The Hybrids of Rhododendron Racemosum
by G. G. Nearing
Fig. 2. A massed planting of R. racemosum
R. Henny Photo
Of all dwarf rhododendron natural species, one of the hardiest anti most attractive is R. racemosum. The form of it which is founded on Forrest's collection 19404 is rated on a par with R. repens (Forrestii), williamsianum, haematodes, impeditum, moupinense. Not only is it hardier than any of these, but it makes a more serviceable garden plant, with few drawbacks. Other races of R. racemosum have proved less hardy, not so dwarf, nor so outstandingly ornamental, though an earlier race did receive the British F.C.C. in 1892.
Fig. 3. R. racemosum F. 19404
R. Henny photo
The best type of my selection grows only a foot high in a dozen years, while spreading rather wider than a foot, and hides its miniature leaves every season under masses of brilliant bloom varying from pale pure pink to rich rose, with never any hint of purplish shades.
Small wonder that so fine a plant soon attracted the attention of breeders both here and abroad. To envision as its progeny a magnificent array of dwarfs in various colors and of diverse habits is natural and inviting. R. racemosum cannot of course be bred with any except other scaly species, at least not with the slightest expectation of success, but among the scaly Rhododendrons are so many different types, some very handsome, that the field looks exceptionally attractive.
First to mind come the sixty or more species of the Lapponicum alliance, nearly all low growing, and many hardy. Immediate disappointment appears to have followed every attempt in this direction, for racemosum refuses to cross with them at all, or if anyone has succeeded, the records make no mention. My own efforts have proved futile.
More promising was the production at Kew in 1926 of 'Spinulosum', a hybrid with R. spinuliferum, straggling, tender species of some merit, one of the reddest in the scaly section. 'Spinulosum' however proved not much hardier or better than spinuliferum, and on the whole less attractive than racemosum.
By 1928 Magor had crossed R. racemosum with R. keiskei, our hardiest, dwarf yellow from Japan, and bestowed upon the offspring the hideous name 'Keiskrac'. Joseph B. Gable and I raised it from seeds sent to him by Magor, and hoped for a hardy intermediate between these two hardy species. Free-flowering plants in shades of pink resulted, much like racemosum but somewhat larger and, another disappointment - less hardy than either parent. All the flower buds above snow line and much of the top growth are killed by any cold winter. We crossed our hardiest strains of racemosum with winter-tested keiskei, and still the offspring came hopelessly tender. To make matters worse, the flowers had no stamens, nor would they set seed when first we pollinated them. Later a couple of plants did produce some seed. Hardgrove has one of them, Gable another, and we are all growing second generation seedlings in the hope that hardiness may reappear in various recombinations.
Gable soon came forward with 'Conemaugh' (racemosum x mucronulatum), 'Codorus' (minus x racemosum) and 'Conestoga' (racemosum x carolinianum). Of these the best by far is 'Conestoga', with generous flowers in pretty clusters, and an excellent shade of pink. But again hardiness was lacking, at least in northern New Jersey, though sufficient for southern Pennsylvania. These two entirely hardy species combined to give a hybrid not satisfactory in much of the Northeast.
At my suggestion, both Gable and I raised great numbers of seedlings from 'Conestoga', only to meet with disillusionment. He wrote me finally that he was giving up the attempt, since nearly all the offspring turned out worthless and tender. But I persisted. After throwing away a few hundreds, or perhaps thousands, I set aside a dozen or so which showed some promise.
Fig. 4. R. 'Windbeam'
One grew so vigorously and showed such evidence of hardiness, that about 1944 I began to propagate it under the name of 'Windbeam', after a nearby mountain. With the habit of carolinianum, but much smaller leaves, slenderer twigs, and comparatively large, handsome flower clusters, it developed slowly into a dense, spreading bush now nearly five feet high at the age of about twenty years. It is certainly one of the most floriferous of all rhododendrons, and as it sets few seeds, does not require any picking of the faded flowers. The buds open with an apricot tint soon lost, and the display is pure white for three or four days, after which it takes on a soft pink.
Unlike many varieties which bloom heavily, the flowers do not appear unduly crowded, nor is the plant bent by their weight. The clusters pose neatly erect, smothering the foliage, which is fully evergreen, free from disease, and pleasing at all seasons. Hardiness compares with either racemosum or carolinianum, but full information on this important trait must await trials in regions other than northern New Jersey and Long Island.
Cuttings root well but slowly, preferably without heat. R. 'Windbeam' is still very scarce, Donald L. Hardgrove, who bought the original plant from me, still holding almost a monopoly of it, though others are now building up stock.
