JARS v43n4 - Growing Rhododendrons In The Gulf South

Growing Rhododendrons In The Gulf South
John T. Thornton
Franklinton, Louisiana

Franklinton is located 50 miles north of New Orleans in the piney woods area of southeastern Louisiana. It is located on the border line between USDA hardiness zones 8 and 9. The soil is mostly strongly acid clay with poor internal drainage characteristics.
Rainfall averages 65 inches per year. July and August are the wettest months averaging 7 to 8 inches each. June, October, and November are usually the driest months. Droughts can occur any time of the year, but they are seldom as severe as in other parts of the country.
Temperatures average about 90°F (32°C) for the high and 72°F (22°C) for the lows during June through September. Average lowest January temperature is about 20°F (-6°C) with record low temperatures being about 5°F (-15°C). First frost occurs about November 20; last frost occurs about March 20.
Summer relative humidity is usually very high, although occasional periods of very hot low humidity weather occur. Autumn, winter and spring relative humidity varies from high to very low.
The deciduous azaleas Rhododendron canescens , R. serrulatum and the mountain laurel, Kalmia latifolia , are native here. Rhododendron minus and R. chapmanii grow at low elevations 200 miles east of here. The Southern Indica azaleas are grown extensively.
Successful rhododendron growing here requires the selection of proper varieties along with proper cultural practices.

Cultural Practices
A built-up bed is necessary unless the planting site has light sandy soil or is located on a hillside or hilltop. The built-up beds can be made of topsoil or well-rotted pine bark. Organic matter soil mixtures usually are not satisfactory because they often develop a waxy coating that resists wetting in dry weather. Also, organic matter rapidly decomposes in our hot humid climate and causes built-up beds to rapidly disappear.
At least a six inch built-up bed (if soil), or an eight inch built-up bed (if bark), is necessary. Sand or gravel under built-up beds are counterproductive because they interfere with movement of moisture from deep within the soil during dry weather. Built-up beds of pine bark may be held in place with landscape posts or stones. Pine bark decomposes about ten percent per year, and beds may be maintained by top dressing with additional pine bark.
Heavy clay topsoil is better than light sandy soil for making built-up beds. Heavy soil holds moisture better than light soil. Also light sandy soil may encourage the root system to grow too deeply. These deep root systems may suffer die back during wet seasons and allow root diseases to occur. About 50 gallons of soil is necessary for a large growing rhododendron.
Acid forest soil is ideal for built-up beds. Topsoil from farmlands sometimes contains harmful herbicides and topsoil from barnyards may contain harmful amounts of chlorides or lime.
Care should be taken not to smother the roots of rhododendrons planted in heavy top soil. A small amount of sandy soil or leaf mold may be placed immediately around the roots. Purchased plants growing in organic matter should have much of this material removed before planting so there is good soil-root contact.
Pine needles make excellent mulch for rhododendrons. Hardwood leaves or pine bark are also satisfactory.
Rhododendrons are easier to establish here if planted in November since the roots grow vigorously in cool weather. Rhododendrons perform best in this area when grown under a light-to-medium shade. Tall pines make excellent shade trees. A shade to the east, south and west with open sky above is ideal. With open sky above, plants radiate away more heat at night, get cooler and collect more dew as compared to plants growing directly under tree branches.
When conditions of shade cannot be made ideal, western (afternoon) shade is the most important. The arc of the afternoon sun travels far to the northwest in southern latitudes, so that western and northwestern shade is necessary.
Foundation plantings on a south or west side should be avoided unless there is adequate shade. Foundation plantings also may suffer from being too wet or too dry. Rhododendrons may be grown in planters in pine bark. These planters may be moved to provide ideal shade during the year, also planters may be moved to protect from late frosts.
Rhododendrons grown in soil require little or no fertilizer. Plants grown in pine bark should be fertilized in early spring using cottonseed meal at the rate of one tablespoon per foot height of plant or azalea and camellia fertilizer at the rate of one teaspoon per foot height of plant. Pine bark is deficient in sulfur. Chlorosis due to sulfur deficiency may be corrected with Epsom salt. Rhododendrons are very sensitive to chlorides in commercial fertilizers and care should be taken never to over-fertilize.
Rhododendrons should be watered only when needed. They should never be overwatered. Established plants may be watered by wetting the leaves and mulch daily during droughts. This is done preferably in the early morning when the water and temperatures are cool. Multiple wettings per day should be avoided since this often leads to dieback. Plants may also be watered by thoroughly wetting once a week with a sprinkler. Never over-water with a sprinkler. A drip irrigation system may also be used wetting the roots for a short period of time once daily.
Spider mites are an especially bad problem with plants containing R. griersonianum or R. griffithianum blood. They may be controlled with Malathion. Phytophthora root rot is best controlled by good drainage, proper watering technique and by the selection of plants with resistance to this disease.
Rhododendron chapmanii and R. minus are often hard to establish due to their high susceptibility to root rot. Drenching these plants with Subdue at planting time seems to greatly reduce this problem.

