QBARS - v28n4 Growing Rhododendrons in a Hot Climate

Growing Rhododendrons in a Hot Climate
Arthur W. Headlam, Bentleigh, Australia

It occurred to me that readers of the Bulletin, whose problems in the main are on the lower end of the temperature scale, may be interested to hear of some of our troubles when temperatures exceed the century mark.
We live at Bentleigh, a suburb of Melbourne, and our garden covers an area of slightly less than a quarter of an acre. Some sixteen years ago we became interested in rhododendrons, and decided to remove a number of the less attractive trees and shrubs to make way for some rhododendrons, leaving, however, a few trees to provide a certain amount of shelter. Amongst these were a pink and white flowering cherry, a flowering peach, a crab apple tree and a standard cherry, which makes an attractive sight when in flower, and later when the leaves appear provides some shade for a number of azaleas growing under its branches.
Soon the available space was filled with rhododendrons, and it was not long before our fruit trees fell victim to the axe to make way for more rhododendrons. Had all of the rhododendrons we planted 16 years ago survived, the garden would by now be a veritable jungle, but as is almost invariably the case with novices, we made a number of mistakes in setting as well as not purchasing the hardier varieties better to withstand our particular climatic conditions.
Melbourne, generally by oversea standards, has a relatively mild climate, similar in some respects to that of San Francisco. Winter temperatures rarely fall below freezing point, and the few frosts encountered usually in July and August are certainly not severe enough to damage rhododendrons. It is during the summer months that the most difficult conditions are encountered, when temperatures usually exceed 100° F., for up to 10 to 12 days each summer. On one isolated occasion, temperatures of 108, 109 and 110° F., were recorded on three successive days; fortunately these extremes do not occur very frequently Rainfall averages around 30 inches, spread fairly evenly over the year, resulting in rather wet winters and dry summers, which combined with hot drying winds from the inland, result in the rate of evaporation exceeding the rate of precipitation. Regular watering is necessary for four or five months of the year. Conditions can vary from year to year. We have on occasions had as much as nine inches of rain in February, our last month of summer, which is probably the most difficult month for gardens, particularly when the rainfall for the month may be as low as 15 to 20 points (the long term average for February is 193 points).
Our soil is a very fine loam overlaying clay, and has a habit of becoming very hard unless continually worked. As this is impossible with rhododendrons and azaleas, it has been necessary to work in before planting, copious quantities of leaf mold, compost, coarse gravel etc., to provide better drainage and aeration.
Possibly one of the greatest causes of failure of rhododendrons growing in Melbourne suburban gardens is that they are almost invariably propagated and grown on in the rich, acid, moisture-retentive soil of the Dandenong Ranges. Unless the root ball is soaked in water or hosed to remove most of the heavy soil, the roots are extremely reluctant to leave the soil to which they have been accustomed and spread into a different soil.

