QBARS - v20n4 Rhododendron Experiences South of the 30th Parallel

Rhododendron Experiences South of the 30th Parallel
Arthur I. Coyle, Houston, Tex.

At the Editor's request we are reporting on our experience with growing rhododendrons in the Houston, Texas, area of the Gulf Coast. Here we normally have a climate which varies from hot and humid summers (90 to 105° F) to mild winters which never have temperatures lower than 10° above zero. A climatological summary of the 1965 Houston, Texas, weather is set out below. Data supplied by the Universal Weather Service.

Month of J F M A M J J A S O N D
Highest temp. °F 82 80 85 87 89 95 100 98 97 89 86 79
Lowest temp.°F 23 26 28 50 54 67 74 69 60 45 39 37
Avg. monthly temp.°F 56.0 55.1 58.7 73.5 77.1 83.0 85.1 83.5 81.2 69.4 69.1 58.9
Departure from normal, °F +2.4 -.07 -2.6 +5.0 +1.1 +1.4 +2.1 +0.3 +2.0 -2.0 +8.3 +3.2
Total prec., inches for month 1.87 3.27 0.81 0.95 6.53 3.06 1.57 2.29 3.56 3.09 4.82 6.15
No. days with excess of 90° or higher 0 0 0 0 0 20 27 27 18 0 0 0
No. days with excess of 80° or higher 1 0 8 22 30 30 30 31 28 14 10 0
No. days with minimum of 45° or lower 18 11 14 0 0 0 0 0 0 2 1 9
No. days with minimum of32° or lower 4 2 2 0 0 0 0 0 0 0 0 0
Avg. humidity at noon 66% 63% 61% 63% 59% 62% 59% 57% 62% 53% 61% 65%

The year 1965 was still experiencing the 15-year drought. Prior to 1950 the average rainfall was 56 inches per year. Precipitation for 1966 promises to return to normal.
Our experiences with the culture of rhododendrons ranges from some 20 years of observation of rhododendron culture during our travels in the Northeast to about 3 years of actually growing rhododendrons in Houston, Texas, and elsewhere in Texas and Oklahoma. Several years ago we investigated a number of failures of rhododendron plantings here in the Houston area. We found that all of these failures turned out to be plantings of 'Roseum Elegans'. Here, we find 'Roseum Elegans' to be most susceptible to root rot. This is especially so when planted in soils containing any appreciable amount of clay.
The greatest stumbling block to our getting started with rhododendrons in this area was the great difficulty we had in obtaining plants. About 4 years ago we started sending out letters to many rhododendron nursery people all over the country, trying to buy plants. Most of the people to whom we wrote either ignored our inquiries or wrote back and told us we were crazy to try to grow rhododendrons in the Southwest because they would not grow here. Finally, some of our letters arrived in the hands of the foremost hybridists and nursery people in the rhododendron field. These men took notice of what we were trying to do. Instead of skepticism, we received many letters of encouragement, suggestions on possible soil culture for this area and lists of plants which might grow here. Some of these fine people in the nursery business forwarded catalogues and/or shipped us plants with large root balls, which is so necessary for getting rhododendron established in a high temperature area. We, of course, surrounded ourselves with all of the literature on the subject, but it took the added personal touch from some of these experts in the field to help make our efforts a success. We are especially indebted to Dr. J. Harold Clarke and David Leach for the information they supplied us regarding the latest methods of controlling the two most dreaded fungus diseases which may prevent the growing of rhododendrons in a hot climate - namely Phytophthora cinnamomi and Rhizoctonia solani . That kindly Norwegian and hybridist, Halfdan Lem, has given us many rhododendrons to try in this area and he has written thousands of words of advice to us regarding the culture of rhododendrons.
This report is not one where we claim to be an expert on growing rhododendrons in a high temperature area. We feel sure there is much more to learn. We believe, however, that these beautiful plants can be grown in our area by using a little effort.
The four big requirements for growing rhododendrons in the Southwest Gulf Coast area are:

