JARS v37n4 - Heat-Tolerant and Sun-Tolerant Rhododendron Hybrids Developed by Koichiro Wada (1907-1981)

Heat-Tolerant and Sun-Tolerant Rhododendron Hybrids Developed by Koichiro Wada (1907-1981)
Tomoo Wada, Yokohama, Japan
Edited by Frank Doleshy

The aim of this article is to review the hybrids developed by Koichiro Wada, my father, during more than 60 years as a rhododendron enthusiast. These hybrids are so numerous that he himself might find it hard to explain the many trials leading to major developments, and a full review would be too long for a single journal article. Consequently this article includes only the cultivars which at the time of writing the author considers distinctive. And, to save space, descriptions are rather brief.

The Challenge
To understand the rhododendron hybridizing work of Koichiro Wada, the reader should be aware that for many years most Japanese horticulturists had found rhododendrons difficult to grow in their gardens. Most of these gardens were (and are) in low-elevation parts of Japan, where some azaleas thrive but the thick-leaved evergreen rhododendrons usually had been a disappointment, surviving only briefly.

Of these latter plants, the charming native species were well known to Japanese horticulturists. Yet even today, to see any such species as R. yakushimanum growing and flowering nicely, it is necessary to visit England, New Zealand, or the U.S.A. - unless one is prepared to climb the mountains of Yakushima.

The so-called hardy hybrids, introduced to Japan from Western countries, were bred to endure reasonably cold weather. Yet most of them did not survive in Japan, since hardiness here is principally a matter of withstanding the summer heat.

Koichiro Wada learned of these problems through his own experiences, and he responded by entering into the demanding work of rhododendron hybridizing. Thus he showed his determination to combine beauty and satisfactory garden performance.

The Early Days
Koichiro was born in 1907 in Numazu, a medium-sized city facing the Pacific Ocean, south of Tokyo and Yokohama. His curiosity about horticulture appeared early, while he was but 10 years old. Two years later, on an early spring day of 1919, he first obtained a rhododendron for himself. Near his home city was Mt. Amagi, densely inhabited by the plant he would later name R. metternichii var. metternianum - also known to botanists as R. metternichii var. kyomaruense but popularly called R. metternianum . A seedling from the natural stand was perhaps his "first plant". Although this flowered nicely in his garden, it did not survive long, and the 12-year-old owner did not yet have the knowledge necessary to understand what had happened to it.

His next step was to try the hybrids of England. Starting to import seeds from various English sources when about 16 years old, he obtained rather huge quantities. Many germinated, yet with only one or two exceptions the resulting plants did not survive long. From these trials, as well as his earlier attempt to grow a native plant, the lesson was clear: tolerance of heat and sun are essential characters in rhododendrons for the gardens of lowland Japan.

His own hybridizing work began about 1927, when he was 20 years old. During the first 20 years of this work, he frequently used English plants as parents. And, because of his own experiences, he avoided the use of R. metternichii and its allies. Yet he made no remarkable progress toward his objectives until he finally did bring in R. metternianum as a parent.

The First Success: Descendants of R. metternichii and Allies
Growing the seedlings from a cross of R. metternianum with R. arboreum , Koichiro was much surprised to find that these had an appreciable tolerance for heat and sun. This was his first real success with rhododendron hybridizing, and from the offspring he selected one cultivar to be called 'First Step'. Flowers were reddish pink, and foliage was metternianum-like . Also, besides summer hardiness, it had the merits of flowering when still rather young (3-4 years from the seedling stage), flowering in early spring (April), and producing a relatively good trunk and branching pattern.

To obtain such a plant, the parents had been selected with care. The R. metternianum had flowers of darker pink than usual, and the R. arboreum was a lowland form, tolerant of conditions there. The resultant 'First Step' was stronger than either parent, but Koichiro did not think the flower color sufficiently red. Therefore, although already about 40 years old, he considered this achievement a starting point rather than a final victory.

