QBARS - v30n4 Deciduous Azaleas at Winterthur

Deciduous Azaleas at Winterthur
Hal Bruce
Garden Taxonomist at Winterthur

Deciduous azaleas have never been as popular as their evergreen relatives among gardeners in the Middle Atlantic States, in spite of the large number of both species and hybrids available to the public, and in spite of the undeniable beauty of many of these. The reasons for this are not difficult to determine: The climate here enables us to grow a good many of the evergreen section, and these are, superficially at least, nearly ideal garden plants - compact in growth, brilliant in flower, and interesting throughout the year because they retain much of their foliage over winter.
With a few conspicuous exceptions, deciduous azaleas are mainly one-season plants, spectacular in bloom but lacking interest during the balance of the year. Nevertheless, interest in them seems to be on the rise, judging from the number of Exbury Hybrid seedlings one sees for sale in garden centers. One hopes that the Exbury craze will lead the amateur to experiment with some of the exceptions mentioned at the beginning of this paragraph - R. atlanticum , with its compact habit and excellent foliage, for example, or the very late-blooming R. prunifolium , or the Royal Azalea, R. schlippenbachii , spectacular in flower, summer leaf, and autumn color.
Deciduous azaleas in great variety have grown at Winterthur for many years, some plants by far antedating garden records. There are, for example, huge specimens of Ghent Hybrids with trunks as thick as a man's calf, as well as veritable thickets of R. nudiflorum which were probably part of the native woodland flora when the gardens were first designed. Three out of the four subseries of deciduous species are well-represented, only the rare, monotypic Nipponicum being absent.

I. Subseries Canadense
All four species of this small subseries grow (or have grown) at Winterthur. The Rhodora ( R. canadense ) is comparatively unattractive in its typical mauve form, but the white variant called albiflorum is a pretty little thing, with an abundance of small, white flowers whose segments are so deeply cleft that the cluster seems made of lace. It blooms early, about the last week of April here, and grows only three feet or so tall. Its one drawback is that it demands cooler summers than those in Delaware. Our plant grew well enough for some fifteen years but recently succumbed.
The other American member of the subseries, R. vaseyi , does well here. There are several large plants of the typical pink form near the Museum which are obviously quite old, possibly planted when that area was landscaped in the 'thirties. The pure white form called 'White Find' has grown here since the late 'fifties. This species rivals schlippenbachii , which it rather resembles, in showiness, with large, flat, butterfly-like blossoms around the first of May.
One of the rarest and most beautiful of all azaleas is R. pentaphyllum . It makes a large shrub with slender coppery-brown branches, at whose tips whorls of fresh green foliage poise elegantly. Here it blooms very early, close on the heels of mucronulatum , and its large pink flowers are very beautiful. The plants at Winterthur do well, though young plants are slow-growing and reluctant to bloom. One of its chief drawbacks is its rarity. Plants are difficult to find.
A note on the history of this species at Winterthur may be of some interest. The first plant came as "quinquefolium pink" in the late fifties from Henry Hohman of Kingsville Nursery. It was set out with the rest of the year's acquisitions in a test garden called the "old nursery," atop a wooded hill. There it grew quite unnoticed until one mild late March day in the early 'sixties, when I trudged through the snow of the day before in order to check on the condition of an early clone of R. mucronulatum growing there. "Quinquefolium pink" was by this time some three feet tall, with myriad slender, upright branches. As I walked into the nursery I saw that a large, bowl-shaped pink flower at the end of a long, curved pedicel swayed gracefully from the tip of each of these twigs. The effect against the snow was absolutely breath-taking.
The books quickly revealed the plant's true identity. Knowing that no one that day had been to the nursery before me (the snow on the road told me this), and knowing that H. F. du Pont liked to know what was blooming, especially in the way of new plants, on his grounds, I sent the flowering twig used for identification purposes to him via one of the footmen and went on about my business. An hour or so later one of the grounds crew caught up with me. Mr. du Pont wanted to know more about this fantastic new azalea and nobody could tell him anything. What was it? Where was it? Where did we get it, and could we get more? By the time I had answered the first three questions plans were already afoot to plant a large area next to the old Magnolia soulangeana at the head of the March Walk, both the azalea and the magnolias being similar in color and blooming concurrently. I have seldom seen more enthusiasm about a plant.
The project was fraught with difficulties, alas, from the beginning. Diligent search located only two plants besides ours on this side of the Atlantic. One, on the West Coast, was not for sale. The other, a six-foot specimen that had never bloomed, belonged to Joe Gable. It was not for sale either; he insisted on giving it to us! Maybe we could make it bloom, he said. Those who knew Joe Gable will recognize the generosity that was so typical of him in that act.
The rest of the story is brief. We ordered a number of plants from a foreign nursery. Walter Petroll managed to pull the majority of these pitiful, bare-rooted specimens through, but when they bloomed they proved to be R. reticulatum . Joe Gable's plant bloomed for us, and though it was not as fine in flower as our original plant, it did provide the cross-pollination apparently necessary for our first crop of seeds. Seedlings are so slow-growing, though, that the project slowly petered out. On one nurseryman's advice we tried substituting R. albrechtii , but it is no substitute. Finally the holes in the planting were filled with the lepidote hybrid 'Conewago.'
R. pentaphyllum has not lived up to the bright promise of our first encounter with it. It is rather fickle with its flowers, some years blooming abundantly, some sparsely, some years accompanying R. mucronulatum , some following it by two weeks. Perhaps it will settle down as the plants age. Even with these defects, it is a beautiful azalea.
R. albrechtii , supposedly very similar to pentaphyllum , is really quite different. It is much lower in stature, later blooming, and different in color, a rather harsh magenta pink, unlike the soft, pure pink of pentaphyllum . At Winterthur it grows but is not happy. Our summers are probably too hot for it.