  1. Have sufficient shade available from east to west for protection against the direct rays of the sun. If shade is not available it must be made available.
  2. Have water available which does not have a pH higher than 8.5. Softened water from a water softener will kill any acid-loving plant, including all rhododendrons and azaleas.
  3. A light friable acid soil is a must. By friable we mean the soil should be loose either wet or dry. The pH of the soil should be from 4.5 to 5.5. If acid soil is not a natural occurrence in the planting area, then it can be made so by the addition of acid German peat moss and sulphur, or best of all-iron sulfate.
  4. A strict observance of the soil culture, as set out hereafter in this report, is recommended in order to be assured of success in growing rhododendrons in this area.

Frankly, we believe one should forget the idea of trying to grow rhododendrons in this area if the above mentioned conditions are not met.
At our home here in Houston we are presently growing 41 different rhododendron hybrid varieties and a total of 60 rhododendrons. Ages of the plants range from 2 to 5 years old. In addition to the rhododendrons growing at our home, we have about 10 rhododendrons growing elsewhere in Houston and at other locations in Texas and Oklahoma.

Types of Rhododendrons to Plant in a High Temperature Area
The selection of rhododendrons for growing in a high temperature area is related in some cases to the cold hardiness of the plant, with the theory in mind that cold hardiness and heat tolerance go hand in hand. This works out fine for hybrids of the Ponticum series, for many of them will tolerate both cold and hot weather. The Fortunei series hybrids are also at least heat tolerant. Recently, however, we were told by a noted Long Island rhododendron grower and hybridist that the Fortunei hybrids will not tolerate direct rays of a southern sun because their leaves turn yellow and the plants are killed unless shaded. Our own experience with our few plants of Fortunei blood shows this to be true. This is actually no serious problem because none of the Ponticum series hybrids which we have tested will stand a full Texas sun either. Shade is no problem in the south, because it can be obtained quickly one way or another. The happy combination of R. griffithianum and R. maximum resulted in the clone, 'Mrs. T. H. Lowinsky', a vigorous grower under high temperature conditions. Let's hope that more crosses between species of the Fortunei series and the Ponticum series will do equally as well.

Our experiences with the species show them to be much more difficult to grow in this area than the hybrid varieties. There are only four which we would recommend at this time. They are, R. catawbiense , maximum , smirnowii and argyrophyllum var. nankingense . We grow each under full shade. Other species which we hope to report on later are of the Triflorum series and the Arboreum series which, while showing great promise as heat resistant, are beginning to have some root rot troubles. We believe this can be corrected by using still sharper drainage and absolutely no clay. Rhododendron crinigerum definitely will not stand our heat. Its leaves burn off even under full shade. There is no doubt that the species which we have tried require much sharper drainage than the hybrids. A few of the hybrid varieties also require very sharp drainage such as 'Roseum Elegans' and 'Margaret Dunn Golden Belle'. Although we have by no means tested all of the so called Iron Clads we believe most any one of them will grow in the Southwest. There are also a number of the more tender ones which will do well here.

Rhododendron Hybrid Varieties Which Are Doing Well for Us
'Catawbiense Album' H-1 'Roseum Superbum' H-2
'America' H-1 'Album Elegans' H-1
'Nova Zembla' H-1 'Roseum Elegans' H-1
'Catawbiense Grandiflorum' H-2 'May Day' H-4
'Charles Bagley' H-2 'Margaret Dunn Golden Belle' H-4
'Anna Rose Whitney' H-4 'Mrs. T. H. Lowinsky' H-3
'Loder's White' H-4 'Anna' H-3
'Jean Marie de Montague' H-3 'Marinus Koster' H-3
'Big Ben' 'Holy Moses'
'Walloper' 'Vulcan' H-3
'Smoky No. 1' 'Betty Wormald' H-3
'English Roseum' H-2

NOTE: 'Roseum Elegans' must have half day sun here to set bloom buds. 'Roseum Superbum' and 'Mrs. T. H. Lowinsky' are the most vigorous. Most of the above varieties have bloomed here twice, a few only this year, all appear to be setting bloom buds for next year's bloom. R. catawbiense and R. maximum have bloomed twice here.