Going onward to a second generation, the selfing of 'First Step' yielded 'Michiko', which inherited the parent's vigor and had flowers of true red. Also he crossed 'First Step' with R. metternichii aff. - his own selected cultivar of R. metternichii . Among about 100,000 seedlings from this latter cross, he looked for both vigor and flower quality, and his selections were 'Wada's Triumph', 'Brilliant Triumph', 'Giant Ball', and 'Kyoho'.

'Wada's Triumph' and 'Brilliant Triumph' have a very early flowering season, usually beginning by the end of March and continuing until the end of April. 'Giant Ball' and 'Kyoho' are notable for their relatively large, spherical trusses, 18-20 centimeters (7-8 inches) in diameter if grown in fertile soil.

Another plant as old as 'First Step' is 'Akebono', the result of a simple cross between R. metternichii aff. and R. metternianum . This remains popular because of its delicate flowers.

Simultaneously with these lines of work, Koichiro tried crossing the darker pink form of R. metternianum with some of the Western hybrids, searching for further flower variety. Representative of these efforts is 'Tomoo', obtained by crossing R. metternianum and 'Sarita Loder' ( R. griersonianum x 'Loderi'). This 'Tomoo' is vigorous, with a good habit of growth and rather large griersonianum-red flowers.

The various kinds of hybrids so far discussed are all derived at least partially from R. metternichii or its allies, and their foliage reflects this ancestry. In Japan, such foliage is an advantage, since it is particularly well liked by horticulturists.

Continuing from the second generation progeny of 'First Step', Koichiro promptly produced third and fourth generations. Among these is 'Kooen', with shining red flowers, from a cross of 'Michiko' with 'Vulcan's Flame'. Quite hardy in all seasons, this is especially able to cope with summer heat and sunshine. More recently, the author selected 'Tosca' from seedlings of the same cross. This should become popular for its flowers. 'Taeko', with delicate pinkish white flowers, is a cross of 'Michiko' with the R. arboreum x R. griffithianum hybrid; again, the flowers should make it a favorite.

photo by Tomoo Wada

Koichiro did most of the so-far-described work - involving R. metternichii and allies - during the 1947-1960 period, at Yokohama. Having moved there from Numazu after the Second World War, he established a nursery on an open slope facing south. This site, with its summer temperatures and direct sun, provided a severe test for the plants.

Introduction Of The R. williamsianum Habit
Japan being a land of bonsai, there is a strong preference among Japanese horticulturists for shrubs of semi-spherical or dome-like shapes. Since rhododendrons generally do not accept heavy pruning, the rounded shapes can best be obtained with plants that tend to grow this way naturally. R. williamsianum seemed to be the key to producing such plants, provided that its habit could be incorporated into heat resistant hybrids, and Koichiro's work on this problem possibly extended from 1947 to 1965.

An initial attempt was the cross of R. williamsianum with R. fortunei . 'Takao' was a cultivar thus obtained it had the R. williamsianum habit but not the necessary hardiness. In the F2 generation, 'Wada's Success' was produced by crossing 'Takao' with an arboreum hybrid. This cultivar is quite distinctive because of its sun tolerance and wide range of temperature tolerances, from -18°C. (0°F.) to 36°C. (97°F.). Moreover, under the sway of its R. williamsianum ancestry, it grows naturally into an almost semi-spherical or dome-like shape.

'Benihime' will be made available as another hybrid with the same habit; this is a cross of 'Takao' with 'Earl of Athlone'. While it did not inherit the red color of 'Earl of Athlone', its large, pink flowers were enough to justify its selection and naming.

Still another cross of 'Takao' was with 'Arboreum Hybrid Seedling’ ( R. arboreum x R. ponticum ); this yielded 'Shyuzengi', a tree with maximum height of possibly 6 meters (20 feet), forming a huge dome when grown under ideal conditions. It flowers quite young and quite generously, from mid-March.

