QBARS - v35n2 Rhododendrons and Hot Weather

RHODODENDRONS AND HOT WEATHER
George W. Ring, Fairfax, Virginia

In a summary of the Breeder's Roundtable at Hyannis in 1980, I had the temerity to state that most of the hybridizing of rhododendrons is aimed at extending their cold hardiness, and that what work there was being done with rhododendrons for hot climates was mainly concerned with improving azaleas. I suspect that it was this statement plus our past record breaking hot summer that has gotten me into trouble; for, here I am writing about rhododendrons adapted to hot weather.
Many people are interested in rhododendrons which can take summer heat and still be attractive plants, in or out of flower. We, all, probably want to know the identity of heat resistant plants and, in the back of our minds, we would like to know why some rhododendrons do better in hot conditions than others. I will try to point out some rhododendrons which have been found, through experience, to be better performers in hot weather than the average. I will also touch on the "why" - but information in this area is scarce and so my discussion must not be taken too seriously. We, all, surely need to know more about "why" some rhododendrons can survive the rigors of hot weather; but especially, hybridizers need to know.
The most heat tolerant rhododendron known to me is the replica on the gate to the Royal Botanic Garden at Edinburgh. Being cast iron, it is immune to heat, cold, insects, and other causes of distress in some rhododendrons, more or less. Although I would like to have the replica as a garden ornament, living rhododendrons are much more exciting with their constantly changing character and inspiring flowers.
A large majority of rhododendrons in the wild (species and natural hybrids) are found in cool moist climates nearly matched at the gardens at Edinburgh. However, in other areas of the world where rhododendrons are now being grown, cool moist periods are limited to the spring season. Many parts of the northeastern United States with cool spring seasons have winter conditions much more severe than most Asiatic species are accustomed to in their native habitat; and so there has been with much success considerable hybridizing to develop more cold hardy rhododendrons. Even with the great progress already made in this direction, a wider range of plant and flower character in cold hardy rhododendrons is still needed and further improvements should, and are taking place, with many new, tough varieties coming in the future. With these developments of new rhododendrons having more cold hardiness, improvements in heat resistance should be expected, but not in every case, as will be discussed later.
After studying the available literature on heat tolerant rhododendrons, one must come to the conclusion that the major effort has been aimed at identifying those kinds already in existence which perform better than the average in hot climates rather than developing new varieties specifically for hot climates. A little later I will discuss some of the identified varieties.
The most work on developing new heat resistant varieties is reported by K. Wada, who has been particularly interested in rhododendrons which can survive the heat and humidity of summer in the Yokohama area. He has worked with both broadleaved rhododendrons and with evergreen and deciduous azaleas. Also, Dr. John Thornton in Louisiana is actively hybridizing for heat resistance after isolating suitable species and hybrids for parents. There are no doubt others but they are unknown to me at this time.
Heat resistance means different things to different people depending on where they live. The requirements vary considerably for Long Island, Dallas, Washington, D.C., and the interior valley in California. For example, not only does the degree of heat tolerance required vary with those areas but so does the required cold tolerance. A heat tolerant rhododendron such as R. leucaspis in southern California may not survive the cold winters of Washington, D.C. To be sure, heat tolerance is not a plant characteristic in itself but a name given to a rhododendron's collective resistance to physiological processes, insects, and bacteria associated with hot weather. Some specific types of hot weather stress in rhododendrons are:

1. Sun scald of leaves (sunburn?)
2. Collapse of cells for lack of moisture
3. Death of roots at high temperatures
4. Development of fatal or disfiguring disease associated with hot weather, such as Phytophthora cinnamomi, powdery mildew, petal blight, and others
5. Attack by insects associated with hot weather
6. Other problems yet to be identified.

To minimize these problems, it may help to examine some of the factors involved in the heat tolerance of rhododendrons. There seems to be a surprisingly long list, falling into three categories, all interrelated to the heat tolerance of rhododendrons. These three categories are:

1. climatic environment
2. planting medium
3. plant characteristics.