Unnamed Lem Hybrid Seedlings and Rooted Cuttings From Selected Steedlings.
Now 2-1/2 to 3 years old.
haemaleum X 'C. P. Raffill' 'Dido' X smirnowii
forresti var. repens X 'Letty Edwards' catawbiense album X 'Fabia' (Wizard A.E. )
'Fabia' X bureavii 'Fabia' X ('Tally Ho' X smirnowii )
'C. P. Raffill' X williamsianum ungernii X ( ariculatum X 'Romany Chal')
catawbiense X 'Fabia' ('Wizard' X 'Ole Oleson') X smirnowii
'Albatross' X 'Margaret Dunn' 'C. P. Raffill' X ( fortunei X yakushimanum )
'Vulcan' X discolor 'Rosa Mundi' X sutchuenense
'King of Shrubs' X decorum ['C. P. Raffill' X ( smirnowii X 'Azor')] X 'Albatross'
'Margaret Dunn' X 'Anna' 'King of Shrubs' X 'Souv. of Anthony Waterer'
('King of Shrubs' X 'C. P. Raffill') X smirnowii

NOTE: All of the above varieties are being grown in partial shade (little direct sunlight). Some indicate they would take more sun. The variety 'King of Shrubs' X 'Souv. of Anthony Waterer' should be in every garden even if it never bloomed. It is very vigorous and disease resistant and the beautiful long and narrow leaves are without a single blemish.
We would like to warn newcomers to the rhododendron field to try to obtain rhododendrons at least 3 years old with a large root ball for October or November planting only, in a high temperature area. Know what you are buying, buy by name, not by color. The large plants are much easier to grow. They are also much more resistant to disease and extreme climates. Fall planting in the south gives the plants sufficient time to increase root growth before hot weather sets in the next summer. Stay away from spring plantings in the south unless one has the time to stand over the plants with a water hose until their roots get set. Special conditions must be made for planting one and two year old plants. These small plants may not be disease resistant or have enough roots to be heat tolerant. Continuous insecticide and fungus spray applications may be necessary on small young plants. They can be grown and we have done so, but small plants require time and care. Buy only healthy plants so as to be assured of some success when planting them in an unfavorable environment.
The Houston soil at our home is mostly clay loam (60% clay and 40% fine sand) and is in an area of pines and oaks with an elevation of 50 feet above sea level. Our front yard contains a sandy loam fill which consists of a mixture of 60% sand and 40% clay. This is too heavy a soil for growing rhododendrons in a hot, wet climate but we have some success with a 50/50 mixture of this sandy loam and German peat moss. A few of our Ironclads do well in this soil mixture but it is not too good for 'America' or 'Roseum Elegans'. We do find that 'Roseum Elegans' will take all morning sun along with 'Nova Zembla' but 'America' and 'George Hardy' have leaf burn when not in full shade.
The 'Nova Zembla', 'Catawbiense Album', 'Album Elegans', 'Catawbiense Grandiflorum', 'English Roseum', 'Roseum Superbum' and 'Mrs. T. H. Lowinsky' will stand more clay in the soil than any of our other rhododendrons and we have had no root rot troubles with them in raised beds of a 50/50 mixture of Sandy Loam and German Peat Moss.
After losing a good many rhododendrons because of root rot from planting in soil mixtures containing too much clay, we started resetting our rhododendrons and azaleas in late spring of this year on still higher beds which contained no clay-just a 50/50 mixture of German peat and sand or perlite. We will finish this program on all of our rhododendron plantings in October of this year before the winter rains set in. The raised beds are 16 inches high while loose, but settle down to about 12 or 14 inches within a few months. Our rhododendron plantings are 5 feet in diameter for each setting and are spaced here and there in our azalea beds to take advantage of existing shade.
Prior to the actual setting of the rhododendrons in place in their new bed we take the following steps.