II. Schlippenbachii Subseries
R. schlippenbachii is one of the most beautiful plants of the genus, a big, bold plant in every way. Around the first of May its leafless branches are covered by three-inch flowers of soft pink. Later the whorls of broad, light green leaves at each branch-tip are handsome, more so when they turn to bright yellow and orange in autumn. This species is extensively used at Winterthur, covering the whole northern edge of the Pinetum in combination with two other Rhododendrons (the pink forms of mucronulatum and the Wheeldon Azalea 'Miss Susie') of similar color but different blooming time (earlier and later, respectively.) The original plants here came from either Sargent or Wilson in the twenties. There is also a form with pure white flowers in our collection.
R. reticulatum is quite different, with smaller flowers of mauve or lavender and smaller, diamond-shaped leaves, It is another early bloomer, beginning in April shortly after mucronulatum . At Winterthur it is planted with the pale yellow Forsythia ovata , the hardiest of the forsythias, whose color is a good foil for the rather heavy purplish shade of the azalea. This species (or complex of several closely related species) shows considerable variation in habit, leaf, and flower color. One plant at Winterthur is especially distinct, with quite large flowers of a rich but rather strident magenta purple.
R. weyrichii is an upright shrub with beautiful foliage and flowers of a strange shade of brick orange-rose which appear after the middle of May. It is quite valuable for late bloom, though it never flowers so heavily here as either of the two preceding species. Very similar in leaf and flower but blooming even later is amagianum . Our original plants were gifts from Joe Gable, who told this writer and Walter Petroll, Head of the Gardens Division here, that old plants in his woods grew with the size and habit of dogwoods.