For the bonsai fanciers, Koichiro developed 'Miniature Pink', a hybrid between R. williamsianum and R. hyperythrum (album). This is like a small R. williamsianum , with hardiness against heat and sun. (Other hardy dwarf rhododendrons are discussed in their own section, below.)

Rhododendron 'Taiyo'
The Japanese term "Taiyo" means the Sun: Koichiro's 'Taiyo' rhododendron (actually a group) is so named because it accepts strong, direct sun without any difficulty. Indeed it appears to luxuriate in this bright light and associated high temperatures, and it is the most summer-hardy of the rhododendrons he developed.

'Taiyo' - 18 year old plant
'Taiyo' - 18 year old plant
photo by Tomoo Wada

How was it developed? In a sense, just by happening! But this was after years of work had provided the foundation and set the stage. As Koichiro's hybridizing work progressed, he employed more and more complicated schemes of crossing. Finally, these crosses brought together so many lines of descent that even the hybridizer could not predict the results with any precision. From such complex parentage, one batch of seeds produced plants with a remarkable growth rate. Three years after the first spring, they were already about 70 centimeters (28 inches) tall. In the Yokohama nursery, fully exposed to strong sun, these plants continued to grow rapidly; 15 years after germination they reached an average height of 4.5 meters (15 feet).

Such growth is not exceptional among some of the larger rhododendrons ( R. fortunei , for example) in an ideally cool and moist environment. Yet it is amazing growth in the hard climatic conditions of Yokohama - and these conditions are evidently just what the plant needs, since it did not grow well when tried in a cool, moist situation.

In this plant, with its complicated inheritance, a strong influence from R. metternianum can be seen. However it is so hardy in comparison with R. metternianum that the difference cannot well be explained as a matter of heterosis or "hybrid vigor". Perhaps some kind of mutation has also taken place.

Koichiro was fortunate to obtain this plant as a happenstance result of his increasingly complex hybridizing program between 1960 and 1965. Yet he knew he could not hope for a series of similar accidents, producing a wide range of such hardy rhododendrons. Therefore he turned more definitely to the idea that hardy cultivars can best be obtained by starting with hardy parents, whose genetically dominant characteristics include tolerance of heat and sun.

Trials With R. hyperythrum
As now widely recognized, R. hyperythrum is noticeably resistant to heat and sun. And, fortunately, such resistance can be transmitted to its offspring as a dominant trait. This finding encouraged Koichiro a great deal, since with hardiness a dependable trait of R. hyperythrum hybrids, he did not have to worry about it while working toward flower quality and other ornamental characteristics.

When starting work with this species, his strongest desire was to obtain hybrids which combined heat and sun tolerance with good yellow flower color. He crossed the white form with numerous sources of yellow, and one resulting cultivar is 'Wada's Yellow'. While the flowers are a light yellow rather than a true deep yellow, he nevertheless valued it as the only yellow hybrid with appreciable tolerance for heat and sun.

With the objective, instead, of obtaining large white flowers, 'Loderi' was crossed with R. hyperythrum (album). From this came 'Yukinoyama', which means "snow mountain".

One problem with R. hyperythrum as a hybrid parent is its strong dominance, causing most of its offspring to have flowers and foliage similar to its own. This influence can be diluted in F2, F3, and F4 generations, which will perhaps yield unique and ornamentally valuable flower variations. However, the R. hyperythrum hybridizing program, started about 1960, was interrupted by the death of the hybridizer.

The seedlings (about 2000) left behind appear to offer many opportunities for development and selection of cultivars with strongly fragrant flowers, as well as flowers of pure white, deep yellow, and various other colors - all associated with ample summer hardiness. Selections recently made by the author include 'Elena' and 'Maioogi', both having white flowers with a dark red blotch. These can perhaps substitute for 'Sappho', which does not live long in lowland Japan.

