Yellow Rhododendrons for the Northeast
G. G. Nearing, Ramsey, N.J.
From talks before the New Jersey and New York Chapters
Searching for the widest possible range of flower color, rhododendron breeders have not neglected that most important part of the spectrum centering around yellow and including orange, salmon, straw and cream. These shades, in themselves attractive, are, with the exception of orange, among the most satisfactory for harmonizing with other colors. About 300 hybrids of such coloring have been recorded, not including the javanicums and such other greenhouse subjects.
But unfortunately there is no yellow species entirely hardy in the Northeast except those yellowish forms of R. carolinianum now being isolated, and the hardiest strains of R. keiskei. Among the elepidotes, the large-leaved rhododendrons most familiar to all of us, there is no hardy yellow species.
Therefore we must cross our yellow species and hybrids with hardy plants until the combination of yellow flowers and hardiness turns up in the same offspring. Breeders have been working at this project for a century without complete success. The hardiest of the yellow hybrids are neither very hardy nor very yellow. Shall we then give up in despair, or shall we continue to try new avenues of breeding?
The most nearly hardy so far is Goldsworth Yellow, which with me succeeds in wintering a few buds nearly every year, and does not often have growth cut back by the cold. Its shade of yellow leaves much to be desired, but in lieu of better, any of us would settle for a completely hardy plant of the same color.
Its parentage is now recorded as 'Jacksonii' x campylocarpum. 'Jacksonii' is a bright pink hybrid of caucasicum x 'Nobleanum', while 'Nobleanum' is caucasicum x arboreum. Now I happen to remember that many years ago, when breeding statistics were generally kept secret, there was a good deal of speculation as to the ancestry of 'Goldsworth Yellow'. Finally Lionel de Rothschild came out with a guess that it might be caucasicum x campylocarpum, and because of his standing, nearly everybody else seemed to agree. The foliage and flower shape suggest caucasicum with near certainty. But I have grown thousands of seedlings from various crosses with campylocarpum, and never has one failed to show in the foliage evidence of its descent from campylocarpum. There are certain species which imprint their character on the leaf of every hybrid, and campylocarpum is one of them.
The leaves of 'Goldsworth Yellow', to my eye, could not inherit from campylocarpum. On the other hand, 'Goldsworth Orange', dichroanthum x discolor, bred in the same nursery has foliage rather similar to 'Goldsworth Yellow'. What more probable than that both these hybrids are bred from dichroanthum? My seedlings of crosses with dichroanthum usually agree more or less in general appearance with 'Goldsworth Yellow'. I should prefer to guess the parentage as 'Cunningham Sulphur' x dichroanthum.
By almost universal agreement, R. campylocarpum is accepted as offering the best possibilities for breeding an early-season yellow hybrid, while R. wardii, not very dissimilar, suggests equal likelihood of offspring to bloom somewhat later. Both appear in the parentage of our yellowest and most nearly hardy hybrids other than 'Goldsworth Yellow'.
Campylocarpum hybrids such as 'Canary', 'Carita', 'Damaris' 'Letty Edwards' and 'Mrs. Ashley Slocock' are truly fine plants, and if we could duplicate any of these in completely hardy form, we should have a prize indeed. There are two avenues by which we can breed toward this end. The first is to cross campylocarpum and its best hybrids with the hardiest plants in this section of the genus. No doubt every breeder has thought of that, but the hardiest of all, R. catawbiense, has undoubtedly smothered the yellow genes under its glaring purple-pink, thwarting the best-laid plans in every case. But now that Powell Glass has given us a white catawbiense, I feel sure every modern breeder is repeating the unsuccessful crosses of the past with good hope of success, Gable's 'Catalgla', though showing probably the finest flowers of all these whites, is an exceptionally straggly plant, so I prefer my 'Catanea', originally from Gable, a fairly symmetrical specimen. On this and on a 'Catalgla' x fortunei from Hardgrove, I have been crossing every available yellow. Hardgrove's 'Catalgla' was probably not Gable's, but a white he obtained from me, and the R. fortunei very likely the hardiest plant of my second generation bred for hardiness.
