. The plant; however, is less hardy and damage caused by the coldest winters includes leaf burn and bark split with branch loss.
'Ross L. Dexter'
'Ross L. Dexter' has one of the most lovely globular shaped trusses. The pink, maroon speckled, flowers are well presented on a rosette of large blue leaves. 'Ross L. Dexter' initially appeared to be hardy to about -12°F, but this year it bloomed after -16°F. The plant needs both wind and sun protection.
| 'Ross L. Dexter'
Photo by Astrida Valigorsky
'Dexter's Brandygreen' bloomed well this year after a -16°F winter, proving it was hardier than originally thought. Brandygreen has pink flowers which eventually turn to a buff color. It has the longest blooming period of any of the Dexters: two-and-a-half to three weeks. The plant also receives the first prize for rapidly outgrowing its planting site. Branch growth is fifteen to eighteen inches a year and mammoth plants can be found on Cape Cod. Brandygreen is, of course, most prized for its large "brandygreen" leaves.
'Powder Puff' is perhaps hardier than the -15°F that it is rated. The blossom is pink upon opening and later turns lavender.
C. The Classic Dexters
Bud hardy to -17°F
3. 'Parker's Pink'
4. 'Ben Moseley'
5. 'Sandwich Appleblossom'
'Scintillation' was the first Dexter that I attempted to grow in the garden. It was obtained through Weston Nurseries. There, the yardman who sold the plant to me predicted that it wouldn't survive in the Berkshires, and for the first couple of years, I thought he might be right. The principal problem was foliage desiccation due to wind. As the garden has matured, this problem has been eliminated. In addition, 'Scintillation' prefers to be sited in complete winter shade.
| 'Gloxineum' after -25°F, 1988
Photo by Jon M. Valigorsky
A plant that combines some of the largest flowers of the East Coast hybrids with outstanding foliage, 'Gloxineum' flowers average four-and-a-half inches in diameter and are exotically speckled on all lobes. In full sun it develops large, rugose, downward curving leaves; whereas, in shade the leaves are flatter and even larger. Its habit is quite upright and non-branching, as a young plant. It has stout upright limbs able to tolerate heavy snow loads. As it grows older, the plant begins to branch well and broaden out. When this happened to my oldest plant, it had to be moved. At twelve years of age, it had a root ball which exceeded five hundred pounds in weight and which had grown to a depth of sixteen or twenty inches. It was dragged across the lawn to a harsh southwestern exposure under a 150 year old sugar maple. The plant's location gave it almost no protection from the northwest wind, but it has wintered well at this site, A little over a year later, on October 4, 1987, ten inches of wet snow fell in the Berkshires. Several limbs of the sugar maple gave way and landed on 'Gloxineum'. Amazingly, none of the rhododendron's branches broke, and when the maple branches were removed it sprang back into shape. This experience was repeated all over the garden and attests to the great flexibility of elepidote wood. 'Gloxineum' crossed with 'Catalgla' has resulted in robust seedlings with large and numerous pink and white flowers which are hardy to -20°F. These seedlings are still under evaluation.
'Parker's Pink' is almost a perfect H-2 hybrid and is hardy to -17°F, although it has been listed as hardy to -25°F in some catalogues. A leading attribute is excellent flowering and bush density in moderate shade.
One of the best Dexters and widely recognized to be hardy to -15°F.
'Sandwich Appleblossom' has one of the most beautiful, pink and white ruffled, trusses among eastern rhododendrons. A leading attribute of the hybrid is the length of bloom; it can last up to two and a half weeks. 'Sandwich Appleblossom' does require wind protection. In the past there has been a great deal of confusion surrounding the name and identity of this plant. Many will remember this plant as Dexter's Pink from the cover of Hybrids and Hybridizers. Finally - and rather appropriately - Jonathan Leonard registered the plant as 'Sandwich Appleblossom'.
D. The Almost Iron Clad
Bud hardy to -20°F
1. 'Merley Cream'
2. 'Dexter's Victoria'
3. 'Newburyport Belle'
A hybrid which is widely regarded to be hardy to about -20°F. The flowers of the plant are a rare cream white, and the plant may be a useful parent when hybridizing yellow.
Dexter's Victoria' is characterized by rather small, frilled, lavender colored, flowers and dense foliage.