Search For Hardy Dwarf Rhododendrons
Koichiro's own preference was for larger rhododendrons. Most of his hybrids easily grow to a height of 4 meters (13 feet) or more, while 'Taiyo' will attain 8 to 10 meters (26 to 33 feet) at full growth. During the last 10 years of his life, though, he sought to develop hardy dwarf rhododendrons which would meet the increasing demand for them among Japanese horticulturists (a demand not quite met by most of the semi-spherical or domelike R. williamsianum hybrids).

The key to obtaining these hardy dwarfs turned out to be R. simiarum , which he crossed with R. yakushimanum , R. hyperythrum , 'Loderi', and many others. The cross with 'Loderi' clearly illustrates the interesting effects of using R. simiarum as a parent; in this case the resulting hybrid produced flowers about 9 centimeter (3½ inches) in diameter when the plant was still only about 20 centimeters (8 inches) tall. Because of continuing slow growth it can accurately be called a dwarf, and it is quite hardy. The entire group of R. simiarum derivatives is worthy of its own separate report, but such a report must be deferred until more is learned.

How Hardy Are They?
The health and life span of a rhododendron plant is influenced by many factors: not only the temperature and the amount of sun, but also the humidity of the air, the wind velocities, the soil conditions, and the amounts of rain (or of artificially applied water). In addition, diseases such as the Phytophthora infection can have a detrimental effect.

With such complex interaction, the effects of any one factor cannot be isolated completely from the effects of others. Therefore if we decide that a plant is (for example) sun-hardy, we are making an empirical judgment. This judgment is at first very tentative and can only become more reliable after repeated trials under varying conditions.

This need for repetition and thoroughness was clearly understood by Koichiro, who made a huge number of trials and retrials under all conditions including the most difficult, and who continued individual trials over periods of several or many years. Therefore his observations can be considered reliable.

Some of the Wada rhododendrons are now commercially available in Japan. Also during an introductory period, approximately 5000 plants have been distributed for testing to approximately 1000 Japanese horticulturists living in various parts of the country. Their reports should further substantiate hardiness, also should yield information on the adaptability of particular cultivars to the various climatic zones.

These reports will undoubtedly confirm that 'Taiyo' is the hardiest in difficult summer climates. Its temperature tolerances of 34° -36°C. (93°-97°F.) are well confirmed, and much higher tolerances seem certain but are not yet documented. This cultivar, moreover, needs neither shade from direct sun nor any watering other than that supplied by normal summer rains of Japan.

Other metternichii-group derivatives, such as 'Wada's Triumph', 'Giant Ball' and 'Michiko', are not as tough as 'Taiyo'. If exposed to the conditions that 'Taiyo' withstands so easily, they survive but show signs of stress. Consequently 32° C. (90°F.) is considered the safe limit for these latter, and they grow best where given some shelter from sun and wind.

For 'Wada's Yellow' - the R. hyperythrum hybrid - 32°C. is likewise considered the safe limit, and is only tolerated with sun and wind protection.

These results illustrate the author's still-rather-rough scale of hardiness. 'Taiyo' is rated as C1, most of the metternichii-group derivatives as C2, and 'Wada's Yellow’ as C3. Rhododendrons rated less hardy than C3 cannot survive in the lowland climatic conditions of Japan; these include all Japanese native species - except certain azaleas - and most hybrids from Western countries. Yet some of these hybrids were thought worthy of trial, and the hardiness ratings developed at Yokohama were: 'Vulcan's Flame'-C3; 'Anah Kruschke'-C2/3; 'Purple Splendour'-C3; 'Anna Rose Whitney'-less hardy than C3.

To illustrate the reactions of these Western cultivars to the Yokohama climate, 'Vulcan's Flame' is a good example; this grew rather well when still young, then with increasing age it started to show signs of stress and finally perished. Such reactions cannot be seen in the course of a year or two, and a period of 10 years or more may actually be necessary for definite evaluation.

