Heat and Dry Air Tolerance of Rhododendron
K. Wada, Yokohama, Japan
People who want to grow Rhododendrons in less favored climates are usually aware that they must select plants which are sufficiently winter-hardy. Also, if they have read much about rhododendrons, they are aware that winter-hardiness is not simply a matter of lowest temperature but is also much affected by the climate during the autumn period of hardening, as well as the ground moisture and air humidity during winter itself. Thus the selection of plants which will survive the winters in a particular garden is something of a trial and error process. Heat tolerance is another requisite for growing rhododendrons in less favored climates.
The heat tolerance of a particular variety in one's garden may be checked easily and precisely, using the following method:
Carefully dig up the variety in question at the end of a long spell of summer heat, generally at the end of August, and examine the hairy roots.
If these roots are white and fresh, the variety is easily able to tolerate the heat conditions where it is grown.
If these roots are pale brown but look alive, and are making little or no new whitish growth, this is a sign they have suffered from heat during the summer. Such kinds are not sufficiently heat tolerant for the place where grown, and they cannot make as good a show of flowers as in more favored climates. Their foliage color tends to become yellowish during a spell of heat and they are thinner in growth, with fewer twigs. These kinds will gradually produce white hairy roots as cool autumnal weather comes, and they may recover from their suffering eventually, but the repair of the heat damage consumes strength which could otherwise go toward good growth and a good show of flowers the next spring.
If the hairy roots are darker brown and thoroughly dead without whitish growth, the variety is completely unsatisfactory for the place where grown. Such a summer-tender variety may narrowly survive the first summer, slowly reproducing white hairy roots when cool autumn weather comes, but may not be able to survive another summer after loss of vigor from the first such season. 'C.I.S.' is one of the varieties with the least heat tolerance here and may perish in the first or second summer without white hairy roots. On the other hand, if killed by Phytophthora cinnamomi, white hairy roots remain alive for a while, so the cause of death can be identified if the plant is dug up soon after the foliage has withered.
To check the heat tolerance of various rhododendrons more methodically, plant them in pots during the spring and observe the condition of the hairy roots at the very end of summer. Knock the root balls out of the pots and, if you cannot find any white hairy roots, the varieties are not tolerant of your summer temperature and not suitable for your garden. If you find only a limited quantity of white hairy roots, the plants can survive but cannot produce as good a flower show as in cooler summer areas. Perfectly heat tolerant rhododendrons for your climate should have white hairy roots throughout the whole year.
Another factor affecting summer survival is dry air tolerance. This has not been given much attention in the literature and is perhaps confused with heat tolerance. In reality, this is a separate matter and is related to the fact that wild rhododendrons are nearly always found in areas of high humidity, whether warm or cool. Not surprisingly, their leaves are generally of such physiology that they operate well in humid air but badly in dry air. Yet, this is a characteristic which varies from variety to variety and should be considered in selecting plants for our gardens or breeding new ones.
An example of a cultivar which tolerates heat but not dry air is 'Jean Marie de Montague'. With us, in Yokohama, it does not grow satisfactorily under outdoor conditions. But, in a greenhouse, with restricted ventilation and higher humidity, and, inevitably, higher temperatures, it grows well. Digging it for examination at the end of summer, we find white hairy roots, indicating that the variety is sufficiently heat tolerant for us and that the problem is one of not being able to stand the dry summer air. The kinds we must look for, or breed, for Yokohama and similar places, are the ones that do not fail either of these tests.
I have been breeding "rhododendrons for our climate" in my Yokohama nursery for nearly 30 years. This nursery is on rather steep ground, facing south, and receives full sun the whole day long and a full amount of wind. The soil is volcanic and very good for rhododendrons, but the other environmental factors are bad, with much heat and very low humidity during the summer.
I have raised more than three million seedlings during these 30 years and exposed them here for selection by this very unfavorable climate. A very few have survived, and these are now making small jungles. Some are growing like evergreen oak, but others are succumbing even when more than 10 years old, presumably because of some final environmental test which they failed to pass.
I recently purchased a small plot of land in the Mt. Amagi Plateau, at an elevation of 3000 feet above sea level, about 50 miles south of Mt. Fuji in the Izu Peninsula. This highland is surrounded by the Pacific Ocean except to the north and has high precipitation and very high humidity. The average rainfall is around 150 inches, fairly well spread throughout the year but generally with a peak in June and July, and also with the highest degree of humidity during these months. This past July, humidity was nearly 100% day after day, and my planting of Exbury Azaleas suffered badly from rust on their leaves through this wet period. But evergreen rhododendrons were very happy, responding both to the moisture and to the air temperatures, which are generally 10 degrees Fahrenheit lower than at my Yokohama nursery.
Beginning last year, I started to test various kinds of rhododendrons at this place to see how they would differ from those grown at my Yokohama nursery. Many European and American hybrids which were not happy or could not survive the summer satisfactorily at Yokohama have done quite well and shown great improvement in this Amagi Plateau nursery. But, very interestingly, hybrids bred in my Yokohama nursery and selected by the climate there have not shown any difference in growth and rather seem to prefer Yokohama, with its hot, dry summer weather. Therefore, it seems, these survivors of the difficult Yokohama testing are no longer limited by heat or dry air in any significant way, and they can grow without impairment of quality in a summer environment in which most rhododendrons could not survive.
These results indicate that it is highly worth while both to breed for the ability to withstand heat and dry air and to test our existing plants for this ability. Through such testing, the American Rhododendron Society could do much to encourage the spread of rhododendron cultivation into less favored areas.
Incidentally, Rhododendron chapmanii, native in the U.S.A., is one of the most dry-air-tolerant species of the world. Another is R. mucronulatum, native to Korea. Hybridizing programs which include these species should produce new lepidote hybrids of value for the areas of dry air. On the elepidote side, a species from Formosa, similar to R. hyperythrum but not yet botanically named, is one of the few which tolerate dry air and would seem to be an interesting parent for more hybrids with this characteristic.