The first to flower (1962) was 'Catanea' x 'Moonstone', pure white with perhaps a hint of cream. The plant budded at ten inches and wintered its bud in the open, two good omens. The campylocarpum in its parentage shows somewhat in the foliage, so I pollinated it with campylocarpum x decorum, the nearest I had to campylocarpum itself, and a clear pale yellow. Caterpillars ate most of the capsules, but I have a few seeds. A second plant is now budded while even smaller, but its totally different foliage approaches R. williamsianum, as does the character of its bud. Other seedlings of this cross are coming along, and I mean to carry on this and similar lines of breeding until thousands of plants, including every conceivable combination, have been raised.
R. 'Moonstone', campylocarpum x williamsianum, I have grown in various forms, including crosses on my own plants. The best of these proved nearly hardy at Ridgewood, and one plant even bloomed somewhat at a later date, in the open at Mountain Lakes, N. J. With both parents rated at the very top among rhododendron species, and with its yellow flowers, often orange in bud, 'Moonstone' offers excellent possibilities for breeding superior yellows.
Hardgrove's fortunei x croceum was so fine a yellow that all who saw it several years ago must have been greatly disappointed to heart that it died. He tells me that he now has similar hybrid still better, and if so it must be very fine indeed. He gave me a different seedling of the fortunei x croceum cross, not so deep a yellow, which I have used extensively. But it was not hardy. R. croceum is now considered to be a form of wardii, an extremely variable species. Leach rates the wardii at Edinburgh as the finest yellow in cultivation. He sent me pollen from it, which I used on 'Catanea' and other plants, sharing the seeds with him. An abundant crop of seedlings both at Brookville and at Ramsey, proved exceptionally subject to disease, and I doubt if more than two or three out of many hundreds have survived. So improvement of color must probably come from another direction.
A plant of wardii x discolor which flowered for Gable many years ago suggests a hardy yellow with larger flowers, though the color on this particular plant is nearly pure white. I rooted a cutting of it which is now large enough to flower, but has not yet set buds. Open-pollinated seedlings from the original plant proved fairly hardy, but few have been able to carry their buds, and those flowered white. The evidence of R. wardii in their foliage however is strong. Thirty or forty of these seedlings have not yet budded or have not proved lacking in bud hardiness. One of them may still open yellow flowers. In fact it is frequently in the tail end of a batch of plants that the best forms appear.
Fig. 18. R. fortunei
C. Smith photo
Since fortunei x croceum has given so fine a yellow, it would seem that there is something in the genetic makeup of R. fortunei which lends itself to yellow strains. The so-called R. chlorops is probably just a yellow strain of R. fortunei, no doubt less hardy than the pale lilac strain growable in the Northeast, yet two or three generations of selection for hardiness might give us a hardy R. chlorops for future breeding.
I have been using R. campylocarpum x decorum in many crosses, with the idea that R. decorum, closely related to R. fortunei, may show a similar leaning toward yellow. But the lack of hardiness in R. decorum makes it less useful. If my hardiest R. fortunei blooms well this coming season, and if its flowers are not all eaten by caterpillars, as they were last season, I plan to explore its friendliness for yellow with all the yellow pollen I can lay my hands on.
A different approach to the problem, one which I am pursuing with due perseverance, is to raise more seedlings of both campylocarpum and wardii, with the idea of selecting from among them a hardier form, this later to be crossed with 'Catanea' and with near-hardy yellows. So far three plants of R. campylocarpum have wintered in the open several years, and one of wardii. The R. campylocarpums are evidently true, but the wardii, though a handsome and vigorous plant, is almost surely a hybrid. None have set buds.
Other yellows which might contribute to the program are not being neglected. R. dichroanthum and its hybrids such as 'Dido' and 'Fabia' are crossed with everything hardy, in search of shades approaching orange. Here two bad faults, unsymmetrical growth habit and drooping flowers must be dealt with, and first class hybrids of a given color can hardly be expected in less than three generations. On the other hand, the tendency toward a colored calyx may be an asset in some combinations.