III. Luteum Subseries
The large Luteum Subseries is second only to Subseries Obtusum (many would say equal to it) in terms of numbers of plants of great garden value. We find in it some eighteen species, of which sixteen are represented at Winterthur, and several hybrid groups, of which we grow Ghent, Mollis, and Exbury/Knaphill hybrids.
Prior to the fifties, the area now known as the Peony Garden contained large numbers of Ghent and Mollis azaleas. They did not prosper there, and were gradually moved into the Pineturn as Mr. du Pont obtained hybrid peonies from Silvia Saunders with which to replace them. The main problem with the azaleas was that they were grafted plants, growing mainly on flavum ( luteum ) understock. Today they grow in a large block in the Pineturn, where they make a pretty sight in spring under planted with blue Scilla campanulata . Most of them are small single yellows and pinks. I suspect that most of these are flavum and nudiflorum understock which have taken over after the death of the hybrid top growth. There are a few true Ghents and Mollises, the latter easily recognizable by their larger flowers in shades of pumpkin and pinkish-orange. Most of these have been here so long that their names have been lost. The Ghent hybrid 'Narcissiflora,' however, is easily recognizable by its double yellow flowers, and the Mollis called 'Miss Louisa Hunnewell', with large, showy, bright yellow flowers retains its name tag.
The Exbury and Knap Hill hybrids first came to Winterthur in the fifties, and we have a better record of varietal names of these. As every rhododendron fancier knows, the blooms of the better clones are really stunning. One might wish, though, for a more pleasing habit (most are graceless shrubs with thick, stubby twigs) and foliage more resistant to the mildew which renders them so unsightly during our summers on the East Coast. We grow a dozen and a half or so cultivars. Of these 'Cecile,' a huge-flowered yellow-blotched salmon, was H. F. du Pont's favorite, and it is a showy thing. Other good clones are the blazing orange 'Gibraltar,' the bright yellow 'Sun Chariot,' and the pure white 'Oxydol.' Rather atypical in this class are 'Royal Lodge,' a late red, and 'Homebush,' a double deep pink. In their comparatively small flowers these two show more evidence of American species in their ancestry than the others.
All the species in the Luteum Subseries with the exception of R. oblongifolium , R. occidentale , and the newly described R. coryi grow at Winterthur, though not all grow with abandon. The Chinese Azalea, R. molle , languishes in our cold winters and hot summers. Its Japanese counterpart, R. japonicum , in contrast does well here, and is an excellent midseason orange yellow. R. flavum is another exotic species which is not well suited to our climate, though it does better than molle . The selection introduced several years ago by the USDA under plant introduction Number 122709 is the most successful clone of flavum here, and it is a good early sharp yellow, with abundant small, tubular blossoms.
Turning to the many American species, we find many interesting and successful garden plants. The Pinxterbloom, R. nudiflorum , grows wild on the piedmont hillsides of Winterthur, and the Swamp Azalea, R. viscosum , probably occurs in woodland swamps, though it is far more abundant a few miles away on the coastal plain. Both species are of course perfectly adapted to our climate and respond well to cultivation. The Pinxter is very variable in flower, some forms being small and pale and others large and bright. The pink flowers commonly have a tube of vivid carmine. It is a nice touch of woodland color in early May and, like all deciduous azaleas blooms much more abundantly when planted in full sun. I have seen wild specimens fifteen feet tall with trunks as thick as a man's arm.
Swamp Azalea blooms in late June and July, with small, tubular flowers of white or, rarely, pink. Though not really showy, the flowers are pretty against the polished deep green leaves of the plant, and exhale a strong, sweet fragrance. It grows naturally in wet soils, but does well in other situations so long as the soil is acid.
On Oak Hill we grow a collection of native azaleas, the nucleus of which are propagations from the selections made by Henry Skinner on his famous expedition in search of wild azaleas in the 'fifties. There is an astonishing range of color in these, and an extremely long season of bloom, beginning with canescens and austrinum in early May and ending with serrulatum and prunifolium in July and August.
R. austrinum is the earliest yellow to bloom, with small but abundant, long tubed blossoms. The tubes are often deep orange, adding greatly to the garden effect. R. speciosum begins a few days later, with slightly larger flowers that are in some forms a glowing scarlet red. The celebrated R. calendulaceum blooms in late May and early June, with the largest flowers of the group. These vary from yellow through flame orange to bright red, the yellow clones usually having the largest flowers. 'Smoky Mountaineer,' selected by the Arnold Arboretum in the late 'fifties, is nearly scarlet. In the middle of June blooms R. bakeri , with glowing orange flowers. The species is very dwarf and floriferous, one of the best garden plants in the genus. Finally, the rare R. prunifolium comes into flower in July with large flowers of deep orange or red. This spectacular species is somewhat more exacting as a garden plant then most, requiring acid soil and plenty of moisture, but it is well worth coddling. It is unfortunately none too hardy. When the winter temperature drops to zero a large percentage of flower buds are killed, but here this happens only on occasion. It seems to do best with a certain amount of shade, and is ideal as a late-flowering woodland shrub. The plants at Winterthur, which are about twenty years old, are more than ten feet tall.
In the pink range the bloom begins with canescens early in May. This species resembles the northern Pinxter but in most forms is paler in color, with almost spherical heads closely packed with small, long-stamened flowers like pink powder-puffs. R. roseum , in bloom a week or so later, also resembles the Pinxter, but has flowers of, usually, a brighter, deeper pink and is generally a better garden plant. Its attractive foliage is noticeably more grayish or bluish than that of the Pinxter.
R. alabamense is a fine early white with fragrant flowers. Most wild collected plants show intergradation with canescens , but the typical form has large flowers of pure white with a yellow blotch. In mid-May the Coast Azalea, R. atlanticum , comes into bloom. Like the preceding, the typical form of this species has large, fragrant white flowers, but commonly intergrades with other species. We have one plant which came as a pink atlanticum which is obviously a hybrid with nudiflorum . In its pure form the Coast Azalea is an excellent plant, low in stature, with attractive glaucous leaves and large flowers. In habit and floriferousness it approaches an obtusum azalea.
The Sweet Azalea, R. arborescens , blooms in early June. Its white flowers are large and fragrant, and its foliage a glossy, attractive, dark green. The Hammocksweet Azalea, R. serrulatum , is very close to the northern viscosum externally except that it blooms in August, the latest of the group to flower, and is much more tender. Like prunifolium , its flower buds winter-kill in low temperatures.
We also grow some interesting hybrids of bakeri with both arborescens and viscosum , which are attractive salmon pinks blooming in mid-June, after the pink species (and indeed most azaleas) have finished flowering. A recent acquisition is called 'Coleman's Early Yellow' and it is just that. It was named by S. D. Coleman, one of the most ardent devotees of this group of azaleas, and is a hybrid of alabamense and austrinum . I have mentioned the attractive foliage of most of these species. They also show considerable autumn color, which makes them garden subjects of more than one-season interest. Perhaps the day will dawn when we will find flowers of Exbury size on shrubs as pleasing as bakeri and atlanticum . From published descriptions of some of the new cultivars of David Leach like 'Scarlet Salute' and 'Tang,' I gather that that day may be at hand.