R. 'Sappho'
FIG.66. 'Sappho' requires no special care
in a warm Australian climate.

What appears to be quite adequate watering for the surrounding soil, and no doubt is, is not sufficient to penetrate the ball of mountain soil which gradually becomes harder and harder. Usually in a year or two the plant goes into a decline and dies. On lifting the rhododendron it is found that the soil has formed into a hard cement-like mass which is quite impervious to water. However, nurserymen are now aware of this difficulty and advocate removing all or most of the heavy mountain soil before planting in a different soil.
There are three distinct climatic zones in our garden. In front of the house facing north, and subject to full sun for most of the day, not even the hardiest of rhododendrons will survive. Around the margins of this lawn (American buffalo grass) has been planted roses and Sasanqua camellias. We have found that Gumpo Azaleas tolerate these conditions extremely well. Planted as a ground cover amongst the roses that are little affected by the heat, and flower profusely in the spring and summer.
The area at the back of the house is also quite hot in summer, but does have the advantage of some protection from the drying north winds. In this area are some of the hardier hybrids, 'Sappho', 'Van Nes Sensation', 'Blue Peter', 'Schubert' and 'Cynthia' as well as deciduous azaleas which do quite well.
It was found that the cherries and other shrubs provided reasonable shelter when rhododendrons were small, but in a few years time as their size increased, the advantage was lost and it was decided to construct a shade house some 40 feet by 50 feet and nine feet high, having slats which cut the light by approximately one third. On the western wall the slats were spaced to provide 50 percent shade, minimizing the effects of the hot afternoon sun. The western side has several removable panels which are dismantled and stored usually from about April to October, thus giving the plants the benefit of some additional sunshine after the worst of the hot weather has passed.
This area provides the most favorable conditions for growing rhododendrons. It seems a pity that so much protection is required when the heat damage is confined to perhaps only 10 to 12 days each summer, but several runs of two or three days with temperatures over 100° F., accompanied by hot drying winds, can have quite a devastating effect on rhododendrons and azaleas, or for that matter, many other garden plants.
It may be of interest to go back to the summer of 1967/8, which followed an exceptionally dry winter. Water storages were totally inadequate to supply a rapidly expanding metropolis, and severe water restrictions were imposed. Sprinkler systems of all descriptions were prohibited, as was the watering of lawns, and watering gardens had to be carried out by means of buckets or watering cans.
It was obvious that every assistance would have to be given if plants in the garden were to survive, and a mulch of four or five inches of pine needles (Pinus radiata) was spread over the garden with quite dramatic results in reducing ground temperatures and conserving moisture. As an indication of their efficiency, a thermometer was was placed with a bulb on a patch of sand in full sun and registered 145° F. Under five inches of pine needles, also in full sun, the temperature dropped to 95° F., and in the shade house under pine needle mulch the temperature recorded was 85° F., additional shade having been provided by temporarily thickening the top with additional slats to achieve about 50 percent shade.
'Lamplighter' and 'Rodeo', located on the east side of a high trellis wall did not actually suffer much leaf burn, but the foliage hung limply downwards and did not recover even during the following winter and spring. These were grafted plants, which at this time were imported from England and Holland, and it appeared that the graft was restricting the flow of sap in times of high rate of transpiration. It was not until some years later that I read Ten Van Veen's advice in his book, "Rhododendron in America". "If you live in a hot climate look for a graft mark on a rhododendron before buying - if the trunk shows that the plant has been grafted, it is not for you. This does not hold true for a colder climate," and my suspicions were confirmed.
Fortunately, during the ;previous season, a friend had pollinated 'Lamplighter' with a good form of a red R. arboreum and a number of seedlings were raised producing a diversity of hybrids, some resembling 'Lamplighter' in foliage and habit, but none quite as good in the flower, whilst others varied in color from light to dark red. One which has survived the heat and water shortages and is of very compact habit of growth, has smaller dark green leaves and produces well formed trusses of deep blood red with a slightly frilled edge to the floret - it flowers some weeks earlier than 'Lamplighter', a decided advantage in our climate, and the trusses have good substance and lasting qualities. 'Astarte' and 'Lady Chamberlain', given some shelter from afternoon sun do quite well and are attractive for their tubular-shaped flowers in loose trusses.
Rhododendrons in the shade house generally fared much better, but the R. griersonianum hybrids were the exception. 'May Day', 'Elizabeth', 'Matador' and 'Tally Ho' went into a decline and died in the following spring. One exception was the R. griersonianum hybrid, 'Damozel', in close proximity to and under identical conditions to 'Tally Ho'. However, this was a layered plant. It seemed little affected by the trying conditions, and is still flourishing.
There are a number of rhododendrons which will just not tolerate our summers in Melbourne, amongst which are R. williamsianum and its progeny, and R. orbiculare , as well as the large-leafed members of the family. Even under the most favorable conditions in the shade house, these cannot stand the high temperatures and low humidity. This is where some of San Francisco's fogs would be an advantage!
A hybrid, R. calophytum x 'Neon Lights', which resembles R. calophytum in foliage and habit growth is doing quite well, except that the leaves never attain the size of those of the same hybrids growing in the Dandenongs.
Rhododendrons which have stood the test of time in the shade house are 'Alice', 'Topsvoort Pearl', 'Crossbill' and 'Broughtonii Aureum', and amongst the species R. veitchianum , ciliicalyx and polyandrum . R. nuttallii has never lasted more than a couple of years. However, I intend having one more try, this time planting it in a container with a very open mix, consisting of fern fiber, peat moss, leaf mold and coarse gravel. This method appears to give successful results with some Melbourne growers, or, alternatively, planting in a large piece of fern log.
R. yakushimanum was not affected by the trying conditions, but some of its hybrids (open pollinated) reacted quite differently. The ones with heavy indumentum were quite unaffected, whilst others which did not inherit the indumentum suffered some leaf burn even in the shade house, and several of these eventually succumbed.
'Seta', 'Bric-a-brac' and 'Alison Johnstone' sited at the foot of other rhododendrons where they were protected from afternoon sun were quite unaffected, but 'Eldorado' in an equally protected position on the east side of a trellis espaliered with camellia 'Showa-no-sakae' completely defoliated and died within a matter of weeks.