Yellow or yellowish forms of R. caucasicum are available in various hybrids, including 'Goldsworth Yellow', and are commonly not far from hardy. 'Cunningham's Sulphur' is perhaps the second hardiest and 'Ochroleucum' one of the shapeliest. However, this line of breeding has been pursued in many nurseries for over a century, and the results indicate that wardii or campylocarpum blood should be added if depth of color is wanted.
Much has been said and written about the elusive R. chrysanthum, which in its true form is a prostrate dwarf with pale yellow flowers. It appears to be fairly hardy but difficult to please and very reluctant to flower. Without flowers, hybridizing is impossible. Patience may yet bring us hybrids with its plant habit, and perhaps a flower more like campylocarpum, produced more willingly. If a communist should escape from North Korea, with seeds in his pocket of R. chrysanthum from the mountain bogs of that region, I think some of us might be willing to chip in and pay his fare, and we might then find something more hardy and more reasonable than the same species derived from Japan as we know it.
Fig. 19. R. lacteum
R. lacteum (Fig. 19) is described as the finest of all yellow species, but one of the most unresponsive to cultivation. Pollen of certain lacteum hybrids such as 'Mariloo' and 'Jason', is often available, and if crossed on hardier things, may eventually bring pleasant surprises. I have a great number of seedlings from pollen of a complex lacteum-fortunei hybrid used on 'Catanea'. There is small likelihood of a yellow in the first generation, but subsequent breeding might bring out this color.
Fig. 20. R. wightii
C. Smith photo
R. wightii, (Fig. 20) related to lacteum and nearly as good, has been crossed with fortunei to produce 'China', a celebrated pale yellow not too far from hardy. It might contribute, as might R. lacteum, plants of more expansive dimensions than those we have been discussing.
Still larger, and with spectacular yellow flowers, is R. macabeanum, probably the hardiest of those yellow tree rhododendrons with enormous leaves and immense flower heads. I have crossed it with 'Catanea' and with 'Catalgla' x fortunei, and some of the seedlings have wintered two or three years in the open. In twenty years or so, perhaps they may bloom. R. falconeri is nearly as hardy, a paler yellow. Hybrids of it are on their way.
In the lepidote (scaly) rhododendrons, the yellowish forms of R. carolinianum and the hardiest of R. keiskei at once suggest themselves. Why not cross these together? This I have been trying to do for many years, with no results so far. R. carolinianum yields great numbers of seedlings readily, but regardless of what pollen has been placed on the flowers, the resulting plants have usually been carolinianum. However, there are exceptions, and I keep trying.
R. 'Wyanokie', a carolinianum hybrid, has a hint of yellow in the white flowers when first the buds show color, It might be more receptive to yellow pollen, and since it is almost as hardy as carolinianum, with a good dwarfish habit, and bearing abundant seeds, I am crossing it with all desirable lepidote yellows. Of course the lepidotes will cross only with other lepidotes, except in rare instances.
Outstanding among these yellows is R. keiskei. Reasonably hardy, it varies from a low, compact mound to an upright shrub, with flowers of a rather uniform lemon yellow. Unfortunately the most picturesque dwarfs are usually the least bud-hardy, and even the hardiest forms tend to lose a few buds in any cold winter, though usually opening a fairly good proportion. As the flowers come in late April, they are often spoiled by frost when open. My rule is to cross every lepidote yellow with R. keiskei, yet the best result thus far comes from a hybrid with the pink R. racemosum. This hybrid, given the hideous group name of 'Keiskrac', is not fully hardy, has no stamens, and usually refuses pollen, but individuals which will take pollen have been isolated, and one of these, crossed back with R. keiskei, produced R. 'Mary Fleming,' a pale yellow with salmon streaks outside the corolla, giving the effect of a pale salmon. It is a very shapely and free flowering dwarf, rather hardier than keiskei.