The truly hardy rhododendrons are those rated C1. Even among the many Wada cultivars, the only ones with this rating at the time of writing are 'Taiyo', 'Kooen', 'Isioroshibi', 'Yakusimiarum' ( R. simiarum x R. yakushimanum ), and 'Hypersimiarum’ ( R. simiarum x R. hyperythrum ). The number of these C1 plants will, however, increase, since many seedlings in the newer generations of Wada hybrids are giving an indication of adequate hardiness.

photo by Tomoo Wada

The mechanisms of heat and sun resistance are not known with any certainty; for better understanding we need to know more about plant form and structure (i.e., morphology) as well as the functions and activities of the plant's organs, tissues, and cells (i.e., physiology). Then, to find out more about the inheritance of summer hardiness, we will have to rely on the geneticists. These various kinds of investigation apparently have a long way to go, and full understanding of the subject appears to be a rather distant goal.

Worthwhile clues can perhaps be obtained from investigation of the normal and abnormal contents of plant tissues, particularly the amounts of water present. And for this investigation, it is necessary to understand the plant structures and organic compounds involved in the accumulation, storage, and release of water. As part of this investigation, attention might be given first to the control of the leaf openings (stomata) which allow escape of moisture, and second to the presence or absence of any mucilaginous or gummy substances which hold water tenaciously, as in desert plants.

George W. Ring (1981) has pointed out that summer-hardy rhododendrons should, and do, have large, vigorous root systems. 'Taiyo' is a good example of this, with its huge, semi-spherical root system - 1.2 to 1.5 meters (4 to 5 feet) in diameter for a plant about 3 meters (10 feet) tall. This mass of roots obviously serves as a good supplier and tank of water for the plant, enabling it to cope with high summertime rates of transpiration. Large roots, however, do not necessarily indicate heat and sun resistance, since a plant without such resistance may respond to a cool, moist environment by producing a large root system.

Heat and sun resistance merge with another character which is related but not identical: dry air resistance. To date, somewhat different lists of rhododendrons have been recommended for damp-summer and dry-summer regions of the U.S.A. (Ring 1981), and further testing may or may not confirm that different plants are really needed for the two types of region. In lowland Japan, the weather is driest during winter, intermediate during spring and fall, and during summer is wet for humans but still quite dry for rhododendrons, since the plants need to compensate for high rates of transpiration induced by heat and wind. Somewhat similar conditions are apparently found along the U.S. east coast.

Concerning daily temperature cycles, various growers have noticed that cool nights seem to help plants endure hot days. Specific reasons for this are unclear but may include the following;

—  Retention by the plants of a metabolism pattern adapted to mountains, where day-night temperature differences are often large.

—  Condensed moisture on leaves and soil surfaces during cool nights.

—  Discouragement of Phytophthora infection.

From a practical point of view, it is important that almost all the Wada rhododendrons are immune to attacks of Phytophthora. This immunity, besides allowing healthy plant growth, makes less worrisome the problem of precisely controlling soil moisture.

The Future
Koichiro Wada died February 13, 1981, at 73 years of age, because of a sudden heart problem. His work and his dreams were bequeathed to the author, who now must carry on. 'Taiyo', already known to be hardy against heat, has now proved able to resist air pollution, air salinity, and the effect of calcium from concrete. These findings open the possibility that 'Taiyo' and similarly hardy Wada rhododendrons will succeed as park or street plants in Japan's larger cities. The creation of such plantings, freely blooming in the busy and heavily populated areas, was a dream of Koichiro from the time of his visits to the gardens of England.

The author intends to select new cultivars from the many seedlings left behind. Among those already chosen are 'Spica' and 'Sophia', each with unique new flower character.

photo by Tomoo Wada

photo by Tomoo Wada

It was an honor for the author that the American Rhododendron Society provided this opportunity to write an article about the hybridizing accomplishments of Koichiro Wada, and acknowledgment should be given to Frank Doleshy for his excellent editing work. In addition, exchange of information concerning the physiological and morphological aspects of plants with Miss Mikiko Nihashi was helpful to the author.

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