Hardiness and Breeding for Hardiness in Rhododendrons
By David G. Leach
President C. I. Sersanous has asked me to contribute an article to the "Quarterly Bulletin" on hardiness, and breeding for hardiness in rhododendrons.
The treacherous nature of this assignment was illustrated to me only yesterday when I visited some friends in our cult who have the good fortune to live along the lake in Erie County, Pennsylvania, approximately one hundred miles North of my home in Jefferson County.
The moderating effect of Lake Erie combined with the greater humidity which it provides make it possible for these enthusiasts to grow such rhododendrons as 'Purple Splendor', 'Gomer Waterer', 'John Walter' and a number of others which are a complete failure for me one hundred miles farther south. But a surprising bit of information concerned R. maximum and the Catawba hybrid 'America'. Both are reliable ornamentals in my Arctic mountain climate whereas they are not entirely satisfactory in the more benign environment of Erie County.
Observations elsewhere have repeatedly demonstrated that hardiness is both treacherous and deceptive. High humidity combined with a long autumn will render some species and hybrids less hardy than they are demonstrated to be in a more severe climate because their tissues do not fully mature for the first cold winter weather.
The treatment the gardener affords his plants has a large bearing on their hardiness. Protection from wind and extreme temperature variation is a vital consideration as can be demonstrated by building a rough board frame around a tender plant. Such shelter will bring through, with floral buds intact, rhododendrons so sensitive to winter conditions that they are killed outright when grown without protection. In the eastern United States plants of doubtful hardiness should not be planted with an eastern exposure. The morning sun shining on them when they are frozen is injurious. Hardiness can also be controlled to some extent by chemical applications. Potassium sulphate, applied at the rate of 25 lbs. per 1,000 square feet, has been demonstrated to greatly hasten the maturation of plant tissues which are in lush growth late in the season.
Fertilizing rhododendrons after the first of July is a risky business in this part of the world and watering during August and September is even more hazardous. Rhododendrons can stand a surprising amount of drought - they are much more resistant in this respect than are deciduous azaleas and they should be permitted to go to the full limit of their endurance here in the east. Leaves can droop and curl forlornly without injury to established plants.
This lesson was vividly learned about twelve years ago when I watered some plants of R. catawbiense about two feet tall because they appeared to be suffering from late summer drought. The stimulus they received so encouraged and continued late growth that nearly every plant of this ironclad species was killed or severely damaged when the bark split under the stress of freezing temperatures at a time when the tissues were turgid with sap of low sugar content. I have since learned even to shield plants of doubtful hardiness and exceptional value against late summer rainfall if they show any evidence of renewed growth activity. For maximum hardiness here in the east we want the plant tissues to be fully matured and then we want to be certain that the earth has ample moisture immediately before the advent of sustained freezing temperatures. The time to water rhododendrons in the fall is around Thanksgiving so that no late season stimulation results.
Rhododendrons are very obviously hardier in age than they are in youth and the plants must be at least three years old to give any indication of their cold resistance at maturity. R. fortunei is noteworthy as a species which increases in hardiness with age. Some of the azalea hybrids of the obtusum subseries which are unsatisfactory ornamentals as young plants will bloom reliably as they grow older, very probably because growth slows down with age and vegetative activity declines.
The length of time a rhododendron has been planted has an important bearing on its hardiness. When the root system has had ample time to develop so that it can fully support a plant and replenish the loss of moisture transpired through the leaves the endurance of a plant is enhanced. Gardeners who want to experiment with rhododendrons of doubtful hardiness are well advised to plant in the spring instead of in the fall.
Rhododendrons vary in their susceptibility to the season of cold. Some varieties which are sensitive to cold weather late in the fall and early in the winter are quite hardy to much colder weather in mid-winter or to delayed coldness in a late spring. R. maximum, generally considered to be ironclad in its hardiness, loses floral buds in seasons of exceptionally early cold weather. It can stand almost any amount of sub-zero cold later in the winter. The Yodagawa azalea (R. yedoense) is exactly the opposite. Dr. Henry Skinner found that it lost its buds when exposed to late spring frosts.
It seems to me that response to the timing of unusual cold accounts for many of the "mysterious" casualties among rhododendrons. A plant may grow happily through eight or ten years of both normal and abnormal temperature variations in fall or winter only to succumb following cold weather unusually late in the Spring. Since rhododendrons do not usually reveal serious injury from cold until mid-summer the gardener by that time fails to associate the timing of the previous cold weather with the death of his plant, and especially so since he can often remember much lower temperatures earlier in the winter.
Rhododendrons vary considerably in hardiness according to their genetic constitution. Mr. Joseph Gable has isolated progeny from a number of species which are considerably hardier than the average of those species as they are sold commercially. By allowing the rigors of his climate to eliminate the tender plants and then using the survivors as parents, the progeny have become appreciably hardier than the average. Mr. Guy G. Nearing found individual hardy plants of R. sanguineum and of R. radicans, both species which are ordinarily tender in his New Jersey climate.
A final suggested reason for the behavior of rhododendrons as they are affected by climate rests in the inherent response of the plants to unusually warm temperatures in winter as well as to very cold temperatures. The dormant periods of most rhododendrons are unaffected by brief periods of higher temperatures but some species and their hybrids do not give a good account of themselves in mild climates because their dormancy is easily interrupted. R. mucronulatum and R. schlippenbachii are perfectly hardy in my 25 below zero climate but they are quite likely to be unsatisfactory south of the Mason and Dixon line because they respond too readily to warm periods in winter and early spring. They are then damaged by the resumption of normal cold weather.