Crossed with R. pubescens a shrub rather similar to R. racemosum but wooly, keiskei produced the Guyencourt hybrids, one of which 'Lenape', is a very pale yellow. This I am crossing back with R. keiskei, for seedlings of the Guyencourts have produced strange things. One of these, 'Ramsey Tinsel', is a very dwarf dull yellow, with 8 to 10 separate, narrow petals, something like a bloodroot. Another had double yellow flowers like miniature roses, but it is not bud-hardy, and the only flower it opened last spring was single. The doubling may have been a freak. Still another is a rather bright yellow with five separate petals, a larger plant soon to be included with the Guyencourts under the name 'Elsmere'. Though 'Ramsey Tinsel' and 'Elsmere' seem completely hardy at Ramsey, they do lose buds in a cold winter. So the search must go on for similar plants entirely bud-hardy.
A cross of keiskei x spinuliferum which Hardgrove gave me, suggests a possible road to deeper shades. R. spinuliferum is red, a very unusual color among the lepidotes, and it is possible, though unlikely, that this shade might combine in such a way as to enrich the yellows with which it is crossed. The hybrid is ochre yellow with a pinkish cast. However, this species is not at all hardy.
Two of the very dwarf yellows in the Lapponicum Series, R. flavidum and R. chryseum, are nearly hardy, and for many years I have tried to cross these with keiskei and other species and hybrids, but without success. 'Yellow Hammer' (flavidum x sulfureum) flowers generously in England, but has not opened its buds in my pit. It might contribute good color if crossed with something very hardy.
But R sulfureum, megeratum, and other yellows of the Boothii Series are so far from hardy that it may require several generations to bring their brilliant shades of yellow into forms growable in the Northeast. I have flowered R. megeratum in my pit, and admired it greatly, but little has come from it. Crossed with keiskei, it produces plants which usually grow for a few years slowly, then die without budding. Hybrids from R. sulfureum are not yet old enough to evaluate.
Yellow forms of R. cinnabarinum, named xanthocodon and concatenans, offer prospects of something nearer hardiness, though with the curse of ungainliness characteristic of that race. I have grown a number of seedlings of R. xanthocodon, and one of these has reached a height of about two feet in the open unprotected. But it has not set buds, and perhaps will not be able to flower without shelter. In spite of its drooping flowers, this may in the end prove valuable, and I 'am rooting cuttings of it because of its unexpected hardiness.
Of the many yellows in the Maddenii Series, R. valentinianum comes nearest to hardiness and may contribute its bright color to hybrids of the future, but the crosses I have made with it have not done well. Three or four generations may be needed to bring its beauty within reach of the Northeast. Often recommended for their yellow flowers are two species of the Triflorum Series, R. ambiguum and R. lutescens. These are rather closely related to keiskei, but less hardy and not so effective. The flowers of R. ambiguum are so strongly tinged with green that they match almost exactly the yellowish green of the unfolding foliage, thus cancelling the effect of being in flower. It bloomed in the open unprotected at Guyencourt, Delaware, but was not successful at Ridgewood, N.J.
R. lutescens has better color, and makes a shaplier bush, but is far from hardy in the Northeast, and I have not recently been able to flower it, even with protection. However, within any species there is more or less variation of the hardiness of individual plants. A much hardier than average lutescens might yet turn up to alter my opinion of the species.
R. hanceanum nanum, an extreme dwarf only a few inches high, though slow to bloom, eventually masses its flowers of a good pale yellow, so spectacularly that we must hope it is nearly growable in cold climates. A hybrid of it with R. keiskei, growing in a pot, has reached a height of nearly three inches, and as it had 21 branches. I took a couple of these for cuttings, in case anything should happen to the plant. Others of the same cross are somewhat larger, but still strongly resemble R. hanceanum. Crosses with 'Wyanokie' and carolinianum album are not quite so convincing, but no doubt some of them will prove successful. A hardy hanceanum may be near at hand.
With these and many other avenues of approach, the problem of adding yellows to the rhododendrons in our gardens may soon become less formidable. But a couple of centuries may pass before dandelion yellow and bright orange flowers can be produced on hardy plants of all the various styles and statures which rhododendrons now offer.