  1. Add 8 inches of coarse sand or perlite on top of the clay soil in an area 5 feet in diameter or larger if desired. Do not dig a hole in a clayey soil. To do so will create a bathtub situation-cause excess water to collect which in turn causes root rot fungi to multiply.
  2. Add 8 inches of German peat moss on top of the sand or perlite.
  3. Spread out over the peat moss, one quart of cotton seed meal, one quart of superphosphate, one cup full of fritted trace elements and a liberal dusting of chlordane and DDT.
  4. The above planting medium is well mixed with a power tiller which is set so as not to scrape up any of the clayey soil from beneath. If plantings are over alkaline soil, work in a heavy coating of sulphur with the alkaline soil twice the diameter of the rhododendron bed. Also add 10 lbs. of gypsum per 100 sq. ft. of area to loosen up the clay. Mix the new planting medium elsewhere and dump the mixed medium on top of the alkaline soil.
  5. Shape the bed to 5 feet in diameter and level off the top. This will result in a mound 16" to 18" high which will settle down to about 12 or 14 inches within a few months.
  6. Scoop out a hole in the center of the new bed, twice the diameter of the root ball all the way down to the grade of surrounding yard area. The removed planting medium should be set aside for later refill.
  7. Add enough sand or washed flint gravel at the bottom of the excavation to raise the bottom of the root ball at least 4 inches above the grade of the surrounding area, and the top of the root ball just level with the top of the new bed. The sand or gravel at the bottom of the hole will prevent the root ball from settling which must not happen.
  8. Set the rhododendron in its new bed-remove the burlap from the root ball-spray the root ball all around with a strong stream of water until at least 3 inches of root ends are exposed all around the root ball. Otherwise the roots may never leave the original root ball to grow into the new soil. The soil washed from the root ball is left in the hole around the root ball so as to inoculate the new planting medium with the beneficial mycorrhizal fungi from the original planting area.
  9. Fill the hole in the new bed with water-then add the medium which had been set aside to fill up the excavation around the root ball. Use a strong stream of water to mix the extended roots into the new soil. This also allows the new soil to settle without too many large air spaces. The bed is then leveled off to the top of the root ball and reshaped to 5 feet in diameter if necessary. Do not pack the planting medium. Let it settle by the weight of water.
  10. Add 5 gallons of aged cow manure over the top of the raised bed. Then add 6 to 8 inches of pine needles and/or oak leaves all around the rhododendron and extending out at least 12 inches beyond the diameter of the new bed. Water the mulch with a spray to soften it so that it will settle to the contour of the new bed. After several light watering to get the mulch to settle-turn on the sprinkler and soak the bed for 4 hours. Peat moss is slow to wet down. Water the rhododendron and bed once a week (if no rain) for the first month. It is very important that the mulch is never removed. To do so will kill a rhododendron in a hot climate. Additional mulch must be added from time to time as it decays. Pine needles last longer than oak leaves. Small mesh chicken wire can be stretched over an oak leaf mulch to keep it from blowing away. Gardeners living in a dryer climate may find it necessary to use only German peat moss without the sand or perlite, especially those who make plantings in deep sand with a low water table. If German peat is not available, then use only the coarse grade of Canadian peat moss. Never use fine peat moss or peat moss with bacteria added. The fine stuff is hard to wet and the other is soon decomposed. The German peat is long lasting.

Insect and Disease Control
We have found it necessary to drench the azalea and rhododendron beds with Lindane for some soil insects which are not killed with chlordane and DDT. We have also used Dieldrin which is very effective. For root rot control we use a Terraclor drench for control of the Rhizoctonia fungi and a Dexon drench for control of the Phytophthora type fungi. Now that we have quit using any clay in our planting medium it may not be necessary to continue the soil drenches for control of these two fungi, but we will continue to do so until we develop more experience on the subject.
We control the die-back blight by cutting off the infected limb below the canker and immediately spraying the plant with 2½ tablespoons of Ferbam - 2½ tablespoons Captan and one teaspoon of Malathion per gallon of water. The liquid Malathion has been satisfactory for our broad leaf rhododendrons and evergreen azaleas but it will kill all the deciduous azaleas. These deciduous plants are not hurt by Malathion in wettable powder form. We always fertilize a blight infected rhododendron or azalea with one tablespoon of ammonium sulfate per gallon of water once weekly for a month. We have never seen a well fertilized rhododendron in a well drained bed, attacked by blight. Other people, no doubt, have more experience on this subject and may not agree.