Fig. 5. R. 'Wyanokie'
Subsequently one more 'Conestoga' seedling showed itself worthy of propagation, and received the name 'Wyanokie', for another mountain near 'Windbeam'. It grows more slowly, but with sturdier twigs and denser, larger foliage. The original plant, without having been cut back, has a height of about two feet and a spread of nearly four, after twenty years or so. Its flowers, not quite so abundant as those of 'Windbeam' but making a fine show every year, are at all times pure white. Like 'Windbeam' it propagates readily though slowly, but because cuttings from the original plant are not available, it will not reach the market in any quantity for at least three or four years.
These two 'Conestoga' seedlings are outstanding among the very few racemosum hybrids fully hardy in northern New Jersey. In general it may be said that primary hybrids between racemosum and any other species are less hardy than racemosum. Prospects for a hardy race of dwarfs from this source depend on further crossing the primary hybrids, either with each other or with related plants.
A number of tender racemosum hybrids await the breeder who cares to explore their possibilities. 'Racil' (racemosum x ciliatum) 1937, does not differ conspicuously from racemosum except in its larger dimensions, but carries the potentialities of its other very different and striking parent. I have seedlings of its back-cross on racemosum now ready to flower. 'Radmosum' (radicans x racemosum) has somewhat the racemosum plant habit, while the flowers more nearly resemble radicans, with a dull purplish tint. Some seedlings from 'Radmosum' (open pollinated, but probably crossed inter se) tend toward the prostrate mat of radicans, but it is too soon to say anything about their flowers or hardiness.
Long ago I used pollen of R. tephropeplum on racemosum, getting something like a more buxom but less hardy racemosum with a tendency not to set fertile seed, though flowering every year profusely under glass protection. A recent cross of racemosum with R. pemakoense I have not yet seen, but am anxious to experiment with the large campanula-like flowers on the tiny mounded plant of that species, so have made the cross myself and am about to sow the seeds, expecting that this hybrid too will fail to weather our winters. What I want to see is the second generation, which cannot be produced until the first has grown and flowered.
About 1933, I flowered the then new R. pubescens, a species very close to racemosum, though botanists have placed it in a different series. The resulting hybrid was hard to distinguish from either parent, and as it is nearly hardy, may have become mixed with my otherwise pure strain of racemosum. R. pubescens, unlike racemosum, tends to give primary hybrids hardier than either parent, as was demonstrated when its cross with R. keiskei, known as the Guyencourt hybrids, showed good winter resistance in this region, carrying their flower buds through temperatures which killed those of keiskei, the hardier parent. The plants reach a size comparable with keiskei.
Pollen of 'Rosy Bell', a two-star hybrid between ciliatum and glaucum, has been tried on racemosum, but the result, called 'Almond Thacker', I have not seen. It should be not unlike 'Racil', the element of glaucum in the parentage offering some difference, but little improvement and small chance of hardiness. It has aroused no echoes.
Another direction was explored by Lord Aberconway in 'Amba' (racemosum x burmanicum) 1933, which does not sound promising and has generated little excitement. The same may be said of 'Arden Fairy', a cross with R. lutescens shown in 1946, while 'Fittra', as recent as 1949, is the result of crossing with a form of R. dauricum, and should be little different from the earlier 'Conemaugh', because mucronulatum is merely a form of dauricum, completely deciduous where dauricum is partly evergreen.
The fact that racemosum hybrids have given few forms which in real merit can be compared with the species itself, should not deter future probing of their possibilities. Remember that in the second generation 'Windbeam' and 'Wyanokie' appeared, with virtues which overshadow the faults of their predecessors. I am at present, pursuing the line of racemosum x keiskei crossed back on both parents, with the hope of obtaining a yellow or yellowish racemosum that is hardy. Hardgrove has already accomplished something in this direction, with 'Springsong' [(racemosum x keiskei) x keiskei], salmon in color. His racemosum x bullatum, still in seedling stage, suggests startling new possibilities if that superb species can be induced to contribute its beauty and intense fragrance without its extreme tenderness, as may occur in a second generation. Another of his most promising ventures is racemosum x (chrysodoron x johnstoneanum), with good yellows in prospect, especially if crossed again with racemosum x keiskei.
I have seedlings of 'Radmosum' ready to flower, offering the possibility that the extreme dwarf, mat-forming radicans may somehow be induced to contribute that quality to future offspring without intruding its less desirable color.
Fig. 6 R. 'Ramapo'
My seeds of racemosum x pemakoense and racemosum x leucaspis, now ready for sowing, suggest little likelihood of plants which in their own generation will prove hardy, but a second generation only three or four years away may put the large flowers of these fine species on dwarfed and more substantial plants. It is evident that many hundreds, perhaps thousands of seedlings must be grown in that second generation before any outstanding forms are likely to appear. But the generation matures in three to five years-not a long time to wait for what good fortune may be ready to thrust into our hands. Many crosses not listed above already exist as seeds or seedlings or as plans for next season's hybridizing, if we could take a census of all breeders' efforts, but those mentioned will give a good idea of the prospects.
Truly, racemosum is stingy of its charms only to those who lack patience.