Taking each of these categories in turn, it seems that some of the climatic factors that affect plant performance in hot areas and, to some extent, in cold areas as well, are:

1. Ambient (air) temperature
2. Relative humidity
3. Soil temperature
4. Rainfall - frequency, intensity, and duration
5. Solar radiation (sunlight) - intensity and duration
6. Wind velocity

There is little information to be found on the degree to which each of these factors, alone or in combination, affect the performance of rhododendrons. What we know seems to have been deduced from trial and error; and therefore, predicting the performance of new rhododendrons is most difficult. A rough prediction might be based on the known performance of those species and hybrids which make up the parentage of a given rhododendron. Even here, there can be offsetting weaknesses and strengths in the combination. For example, it was found in Japan that hybrids of R. metternichii and 'Ruby Hart' were much more resistant to hot conditions than either parent.
Although we may not have quantitative information, it is well known that each climatic factor has a profound influence on the way rhododendrons grow. Wind is the nemesis of rhododendrons as it tends to desiccate the foliage, both summer and winter. It can also remove Btu's quickly and cause rapid freezing in the winter. In very windy locations, it can blow the leaves off of some plants. Large-leaved rhododendrons at some places in Nova Scotia and western Pennsylvania have resembled broomsticks after a particularly windy winter. It should be noted that at least one rhododendron, called 'Australis Borealis', selected by Warren Stokes and grown by Weldon Delp, can retain its leaves with no damage after 50 mph winds at -20°F. Resistance to desiccation by wind is both a heat tolerance and cold hardiness advantage, and where it is a factor in hot weather performance, rhododendrons developed for cold windy areas are likely to have higher resistance to wind effects in hot climates as well.
Solar radiation, of course, has a lot to do with air and soil temperature, and especially when plants are in full sun. Some rhododendrons which can take lots of heat, such as 'Scintillation,' cannot take full sun in Washington, D.C. as the leaves yellow from sun scald. I am not sure whether sun scald is plant cell destruction, as in sunburn of humans, or some other phenomenon. Concerning day length, it would also be useful to know more about metabolism rates of rhododendrons, especially the differences between those which grow rapidly during short growing seasons (alpine) and those adapted to long growing seasons, such as the vireya rhododendron and other species indigent to areas closer to the equator. For example, if some species and hybrids have low metabolism rates, then they might be better able to cope with hot climates with long days, bright sunshine, and along growing season. Perhaps we should examine and compare the metabolism rates of R. chapmanii with that of R. lapponicum to learn more about the relationship of this characteristic to heat tolerance.
Rhododendrons revel in moisture, and for most of us, this comes from rainfall, perhaps with some supplementary watering during low rainfall periods. The water also cools the air through evaporation. The relationship between precipitation and evaporation is also important, for in those areas, usually hot, where annual evaporation exceeds rainfall, salts from in the ground are deposited at the soil surface. Rhododendrons and most salts are not compatible. In moisture deficient areas, sufficient supplementary watering is needed to dissolve and flush away the salts.
It has been reported that there are some partially salt tolerant rhododendrons, mostly evergreen azaleas, being identified and developed in the hotter areas of California.
Some rhododendrons have difficulty growing in high temperature soils because of the death of their roots, or because Phytophthora cinnamomi becomes very active in poorly aerated soils at temperatures between 80 and 90° F. Perhaps a good way to identify those plants resistant to these problems is to grow them in heat collecting black containers with un-sterilized, very moist soil. Hot weather should then separate resistant and non-resistant types. However, this treatment may be more or less severe than conditions in your garden.
There have been a number of observations that most rhododendrons can take a lot of hot daytime weather if nighttime temperatures are cool. We do not know if this is because the cool nights provide a rest period from growing, the average soil temperature is less critical, the contrast between day and night promotes condensation of moisture, or perhaps good air drainage on hillside plantings. Whatever, we usually cannot air condition our plants as we do our houses - but we can mist our plants in the evening or at night and give them a cool night out.
The saying that nobody does any thing about the weather applies to most of our rhododendrons; hence, the search for, or need to develop, heat tolerant rhododendrons. However, there are things we can do to reduce the effects of hot climates, such as:

1. Irrigate (sparingly in the root zone so as to minimize Phytophthora).
2. Plant in shade or partial shade; northern exposures are especially good.
3. Plant in raised beds in well drained mediums.
4. Protect from drying and damaging winds.