R. 'Van Ness Sensation'
FIG. 65. Protected from drying winds
and drought, 'Van Ness Sensation'
grows well in a Melbourne garden.

In the years when there were no restrictions on the use of water, rhododendrons stood the heat remarkably well, the use of sprinklers and watering of lawns created a certain amount of local humidity, and overhead watering of foliage in the evenings did much to revive plan's which had perhaps wilted somewhat during the day. There is little doubt however, that once rhododendrons have had a severe setback such as in the drought of 1967/8, they never fully recover. 'Van Nes Sensation' and 'Hugh Koster', although noted for their hardiness, were two for which the summer 1972/3 proved to be too much.
As the population density becomes greater, with the inevitable increase in atmospheric pollution, it becomes more difficult to grow any but the hardiest of rhododendrons.
'Kluis Sensation', bred at Boskoop in 1946 by Anthony Kluis, is as one would expect, a late flowering rhododendron, and was amongst the many imports brought to Australia some 15 years ago. Its parentage on one side is said to be 'Britannia', and it produces on almost every terminal, well shaped scarlet trusses of medium size in early to late November (last month of spring). Despite the lateness of the season, these stand the heat remarkably well in our garden at Bentleigh. The plant is compact and fairly slow growing, and is well covered with dense dark green foliage. Whilst its late flowering characteristics are probably not ideal for our particular climatic conditions, its habit of growth and sun tolerance should make it a useful rhododendron for hybridizing.
I have tried on a number of occasions to get it to accept pollen from odd late flowers of "Elizabeth' and other hybrids, but without success, alternatively using it as the pollen parent also proved fruitless as it just does not appear to produce any pollen. Perhaps some one else may have more success with this rhododendron which appears to have such excellent possibilities. There is little doubt that with our mild winter climate the solution to our problems is the production of compact growing early flowering rhododendrons - I wrote on the efforts of one of our leading hybridists, Mr. V. J. Boulter, in the October 1971 Bulletin. One of his hybrids, 'Denise' ('Winter Favourite' x 'Chrysomanicum') starts to show color in the buds in midwinter. Its profusion of apricot flushed pink flowers in August will make a pleasing contrast to the many camellias which reach the peak of their flowering season at this time.
Rhododendrons growing in close proximity in small gardens tend to become leggy and have rather unsightly open spaces under the canopy of leaves which go up for the light. However, these spaces provide quite ideal positions for the growing of Malesian rhododendrons in containers which are afforded some protection against frosts and get sufficient sun in the summer to ensure a good bud set.
Indica Azaleas do well in shaded positions and flower over a long period, whilst the Kurumes, which are even more sun hardy add color to the garden, Hino de giri with its profusion of bright red flowers, and 'Kirin' and 'Hino Mayo' are favorites in the pink shades. Even more sun hardy are the colorful Satsuki azaleas with extremely large flowers and the ability of producing such a diversity of color on one individual plant.
Our Malesian rhododendrons, when in flower, are placed under a large porch at the back door; at the moment 'Pink Delight' is a picture with 14 trusses in all to flower. The additional shelter ensuring that each truss lasts for three to four weeks, except of course, when they are hand pollinated. Then the corollas drop within a few days.
Although the climate and other conditions limit the variety of rhododendrons which can be successfully grown in our garden, we are perhaps fortunate that we are only 18 miles by road from the Dandenong Ranges, the home of rhododendrons in Australia and the site of the Australian Rhododendron Society's National Garden at Olinda where it is possible to see rhododendrons in flower for seven to eight months of the year.