The Culture of Rhododendrons
in the Lower South of the United States
Arthur I. Coyle, Waller, Texas
It has been said that rhododendrons are only for the connoisseur. However, the average gardener who will follow planting and cultural instructions can grow rhododendrons anywhere there is good water, high shade, and wind protection.
Rhododendrons are the most beautiful of all flowering shrubs on this Earth. These wonderful plants are fussy and very demanding of the proper culture. The gardener who supplies his rhododendrons with their very demanding needs, will find to his surprise that rhododendrons are really much easier to grow than most azaleas. Many people still believe that rhododendrons will not grow under high temperature conditions such as occur in the southern part of the United States along the Gulf Coast.
Rhododendrons do grow and thrive in the southern part of the United States. Actually, the culture used in the lower half of the South differs somewhat from that used in the upper half of the South and the northern part of the United States. Our statements hereafter deal mostly with culture conditions along the Gulf Coast or south of a line drawn from Dallas, Texas eastward through Macon, GA.
South of the Dallas-Macon line there are, for the most part, soils which no rhododendron will grow in, such as hard pan, fine silty, and very tight soils. Also, south of the Dallas-Macon line are fungi which cause root rots and rhododendron stem and leaf blights. These blights are unknown in the Oklahoma City, Tulsa, Fort Smith, Jackson, Birmingham, and Atlanta areas. Rhododendron hybrids, only of the blight-resistant types, should be planted in the lower South. After testing hundreds of the many hybrids available, we have selected only 30 varieties of rhododendrons which are blight-resistant and which will do well with a minimum amount of care.
It is apparent to us that cold hardiness and heat tolerance of rhododendrons go hand-in-hand for combating the root rots and blight diseases of the lower South. Our selections of rhododendrons are, with few exceptions, tolerant of temperatures below zero. On the other hand, most of the rhododendrons tested by us with temperature tolerance to only above zero temperatures are susceptible to blight diseases.
In recent years, the ever increasing transient population is causing more and more people from rhododendron areas of the North to move South and these people want rhododendrons. Also, the introduction of the townhouse has kindled a desire for container growing of rhododendrons. These beautiful flowering shrubs do well in 30inch diameter redwood tubs in the small shaded back yards of the townhouses or shaded patios near back yard swimming pools. Then too, rhododendrons in redwood tubs can be moved from city to city in any part of the country.
Commercial growers of rhododendrons, in recent years, have started growing rhododendrons in treated wooden baskets. We grow all of our rhododendrons in half-bushel and one bushel treated baskets. These container-grown rhododendrons can be planted in any month of the year, while balled and burlap rhododendrons can be planted in the South only in the fall months of the year. Some manufacturers of wooden baskets advocate planting basket and all. We do not advise leaving rhododendrons in the baskets when planting. In fact, while rhododendrons grow much better in wooden baskets than when planted in plastic or metal containers, we have found some of the wooden baskets will hold water and drown the rhododendrons. This is caused by peat moss and/or pine bark breaking down after several months to colloidal size which plugs the drain courses of the baskets. Therefore, we now always bore drain holes in the sides of the baskets at the bottom, to insure quick drainage of the planting medium.
Rhododendrons do not do well in plastic containers in this area because no air can get through the plastic walls of the containers. Plastic containers could be used satisfactorily for growing rhododendrons if the plastic were perforated with many small holes. This would allow sufficient amounts of air to the rhododendron roots. Our planting medium is sterile and the wooden containers do allow fast root growth of the rhododendrons. We use a 50/50 mixture of European peat and Perlite. Recently, we have substituted a coarse grade of processed pine bark for peat with good results. Pine bark of the type we use requires an adjustment from a pH of 6 to 4.5 or 5. We reduce the pH with iron sulfate. Never use aluminum sulfate for making the planting medium more acid. Thousands of azaleas and rhododendrons are killed each year by the addition of aluminum sulfate to planting mediums. Aluminum ions under acid conditions are very toxic to all of the rhododendron genus.
We use Perlite in all planting mixtures in redwood tubs but coarse sand is better for smaller sized containers, such as one-half bushel, because the sand gives sharper drainage.