Closely related to the weather in how rhododendrons perform in hot climates is the growing medium. Some soil factors which are related to heat tolerance in rhododendrons are:

1. Moisture holding capacity
2. Permeability
3. Capillarity
4. Soil suction pressures
5. Density
6. Nutrient type and level
7. Soil moisture content
8. Mulching

The moisture holding capacity of a soil is that moisture content between soil saturation and the wilting point of the plant. It varies both with soils, and with plant type. Since rhododendrons have low root suction pressures (we think), a smaller amount of moisture in a given soil is available for them than for types of plants having higher root suction pressures.
Permeability is important because it determines how fast rainfall can penetrate the soil, and how fast moisture removed in the root zone of rhododendrons can be replaced by adjoining water. Clean sandy soils and peats have high permeability. Clays have very low permeability.
Soil capillarity is high for clays and exceedingly low for organic materials.  An intermediate value is desirable for pulling groundwater from below into the root zone in areas where rainfall is deficient or sporadic.
Soil suction pressures are related to capillarity and moisture content. Dry clay soils have very high suction pressures, and the low suction pressures of rhododendron roots tend to lose out in the competition with clays for water.
Soil density affects permeability, aeration, and resistance to root growth. Dense soils may also inhibit the well-being of mycorrhizae. In general, then, rhododendrons like loose, well-aerated soils but, as a warning, so do root weevils. In some areas, rhododendrons grow in almost pure clay soils but, in these cases, it is almost always found that these soils, consisting of clay size particles, are "inactive," being primarily either iron oxides or kaolinitic, rather than highly expansive clays such as montmorillonites. It is certainly true that most rhododendrons adapt their root systems somewhat to the type of soil in which they are grown, and will develop coarser, stronger roots in heavy mediums. This does not always mean that they will grow better by comparison.
Soil nutrients are important; however, it is surprising how low a level of nutrients is needed to sustain growth. The effect of nutrients is chiefly on the degree and luxuriousness of growth, resistance to stress, ability to set flower buds, and quality of blooms. Nutrients of native soils vary so greatly from place to place, even in the same garden, that you are pretty much on your own as to what to add, if anything. If you think your soils are lacking in nutrients, your local soil scientist may be able to advise you further.
In hot, long day-length climates, rhododendrons are often grown in other than soil mediums, and these conditions, with proper fertilization and plenty of water, result in many flushes of luxurious growth in a single growing season. Burdette in Clemson uses 2040-12 with eight soluble tract elements, plus ferrous sulphate, magnesium sulphate and gypsum (calcium sulphate).
Newton Edwards in Annandale, Virginia, reports that adding calcium to his growing medium enabled him to grow many of the D. Hobbie hybrids where he had been unsuccessful before. Ross Nelson in San Antonio, Texas, found that root rot and petal blight are not problems, whether from alkaline soils, water, or what, we do not know.
Soil moisture content in cool moist climates is seldom a problem. In hot climates it can be critical with only a narrow margin between too little for the plant and too much, which can encourage the growth of Phytophthora. When irrigation is needed, misting might be the preferred method, since it not only cools the air but provides the needed moisture with a minimum of water at the root zone. The use of wetting agents, such as Aqua Grow, seems to reduce the problem of too much water by increasing soil permeability and by decreasing the thickness of water films on particles of the growing medium.
Although mulch is not a soil factor, a number of studies have shown that mulching improves soil in the root zone (with the help of worms), admits even heavy rainfall to the soil, retards evaporation, and moderates soil from both high and low extreme temperatures. What more could you ask of anything you could do to help your plants? Mulching is probably one of the most important factors in rhododendron plant growth in both hot and cold climates. Everyone does it, even nature.
In summarizing this section on soil factors, there is much that can be done to help rhododendrons adapt to hot climates. Loose, well-aerated soils, or growing mediums such as pine bark, supplying moisture mainly by misting instead of root zone watering, mulching, and fertilization are all likely to result in better performance. Burdette has shown that good water drainage and aeration, achieved with a 4" layer of broken stone 12 to 18" below the root zone, is highly beneficial to rhododendrons in hot climates.
But what if we want to grow rhododendrons in full sun, without watering, and have attractive plants, in and out of bloom? What characteristic should the plants have? Probably the most important is (1) a deep, vigorous root system. Other desirable features are:
2. Heavy leaves, although some types, such as Blue Peter, are exceptions
3. Ability to control transpiration to correspond to moisture supply (leaf scales, hairs, and stomata are effective in reducing transpiration)
4. Ability to go semi-dormant in times of stress (is this related to metabolism rate?)
5. High root suction pressures
6. Ability of cambium layer to transmit water to leaves
7. Resistance to hot weather diseases and insects
8. Presence of surface waxes which reduce transpiration
9. Other?
How do we get these desirable traits by hybridizing rhododendrons? One way is accidentally. (Grow lots of seedlings, then select for the best ones.) Another way is through controlled experimentation and planned breeding. For example, if a hybridizer striving for heat tolerance decides to use R. maximum in a cross, he is likely to have more success by taking advantage of the high variability of this species