Only four species of elepidote rhododendrons have been demonstrated to be fully hardy in a very severe climate such as my own: R. catawbiense, maximum, brachycarpum and smirnowii For ornamental purposes the selected pink, red and white variants of R. catawbiense are much superior to the species type. R. maximum shows less variation but a vinous red is known and a white form with flowers much larger than the typical species was discovered in 1953.
Among hybrids commercially available my own opinion is that R. 'America' (red), R. 'Mrs. C. S. Sargent' (rose pink) and R. 'Catawbiense Album' (white) represent a combination of the best quality and the most reliable performance in mid-season varieties. R. 'Boule de Neige' (white) and Mr. Joseph Gable's R. 'Beaufort' (white with pink edge) are the best early clones.
I would like here to make a plea for the propagation and distribution of R. 'Sefton' which has the largest flowers of any of the commercial Catawba hybrids, blooms very late, and is unique in its reddish plum colored blossoms. It has been perfectly hardy here to 30 degrees below zero and it is almost impossible to buy on the market. R. 'Kate Waterer', a rosy crimson which was last erroneously rated "B" in hardiness is a rhododendron of strong character. It is perfectly hardy, of fine habit and it is a particularly effective plant in the garden out of bloom. It grows like no other Catawba hybrid in the way it unfurls its light green foliage to display the texture of the undersurfaces of the leaves as they curl around their perimeters. The third neglected hardy rhododendron which deserves a better fate is R. 'Abraham Lincoln' (not 'President Lincoln'). Its red blossoms are a purer color than the popular R. 'Charles Dickens' can) cast and Mr. Paul Vossberg has found it to be hardier than any other red Catawba hybrid. It has supposedly disappeared from nursery lists because it was difficult to propagate.
In breeding rhododendrons R. catawbiense is by far the most valuable source of hardiness and it seems to me that even in mild climates hybridists could profit by the use of R. catawbiense album Glass, a white form of the species which does not taint its progeny with a magenta cast. In addition to hardiness it imparts to its seedlings adaptability and a resistance to sun and exposure which some of the newer British hybrids obviously lack. The red variant, R. catawbiense rubrum, should be equally useful to provide the same attributes in breeding red flowered hybrids.
No hybrid of R. catawbiense has yet been produced which is equal in hardiness to the species itself. After very severe cold weather R. catawbiense blooms when all of the Catawba hybrids have lost their floral buds. This is one reason why I question whether any catawbiense hybrid is quite as good as the selected forms of the species for imparting hardiness in breeding. This seems to me to be generally true even though I know that the reasoning is not inevitably sound genetically. I know of at least one instance where R. 'Atrosanguineum', which has been used often and successfully as a parent by Mr. Joseph Gable, produced seedlings which were hardier than the identical cross made with R. catawbiense rubrum, the seed parent. R. 'America' promises to be a good parent. R. catawbiense album is reported to have a propensity toward induced apogamy, the production of seeds without fertilization having taken place. I can only report that seedlings of such crosses as I have made with it clearly show the influence of the other parent. R. 'Roseum Elegans', 'Mrs. C. S. Sargent' and other rose and lavender pink flowered hybrids have usually been very disappointing as parents in the production of hardy rhododendrons. 'Boule de Neige' and 'Cunningham's White', crossed with later blooming hardy hybrids, are producing some valuable new clones which flower early.
R. maximum dominates its progeny in every ornamental respect to a much greater extent than does R. catawbiense, and since it is in itself inferior to the latter as a garden ornament the result is frequently a disappointment to the hybridist. Furthermore, R. maximum is a difficult species for the breeder to use in his efforts to create hardier rhododendrons: it is reluctant to accept foreign pollen and it is necessary to apply pollen daily for five days after the stigma becomes receptive to insure a reasonable percentage of successful crosses.
My feeling is that hybrids of R. maximum are slightly less hardy than are the progeny of R. catawbiense. I believe further that hybrids of R. maximum are less adaptable and less suited to exposure than are the hybrids of R. catawbiense.
In my own work I am using R. maximum only for crosses which are designed to extend the blooming season for hardy rhododendrons. For that purpose I think it is invaluable.
R. brachycarpum has not distinguished itself in the production of hardy rhododendron hybrids. An abnormally high percentage of its progeny are malformed and distorted. Its use on the west coast as a parent seems to have been discontinued because the flowers of its hybrids were too small. Two plants of R. brachycarpum x discolor which flowered here were very poor in the quality of their blossoms, yet brachycarpum crossed with fortunei produced a pleasing hybrid of good quality for Mr. Joseph Gable and its cross with R. decorum is a successful combination of remarkably good plant habit and rather good flowers produced early in the season. It is reported to have produced an interesting group of hybrids with pretty double flowers for an eastern hybridist. R. brachycarpum freely transmits its hardiness to its progeny and I have been surprised to observe in a few instances that it ranks in this respect on a par with R. catawbiense and R. maximum.
R. smirnowii is unsatisfactory as a parent of rhododendrons comparably hardy with the commercial Catawba hybrids. Its progeny are much more sensitive to cold and they retain the parental fault of opening a few flowers in the truss in the fall in mild climates only to lose the balance of the flowers to subsequent winter cold. Out of a fairly large number of smirnowii crosses of which I have direct knowledge only two could be described as successful for severe climates.
Thus the breeder in the eastern United States, after he recognizes the deceptive aspects of hardiness as a critical attribute of rhododendrons, has at his disposal only three species which can contribute a full measure of cold resistance to his hybrids. There are less than a handful of exotic elepidote species of quality from the mainland of Asia which, when crossed with R. catawbiense, maximum or brachycarpum produce fully satisfactory hybrids for the most severe climates. Yet this is no cause for discouragement and in fact we can be grateful that we have at our disposal full means for creating hybrids for the eastern United States which are equal in quality to the originations of England and the Pacific Coast.