QBARS - v28n1 Summer Heat-Loving Rhododendrons

Summer Heat-loving Rhododendrons
by B. Wada. Yokohama, Japan

R. 'Tomo-o'
Photo by K. Wada

All Rhododendron growers, whether commercial or private, are aware that rhododendrons prefer cooler summer and milder winter climates with high humidity centering about 60° F. and without too many extremes. Therefore, the places where planted rhododendrons are now growing most luxuriously do not diverge from these requirements. I believe this is because most bio-chemical substances in the body of rhododendrons function most actively within this temperature range. But it seems that the story is not so simple as there are some kinds of plants on earth which grow better at a higher temperature range than the one stated above and probably there exist other kinds of plants which grow better at a lower temperature range. These exceptional kinds of plants may have some additional biochemical substances which may be able to modify, in accord with the temperature, the functions of the regular bio-chemical substances.
Indian azaleas, or as more generally called Belgian azaleas, are said to prefer the temperature range centering about 60° F. These greenhouse Azaleas are, therefore, now grown commercially in cooler summer areas most profitably. I understand the originating species, Azalea indica, was wild in a cooler summer area at a higher elevation of India. The Belgian azaleas are not very winter-hardy and at the same time are not very happy in a hot summer area. The tolerance of this azalea to temperature differences is narrow.
In Japan, we have some azalea species which grow well at low elevations on the extreme southerly part of Japan, Kawanabe Islands, Ryukyu Archipelago, Erabu Island, etc. These azaleas are botanically called Rhododendron scabrum and R. sublanceolatum . These islands are frost-free during winter and very hot during summer, over 80° F. throughout day and night for over six weeks in mid-summer with very strong sunshine. These wild azalea species and most of their hybrids, Hirado azaleas as they are called in gardens, grow better in a hotter temperature range than that centering around 60°F. Probably they prefer a range centering around 70° F.
In gardens in the Tokyo and Yokohama areas and similar latitudes, the hybrid azalea, called azalea 'Omurasaki', is very popular because it grows stronger and more rapidly than any other known Azalea. I have confirmed morphologically that this azalea is a hybrid of R. scabrum and R. mucronatum . The latter is the well-known white azalea in our gardens, being quite hardy. But the former is not so hardy and prefers a subtropical summer temperature range. The hybrid of these two Aaaleas, 'Omurasaki', is hardier than one of the parents, R. scabrum , and yet is at home in a subtropical summer temperature range.
Therefore, 'Omurasaki' behaves quite differently from other azaleas, such as Kurume, Satsuki, etc. 'Omurasaki' continues to prefer a temperature range centering on 70° F., but with the added acquisition of winter hardiness. In reality, 'Omurasaki' does not do so well in areas 10° F cooler during summer than the Tokyo areas, but it is quite winter-hardy there.
I do not know any species among the lepidote or elepidote species of rhododendron or among the Vireya species which do better in hotter or subtropical summer climates than they do in so called ideal rhododendron climates. I have been breeding rhododendrons in Numazu and Yokohama for nearly 50 years to get hybrids which do well in these climates which have spells of very hot temperatures at night during the summer. The high temperatures continuing through the night are very harmful to rhododendron welfare. It was a serious problem with me. Yokohama may have a climate similar to south of Washington, D. C. and Numazu has a climate similar to Florida.
On the small land on Mount Amagi Plateau, which I recently purchased, I am now constructing a rhododendron garden with greenhouses of my own new design which can provide an ideal environment to make very soft wood cuttings of rhododendrons, etc. most successful. This area is 3,000 feet above sea level and about 10° F lower both during summer and winter than Tokyo and Yokohama. The average annual rainfall is about 130 inches with very frequent fogs and the humidity is very high. It is the home of Rhododendron metternianum . Yokohama has an annual rainfall of about 45 inches. I at first thought that all kinds of rhododendron would prefer this highland climate. Contrary to this expectation, I have come across an exception. To my surprise, my hybrid, named Rhododendron 'Tomo-o', has not done as well in this highland as in Yokohama. The several plants of 'Tomo-o' for test were kept in a frost-free greenhouse in the highland but did not grow as vigorously as they do in Yokohama. I observed that the growth during the hottest days in summer is not as good in the highland nursery as in Yokohama. Therefore, I came to conclude that 'Tomo-o' is benefited by the high summer temperatures in Yokohama which induce it to make continuous growth throughout the summer.
Among, the European and American hybrids, so far tested from the same standpoint, I have found none. They stop their growth during the spells of high temperature in summer or even deteriorate. Their growth was much improved in the highland nursery because of the cooler summer and the higher humidity.
The reasons why 'Tomo-o' grows better in a hotter summer climate may be summarized as follows:

  1. Tomo-o' continues to grow vigorously from spring to the beginning of autumn without stopping its growth even under full sun during the hottest temperature spell in summer, whereas other rhododendrons are induced to stop by the heat spell. The functions of the bio-chemical substances which pertain to growth are so much decreased by the heat spell as to induce growth to stop.
  2. Tomo-o' seems to be a hybrid of long-day character, the typical long-day species, R. metternianum being one of the parents. This species is most related to R. yakushimanum but has more pronounced longday character to adapt to the climates in the more northerly distribution. In cooler summer climates the growth of 'Tomo-o' stops as soon as the days are shortened from the end of June (as R. metternianum does.) But in hotter summer climates, 'Tomo-o' seems to have the additional bio-chemical substances functioning which overcome the effect of the photoperiod and to make it continue to grow even during the heat spells of July and August.

I do not know whether the above theorem is correct or not but 'Tomo-o' is the Rhododendron which does poorly in so-called ideal rhododendron climates and does better where other rhododendrons fail or do poorly.
Readers may be interested in knowing the parentage of 'Tomo-o'. 'Tomo-o' is a hybrid of R. metternianum and 'Sarita Loder' but is picked from among the many seedlings of the same cross. It looks very unusual considering the parentage. It has glossy green leathery foliage with thin fawn indumentum beneath, characteristic of R. metternianum . The habit is compact and very twiggy, forming a shape like Table Pine with a flat head. The color of the flower is brilliant R. griersonianum red without spottings and markings, never paler than the color of a good R. griersonianum and it isn't faded nor burned by sun light. The individual flowers are 3½ to 4 inches across, making a truss of 12 to 15 flowers. As is generally so with all my hybrids, 'Tomo-o' appears to be immune to Phytophthora cinnamoni root rot disease as it was selected by the high summer heat weather in Yokohama.
I have not yet found a summer heat-loving hybrid like 'Tomo-o' among the existing European and American hybrids. But I think 'Ivery's Scarlet' should be tested from this standpoint though its growth is not as much affected by the photo-period as by temperature. I should like to have information from any readers about summer heat-loving Rhododendrons. A list of rhododendron hybrids of this category will delight many gardeners who have never succeeded in growing rhododendrons because of the summer problem.