where adaptability is concerned. During a recent camping trip to West Virginia, I collected some scions of R. maximum growing in full exposure at 3800 ft., with the idea that these are more suitable as parents than those forms growing in their more normal habitat of dense, moist shade.
In California, Carl Deul found a form of R. occidentale growing at higher than normal elevations on the dry side of the mountain to be more vigorous and heat tolerant.
Perhaps the greatest potential in hybridizing to develop heat tolerant hybrids is by starting with identified heat tolerant species and selecting within these species for the best performers. Deul believes that the toughest, most hardy and heat tolerant species are those which have had to adapt (and were able to) to wide swings in climatic cycles.
He also notes that the Vireya species are the most heat tolerant of any segment of the genus. Perhaps the hybridizing technique described by Weldon Delp in "American Rhododendron Hybrids" will permit us to combine the heat tolerance of the Vireyas with some of our most cold hardy hybrids.
In trying to develop heat tolerant hybrids, Mr. K. Wada in Japan worked with species that he found resistant to high temperatures and high humidity in the Yokohama area. Some of these resistant species are:
R. tashiroi
R. metternichii , Aff.
R. metternichii , var. metternianum
R. arboreum , Blood Red
R. hyperythrum (or close to it)
R. scabrum
R. simiarum
R. mucronulatum
R. chapmanii
The last one he rates as one of the most dry air tolerant species in the world.
Some of his notable heat resistant hybrids include those of R. tashiroi , crossed with R. nudipes , R. reticulatum and R. weyrichii , also crosses between R. metternichii , R. hyperythrum and R. arboreum , Blood Red. He notes that although the R. arboreum is quite cold tender, its hybrids are very heat tolerant. Perhaps his most heat tolerant hybrid is thought to be a hybrid of R. metternichii var. metternianum and 'Sarita Loder', called Tomo-O. Mr. Wada has observed that Tomo-O prefers hot, dry climates, and does not do well in moist cool areas. This is another type of rhododendron performance we should strive to understand.
Mr. Wada believes that the ability of rhododendron roots to maintain active growth in hot weather is indicated by the condition of the roots at the end of a hot summer. Actively growing "white" roots are usually found on those varieties which do well in hot weather.
After evaluating many hybrids and species, Dr. John Thornton in Franklinton, Louisiana, has determined that some R. fortunei , R. griersonianum and R. arboreum hybrids are fairly heat tolerant. Many of these are listed in Table 8. An interesting finding is that the form of R. arboreum he grows is susceptible to crown and root rot there while its hybrids are not. R. delavayi , closely related to R. arboreum , seems to be quite root rot resistant. He has decided to use the R. delavayi and also R. hyperythrum as primary sources of heat resistance in the parentage of his future hybrids.
Perhaps the lowland forms of R. catawbiense , as identified by Chris Early, will be useful. There is supposedly a collection of this form of R. catawbiense (insularis) maintained at the University of North Carolina by Dr. W.C. Coher.
R. atlanticum and R. speciosum are recommended by Fred Galle as more heat tolerant than many of the other American natives. For insect resistance, R. arborescens is hard to beat - it is difficult to find either an insect or mildew problem on this species.
Sandra Spencer recommends R. aberconwayi , R. arboreum , R. bullatum , R. bureavii , R. canadense , R. hemitrichotum , R. hippophaeoides , R. leucaspis , R. lutescens , R. racemosum , and R. viscosum as good hot weather species for hot areas of California. About half of these are also fairly cold hardy. Her notes on growing requirements for these, and many hybrids, are excellent.
Another species which may be useful is R. smithii . A form of this species does very well for Don Kellam in Charlotte, North Carolina.
Rhododendrons species in the Ponticum and Fortunei series are considered to be heat tolerant. Their good root systems and resistance to insects may be major factors here. Work on selecting particular forms is needed. Although it does not seem to be mentioned in the literature, some of the Maculiferum subseries of the Barbatum series, such as R. anwheiense , R. strigillosum and others, should be looked at more closely. I am sure that a thorough study of all the species will uncover more that might be useful to the hybridizer working on heat tolerance.
Since heat tolerance in rhododendrons is closely interrelated with cultural conditions, those varieties reported to be heat tolerant by others may or may not be heat tolerant for you. It will depend considerably on whether your cultural conditions and climate are similar, and if not, on how the reported varieties respond to any differences. Articles on heat tolerant varieties would benefit from more information on humidity, soils, rainfall, air temperatures, low winter temperatures, wind, light exposure, and other conditions in order for the reader to relate the performance of recommended varieties to his conditions.
However, those varieties recommended in articles previously published are a starting point, and knowledge of them may be useful information for both the beginning rhododendron gardener and the accomplished hybridizer.
The following nine tables of hybrids and species, regardless of how they have been grown, include plants that have been recommended by others to be more tolerant of warm to hot growing conditions than most other rhododendrons. Of course, some were evaluated over a longer time than others and most varieties have been available for quite a few years. As hybridizers continue to improve the quality of rhododendrons, the lists of heat tolerant varieties may look quite different 20 years from now.