We use only ammonium sulfate as a fertilizer (other than that added initially at planting), at the first sign of leaf yellowing and then only in amounts of 1 tablespoon weekly for a large plant and 1 teaspoon for a small plant. Small plants should have the ammonium sulfate added to a gallon of water. We just sprinkle the tablespoon of ammonium sulfate all over the large plants and water in. Continue the ammonium sulfate treatment until the leaves green up, when the pH is below 5.5. If the pH is above 5.5 add iron sulfate. We try to keep the pH at 4 to 4.5 because of the extra insurance against the Phytophthora fungi. Actually this year we fertilized our azaleas through middle of August and our rhododendrons to July 15. We always add a liberal amount (1 cupful) of superphosphate around and under each rhododendron and azalea on September 15. This helps the plants to harden off and assures more bloom buds. A warning here, it is better to use not enough fertilizer than too much in a hot climate. The tolerance of rhododendrons for fertilizer other than superphosphate is very low when the temperature climbs over 80°F, especially so for plants in sunny locations.
Our present soil culture is slightly different from that used by a friend of ours in Seattle where he uses no soil and only peat and leaf mold. He has a dryer summer and reports no trouble with Phytophthora. One Austin, Texas, rhododendron grower has for years used nothing but raised beds of peat, rotted sawdust and perlite over limestone soil and reports much success with growing rhododendrons.
We have found that a planting medium, well loaded with superphosphate, will cause our rhododendrons to set bloom buds on the first flush of growth which comes early on many of the early and mid-season bloomers and before the weather gets hot enough to cause leaf burn. By holding off the nitrogen fertilizer as much as possible and watering the plants only when they show signs of needing water, the plants usually will not put out any new growth after bloom buds set in this hot climate. Plants treated this way are much more tolerant of direct sun. We never irrigate a rhododendron until the leaves fall below the level position at early morning.
Late season bloomers with new growth that comes on during the hot weather must be planted in full shade here and even then mid-day sprinkling of the new growth will usually be necessary. Actually, we always spray water on the leaves of our azaleas and rhododendrons whenever we have time during the mid-day and late evening.

We are limited for shade and space but have planted more trees to improve our shade condition. We have planted rhododendrons in places in our yard where no shade is available and have improvised by shading with multi-layers of wall canvas or 2 inch lath boards spaced 1 inch apart until the trees we planted get large enough to take over the shading job. The temporary shade is placed about 4 feet above the plants. Actually, we need shade here for some of the plants only while the new growth leaves are maturing. We are hopeful to get started in a new area soon, where shade and space will be no problem. Another way we get around the shade problem is to plant in containers and place them close together where shade is available until new trees are large enough to supply sufficient shade. A 50/50 mixture of German peat moss and perlite makes a good potting medium for us.
It is our hope that this article will encourage more gardeners in the Southwestern-Gulf Coast areas to grow rhododendrons. Correcting the planting mediums is about all that stands in the way of growing many of the rhododendron varieties because the hazard is root rot which can be overcome. A published list of the most root rot resistant rhododendron species and hybrid varieties would be most helpful to all starting with rhododendrons in a high temperature area. High temperature, while a factor, is also one that can be dealt with for many varieties. True, we may have some problems with a few rhododendrons which may tend to break dormancy during our mild winters, but there are enough varieties available which will not give that trouble. We predict that many parts of the Southwestern and Gulf Coast areas will one day be show places for many beautiful rhododendrons.