Bed Planting of Container Grown Rhododendrons
The container grown rhododendron should have its roots slashed (2-inches deep) on three sides of the root ball from the top of the root ball to the bottom after removing the root ball from the container. This stops any circular root growth which will kill any plant.
(1) Prepare a raised bed (turtle back style) 5 feet wide with the center of the bed 10 inches high of loose dirt. This bed will settle after watering to about 6 inches above the grade of the surrounding area. Any kind of soil can be used but the more sand in it, the better. If this raised bed of local soil is alkaline then enough iron sulfate should be added to reduce the pH to about 4.5. Remember, about 40 pounds of iron sulfate per 100 square feet is required to reduce the pH of heavy soil from a pH of 8 to 4.5. Sandy soils may require only half this amount of iron sulfate.
(2) The purpose of the raised foundation bed as described above is to give quick drainage under and away from the rhododendron root ball. This settled foundation bed after a thorough watering is then ready for the rhododendron root ball pad. This pad is placed in the center of the foundation bed and consists of pea gravel 6 inches high and at least as wide as the root ball diameter. This is very necessary because rhododendrons require very sharp drainage and also need a lot of air to their roots in order to thrive. The gravel pad prevents the heavy root ball from settling into the soil of the foundation bed. This is a must for the lower South. Farther north some root growth may take place in areas of quick draining "buck shot" type shales such as in the Forth Smith, Arkansas area.
Place the rhododendron root ball after it has been taken from the basket firmly on top of the pea gravel pad with the plant in an upright position.
The inherent characteristics of the Rhododendron Genus requires a very loose and airy planting medium. Never dig a hole for planting rhododendrons. Rhododendrons planted in a hole in heavy soils in the lower South will quickly die of root rots. High loose beds as hereafter described will give the rhododendron roots the necessary oxygen and quick drainage so necessary for successful growing of these plants. Companion plants such as azaleas, dogwood, etc., do better when planted in a high, loose, raised bed.
(3) Prepare a 50/50 mixture of coarse sharp sand and European peat (a heat sterilized pine bark of a 11/16 inch mesh screening may also be used in place of peat). Canadian peat is too fine and does not last long. Shovel this mixture of sand and peat, or sand and pine bark, around the rhododendron root ball. Slope this planting medium from the top of the root ball to the outer edges of the rhododendron bed. The thickness of this planting medium will be about 16 inches thick at the root ball and slope down to 8 inches high at the edge of the bed when planting bushel basket grown rhododendrons. One-half bushel basket grown rhododendrons will have a planting medium height of only 14 inches from the top of the foundation bed to the top of the root ball.
(4) Mulch the above described bed with 8 inches of pine needles. If these are not available then large-particle sized pine bark will do nicely and hold the bed in shape. Oak leaves and pine needles make the best type of mulch. The oak leaves will feed the rhododendron as they decay and the pine needles after being wet will conform to the shape of the bed and keep the leaves from blowing away.
We are presently building here at Coyle Gardens a rhododendron bed, 8 feet wide and 250 feet long. A foundation bed was first thrown up to 10 inches high in the center with a curved slope to either side. This has settled to about 6 inches above the grade of the surrounding area. The bed is walled with 1" x 8" copper napthanate treated boards. Treated stakes hold the boards in place. This holds the planting medium in place. The rhododendrons are spaced in the center of the bed and are 8 feet apart. Eight low growing azaleas are planted around each rhododendron as companion plants with a different variety of azalea for each rhododendron. A water sprinkler system has been installed along one side of the rhododendron bed with sprinkler heads at 20-foot intervals. The water can be controlled to a fine mist during the heat of the clay by opening one valve.
Sometime back, we erected a large brick planter of loose bricks stacked 6 high around the bed. No cement was used but the bricks are heavy enough to stay in place and hold the planting medium. Here we have various varieties of rhododendrons, mixed azaleas, and other broad-leaf evergreens. Brick planters built this way allow sufficient air to the plant roots with a 50/50 mixture of European peat and Perlite.