1. A Survey of Good Performing Rhododendrons in the Atlanta Area.
2. Raising Rhododendrons in Atlanta, Georgia.
3. Top Ten Hot Weather Rhododendrons, Northern California - Interior Valley.
4. A Few Kings.
5. Rhododendrons After a Hot-Dry Summer in Washington, D.C.
6. Best Heat and Sun Rhododendrons in Annandale, Virginia.
7. Some Rhododendron Species Identified as Heat Tolerant.
8. Rhododendrons in Franklinton, Louisiana.
9. Rhododendrons for Charlotte, North Carolina.

In Tables 1, 2, and 5, only those rhododendrons are listed which were given the highest performance ratings by the authors of the referenced articles in the bibliography. It is quite noticeable that of the listed varieties, many heat tolerant varieties are not suitable for growing in areas having cold winters. Sometime in the future when we have a better handle on more of the factors that affect heat and cold hardiness in rhododendrons, perhaps we will understand why many plants are tolerant of either hot or cold climates, but not both.
Some varieties appear on several of the lists, including 'Mrs. Tom Lowinsky,' 'The Hon. Jean Marie de Montague,' 'Vulcan,' 'Mrs. A. T. de la Mare,' 'Van Nes Sensation,' 'Anna Rose Whitney,' 'Purple Splendour,' Bosley Dexter 1020, 'Holden,' Roseum Pink, 'Caroline,' and others. The last four are also reliably cold hardy for me in Fairfax, Virginia.
Further insight into heat tolerant varieties may be gained from information published in the "Good-Doer" section of American Rhododendron Hybrids. The recommendations of the Azalea, Piedmont, Southeastern, William Bartram, Monterey Bay and Southern California Chapters include many heat tolerant varieties.
Evergreen azaleas have not been emphasized in this article because they are generally much more tolerant of hot weather and, in the hot humid areas of the southeastern United States, are much more often grown than so-called "rhododendrons." The name "azalea," Linnaeus derived from a Greek word meaning "dry" as he noted that many azaleas grew in dry locations. Be that as it may, azalea hybrids, compared to other rhododendrons in their present stage of development, are generally more adaptable. Wada notes that the Hirado azalea hybrids, derived from R. scabrum , are exceptionally heat tolerant. According to him, a hybrid of R. scabrum and 'Mucronatum,' called "Omurasaki", has considerable winter hardiness but
grows well in a daily temperature average of 70 ° F.
What can be done to narrow the gap between the heat tolerance of azaleas and rhododendrons? Perhaps this discussion has suggested a few possible approaches. The subject is certainly a broad one.