Selected Varieties
I have tried to grow several hundred hybrid varieties over a period of 20 years. Many plants that do well in the upper South are not satisfactory here. Plants that bloom after May 1 often have poor quality flowers due to the heat. Only one hybrid, 'Damozel', has been entirely satisfactory. All the others have one or more serious faults. The following hybrids do well with qualifications:
'Damozel' - the best - attractive plant blooms regularly from an early age, flowers are attractive even though it blooms late.
'Van Veen' - similar to 'Damozel,' but suffers die back.
'Anna Rose Whitney' - grows well, slow to bloom, flowers suffer in the heat.
'Grierosplendour' - grows and blooms well, flowers suffer in the heat.
'Bibiani', 'Dame Nellie Melba', 'Ivery's Scarlet' - these early blooming R. arboreum hybrids are excellent except for being tender, very drought resistant.
'English Roseum' - grows well, flowers suffer in the heat, blooms in the fall and winter.
'Roseum Elegans' - same as 'English Roseum'.
'Anah Kruschke' - flowers poor quality in the heat.
'Caroline' - grows well, attractive plants, ten years or older before it blooms regularly, flowers poor quality due to heat.
'Van Nes Sensation' - grows well, blooms in the winter.
'Mrs. A. T. De La Mare' - grows well, attractive flowers, suffers die-back.
'Janet Blair', 'Westbury', 'Great Eastern' - These plants grow well, but flowers suffer in the heat.
'Maximum Roseum' - grows well, very slow to bloom, flowers suffer in the heat.
'Kimberly' - grows and flowers well, flowers attractive, very susceptible to dry weather.
'Albert Close' - grows well; flowers poor quality in the heat.
'P.J.M.' - grows well, attractive winter foliage, blooms in the fall, drought resistant.

During the early 1970's, I attempted to grow from seed most of the rhododendron species listed by the Royal Horticultural Society and by the ARS Seed Exchange. I also obtained and tried many species from nurseries. As a group, selected species or selected forms of species have outperformed the commercially available hybrids.
The following species have performed well:
R. hyperythrum - excellent. A form of R. hyperythrum introduced by Rainier Mt. Alpine Gardens as Creech's narrow leaf form collected wild in Taiwan performs very well for us and is a vigorous grower. We are naming it Dr. John L. Creech. It should be an excellent commercial variety.
R. pseudochrysanthum - Nelson's form from Greer Gardens is excellent, other forms died.
R. formosanum - does well, but is tender.
R. morii - John Patrick form excellent, other forms died.
R. arboreum ssp. delavayi - excellent, drought resistant, but is tender.
R. adenopodum - excellent.
R. adenogynum - does fairly well, is often sold as R. adenopodum .
R. metternichii - Oki Island form, excellent except it is inexplicably tender for me.
R. metternichii var. kyomaruense - does well, other forms of R. metternichii have been less satisfactory.
R. makinoi - does well, seems to be very sun tolerant.
R. maximum - grows well, late forms that bloom in July during our wet season are best.
R. fortunei - all forms do well.
R. decorum - selected forms do well.
R. diaprepes - I have three seedlings believed to be 'Gargantua' that are excellent, flowers are attractive even though they bloom late, however, the plant is tender.
R. ponticum - selected forms do well.
R. minus - excellent.
R. chapmanii - excellent.
R. formosum - excellent, but tender.
R. formosum var. formosum excellent, but tender.
R. ellipticum - excellent, but tender.
R. championiae - excellent, but tender.
R. ovatum - good.
R. hongkongense - good, but tender.

John T. Thornton is a veterinarian in Franklinton, Louisiana and a partner in C & T Nursery specializing in rhododendrons and deciduous azaleas.