Planting rhododendrons in deep sandy locations makes the job easier, but raised beds should still be used, because rhododendrons do best if they get air to the roots from the sides as well as from the top of the bed. Never plant a rhododendron in a hole which has been dug in level ground of heavy soil in the lower South. A hole in heavy soil gives a bathtub situation which holds water and will drown the rhododendron roots.
Shade for Rhododendrons
Some rhododendron varieties will tolerate more sun than others. Blight problems on leaves and stems may be eliminated on some varieties by giving the rhododendrons more sun. This can be accomplished by pruning away some of the overhead tree limbs or moving the plant to a more sunny location. In general, always use high shade which gives good air circulation. Low shade gives poor air circulation and invites fungi problems.
Many rhododendron varieties will stand full sun if the new growth is sprinkled or fogged with water continually during the heat of the day for the three week period required to mature the new growth leaves. Thereafter, the matured leaves are not bothered by the sun. Some rhododendrons such as R. 'America' require full shade at all times, otherwise the matured leaves yellow in the sun and the plant will not thrive. Rhododendron roots will spread out away from the root ball and, after several months, be able to supply more water to the stems and leaves of the new growth. Wilting of the new leaves becomes less and less a problem as the roots extend in the new bed. Planting small plants with large root balls also will prevent leaf burn of new growth on many varieties. Gardeners just beginning with rhododendrons are urged however, to buy plants at least four years old when planting in the lower South because rhododendrons become resistant to most diseases after they are four years old.
It is very important to mulch rhododendrons, including the azalea series, with oak leaves and pine needles and/or bark chips. Bark chips of two inch diameter are excellent for mulching rhododendrons. We use thumb nail size bark for mulching rhododendrons planted in wooden baskets or redwood tubs. Mulches keep the planting medium from drying out and protect the roots from cold and hot weather.
Redwood Tub Growing of Rhododendrons
The planting of container-grown and field-grown rhododendrons is somewhat different in timing. If the plant has been container-grown, it can be planted in any month of the year in the lower South because its root system is complete with feeder roots. However, the balled and burlap field-grown rhododendrons have had all of their feeder roots cut off and must grow new ones which takes time. Therefore, it is safe to plant B&B rhododendrons only in the fall months in the lower South, so they will have time to grow new feeder roots before the coming summer months.
Several years ago, we developed a redwood tub 29-inches wide at the top, 23-inches wide at the bottom, and 16inches high. We had a redwood container company manufacture these tubs for us. Many rhododendrons have been planted in these tubs in the past 15 years with excellent results. We know of rhododendrons which have thrived in these tubs without a change in planting medium for 12 years.
The culture is the same as for bed planting. The step by step planting procedure is as follows;
(1) Locate the redwood container in a shady and level spot, preferably on a raised concrete slab such as is generally used for patios.
(2) Break up a clay pot into big and small pieces and cover the one drain hole in the center of the bottom of the tub.
(3) Add 3 inches of Perlite to the bottom of the tub, covering the clay pot pieces over the drainage hole. Level and smooth the Perlite, then add 1-inch 50/50 mixture of Perlite and European peat or processed pine bark.
(4) Slash the roots of the container grown rhododendron on three sides from the top to the bottom of the root ball 2 inches deep, and set the root ball in the exact center of the redwood tub on top of the planting mixture. Cut off some of the root ball if the root ball stands higher than 1½ inches from the top of the tub. Be sure the rhododendron stands straight. If not, straighten the plant and add more of the 50/50 mixture of peat and Perlite under one side of the root ball so as to keep the plant firmly upright.
If the rhododendron to be planted is a balled and burlap plant, remove burlap from the root ball and wash away all the soil with a strong stream of water. This is a must if the root ball is of heavy soil. Regardless, at least 2 inches of any sandy soil must be washed away from the cut off root ends before planting.
Plant the B&B rhododendron as instructed above. However, in either case, whether the root ball is field-grown or container-grown, if the root ball is too small, then add enough 50/50 mixture of planting medium under the root ball to raise the top of the root ball ~to 11/z inches below the top of the redwood tub.