Bibliography
"Dwarf Rhododendrons at Media Pennsylvania", Virginia Jeffries, ARS Bulletin, Vol. 17, No. 1, January 1963.
"Observations on Heat Tolerance", Ben Lancaster, ARS Bulletin, Vol. 19, No. 2, April 1965.
"Hot Weather Rhododendrons," Francis N. Mosher, Jr., ARS Bulletin, Vol. 19, No. 4, October 1965.
"Breeding Rhododendrons and Azaleas", K. Wada, ARS Bulletin, Vol. 19, No. 4, October 1965.
"Breeding for a Purpose", A. E. Kehr, ARS Bulletin, Vol. 20, No. 3, July 1965.
"Rhododendrons in the Philadelphia Area", F. J. Sholomskas, ARS Bulletin, Vol. 20, No. 1, January 1966.
"Raising Rhododendrons in Atlanta, Georgia", S. Chris Early, ARS Bulletin, Vol. 20, No. 4, October. 1966.
"Rhododendron Experiences South of the 30th Parallel", Arthur J. Coyle, ARS Bulletin, Vol. 20, No. 4, October 1966.
"Azaleas Bring a Fresh Glow to Dallas", ARS Bulletin, Vol. 21, No. 2, April 1967.
"Rhododendrons After a Hot Dry Summer in Washington, D.C.", G. W. Ring, ARS Bulletin, Vol. 22, No. 1, January 1968.
"Rhododendrons in Japan", K. Wada, ARS Bulletin, Vol. 22, No. 3, July 1968.
"A Survey of Rhododendron Hybrids and Species in the Atlanta Metropolitan Area", Robert L. Schwind, ARS Bulletin, Vol. 25, No. 1, January 1971.
"Heat and Dry Air Tolerance of Rhododendrons", K. Wada, ARS Bulletin, Vol. 27, No. 1, January 1973.
"Some Environmental Factors Affecting Rhododendron Wilt Caused by Phytophthora cinnamomi "; Michael J. Trombino, Jr., ARS Bulletin, Vol. 27, No. 3, July 1973.
"Summer Heat Loving Rhododendrons", K. Wada, ARS Bulletin, Vol. 28, No. 1, January 1974.
"A Heat Tolerant Form of R. occidentale", Carl A. Deul, ARS Bulletin, Vol. 32, No. 1, Winter 1978.
"Some Notes on Growing Rhododendrons in a Hot Climate", Sandra Spencer, ARS Bulletin, Vol. 27, No. 1, January 1973.
"More on Heat Tolerance", Carl A. Deul, ARS Bulletin Vol. 32, No. 3, Summer 1978.
"A Few Kings", Ross B. Davis, Jr., ARS Bulletin, Vol. 33, No. 2, Spring 1979.
"Growing Vigorous Rhododendrons", Henry Burdette, ARS Bulletin, Vol. 33, No. 4, Fall 1979.
Personal communication with Newton Edwards, Annandale, Virginia.
"Breeding for Heat Tolerance", Ross Nelson, 1973 Breeders Roundtable.
"How to Succeed in Producing Rhododendron Seed", Weldon E. Delp.
American Rhododendron Hybrids, ARS, 1980, edited by Meldon Kraxberger.
Personal communication with John T. Thornton, Franklinton, Louisiana.
Personal communication with Charles Dewey, Jr., Charlotte, North Carolina.