A tough, vigorous plant that probably is hardier than -20°F. It bloomed fully after -25°F in 1988 and may have its place with the iron clads. The leaves of the plant are simply gorgeous, with a depressed central vein. 'Newburyport,' like 'Wyandanch Pink', may also be a R. decorum hybrid.
E. The Iron Clad Dexters
Bud hardy to -25°F
1. 'Wyandanch Pink'
3. 'Dexter's Purple'
4. 'Great Eastern'
5. 'Bosley Dexter 1016'
'Wyandanch Pink' is the best of the hardiest. This is an extremely vigorous plant. In a milder climate such as Cape Cod it readily becomes a tree rhododendron putting on 15-18 inches of growth each year and develops the largest leaves of any eastern hybrid, sometimes reaching a foot in length. In this cold climate the leaves are smaller and growth is more restrained to 6-8 inches each year. The bush is compact and extremely dense. In the spring of 1988, I moved my larger 10 foot in diameter plant from an ideal shaded northeast facing site to a southwest facing site in full sun. It bloomed after -25°F and set buds in the torrid summer of 1988. Then it went through the snowless winter of 1988-89 with minimal leaf burn, bloomed again, and has set buds for next year. It comes through most winters with almost perfect foliage. My guess is that it is a R. decorum hybrid. Seedlings from a cross of 'Labar's White' with R. decorum 'McLaren' have foliage closely resembling Wyandanch and exhibit excellent foliage in full sun. Used in hybridizing,Wyandanch imparts great vigor to seedlings and is an obvious source of hardiness. There are only a few negative attributes: the plant does not bloom until the age of seven or eight, and branch growth sometimes obscures trusses.
A lavender pink rhododendron which blooms early in mid-season and is mildly fragrant. It is a shy bloomer with an open plant habit. These problems can be corrected by planting in full sun.
The flowers are light purple with a bronze blotch in perfectly spherical trusses. The plant has a spreading growth habit for the first few years and then begins to grow upward. Its foliage is medium green and it is both sun and wind tolerant.
| 'Dexter's Purple', 1989, planted on southern exposure
Photo by Jon M. Valigorsky
Great Eastern is one of the originally numbered Dexters, 109. A grower from the Finger Lakes reported the plant to be hardy to -20°F in a hardiness survey done by Hybrids and Hybridizers. From my own experience it appears to be satisfactory to -25°F with almost full trusses. The shiny foliage seems to be both wind and sun tolerant.
'Bosley Dexter 1016'
A light pink flowered Dexter with slightly glossy light to medium green foliage on a open growing bush. I obtained the two identical looking plants from two widely separated sources. One plant came from the late Dr. Schroeder of Indiana, who rated it as "quite heat tolerant."
Since my personal experience with Dexter rhododendrons is incomplete, I have only listed the plants that I have grown. Less bud hardy Dexters that I have evaluated include 'Ashes of Roses', 'Betty Hume', 'Burgundy Cherry', 'Dexter's Orange', 'Dexter's Springtime', 'Kelley', 'Sagamore Bayside', 'Tan', 'Tom Everitt', and 'Todmorden'. The only plants that I remember killing are 'Acclaim' and 'Adelphia'.
Like all cold climate rhododendron enthusiasts, I have been victimized by exaggerated hardiness ratings; therefore, it is far from my intent to contribute to this confusion. On the other hand, it becomes increasingly apparent to me that the hardiness ratings of Dexter rhododendrons have been too conservative. Except for the plants in group A, all other Dexters can be considered plant hardy to -25° or -31°F. A more objective way to test bud hardiness is in the laboratory, as reported by Harold Pellet and Susan Moe in their article "Flower Bud Hardiness of Rhododendron Taxa" for the fall 1986 edition of the Journal of the American Rhododendron Society. The two tested Dexter hybrids, 'Scintillation' and 'Weston', showed less than 50% pip death at -13°F, with some survival at -18°F. They also recorded the same temperatures for 'Boule de Neige'. Their laboratory experience closely parallels my own with the same cultivars. Yet, 'Boule de Neige' is still generally rated to -25°F hardiness. Hopefully more Dexters will be evaluated in the laboratory in the future.
In giving close attention to the proper site location, soil preparation, and mulching, many of the Dexter hybrids will not only equal, but surpass the performance of the classic iron clads.
Dr. Valiogorsky is especially interested in the performance of hardy rhododendrons.