(5) Packing the planting medium around the root ball in the redwood tub should be firmly done with the hands. Do not place more than one-half inch of the planting medium on top of the root ball. Add 1-inch of thumbnail size pine bark on top of the planting medium for mulch. Pecan shells also make a good mulch for rhododendrons in redwood tubs.
(6) Water the rhododendron several times to get a good soaking of the root ball and planting medium. Thereafter, water only when the leaves droop at early morning. Then water to give a good soaking. If a 50/50 mix of coarse sand and pine bark is used for planting medium, the rhododendron may require more water until the roots have grown into the new planting medium.
The seed pods of a rhododendron should be pinched off soon after blooming because making and maturing seed take so much vitality out of the plant and may prevent good blooming except every other year.
Fertilizing Container and Bed Grown Rhododendrons
Rhododendrons are light feeders, but they do require an acid fertilizer in small amounts such as cotton seed meal or a specially prepared commercial fertilizer of a 1-1-1 ratio. We use a 13-13-13 commercial fertilizer with no filler added. One manufacturer of fertilizer makes a 12-12-12 formulation with dolomite limestone added as a filler. This formulation would be detrimental to a rhododendron. All nitrogen in any fertilizer formulation should be only of the ammonium type, such as ammonium sulfate. The addition of two heaping tablespoons of cotton seed meal plus one level tablespoon of Occidental Chemical Company's ZIPP 1313-13 is usually sufficient for one year's growth, and for setting bloom buds for a rhododendron in a 29 inch wide x 16 inch high redwood tub. Additional chemicals, such as one teaspoon of magnesium sulfate (Epsom salts) and one teaspoon of calcium sulfate, may be added as the organic portion of the planting medium becomes older and depleted of minerals. Always add the sulfate form of calcium and magnesium. Never add the carbonate form to a rhododendron planting. This also applies to the azalea series of rhododendrons. Some rhododendron varieties may require additional nitrogen throughout the year, otherwise the leaves may become yellow. If this happens, when the pH of the planting medium is acid enough, like having a pH of 4.5 to 5, then add one teaspoon of ammonium sulfate per gallon of water at one week intervals to the planting medium, until the leaves turn green. Too much sunlight will cause the leaves of some rhododendron varieties to yellow, and in this case, more shade should be provided for the rhododendron. Do not use high nitrogen fertilizers on rhododendrons, such as the 15-5-10 formulation which is commonly used on azaleas. High nitrogen fertilizers will cause the rhododendrons to go into rank growth and little or no bloom-buds will be set.
Foliage Feeding of the Rhododendron Leaves
We use four teaspoons of Peters 2020-20 in a foliage feeding formula plus one teaspoon of magnesium sulfate, one teaspoon of calcium sulfate, one teaspoon of Sequestrene (iron chelate) per gallon of water with great success. Liquid fertilizers such as Rapid-Gro and Peters 20-20-20 may be used one time for adding to the planting medium of newly planted rhododendrons in order to insure a supply of the necessary trace elements to pine bark or spent peat moss. No more than one application of these two liquid type fertilizers a year should be used in a planting medium because of the rapid build-up of fertilizer salts which are detrimental to the rhododendrons. Fantastic growth of rhododendrons may be obtained with the above foliage feeding method.
Fungi Control - Root Rot
The fungi disease problem associated with rhododendrons such as leaf spots and blights of the leaves and stems and root rots occur mostly on rhododendrons which are under four years old. For this reason, it is a good idea not to force rapid growth on young rhododendrons. Rhododendrons older than four years are not as susceptible to diseases and may not be bothered with disease at all. Here in the lower South, we grow only the most disease-resistant rhododendrons hereafter listed. However, we have noted that any time a rhododendron has an unhappy root system, leaf spots will develop, which may or may not he controlled with fungicides. Also, when the planting medium is improved with better drainage, the disease often disappears and really there is no disease at all, just a condition which had caused disease symptoms. This is the reason we have gone to the 50/50 mixture of sand and coarse organic planting mediums for all of our rhododendron plantings. It is sometimes possible for a rhododendron to overcome the ravages of the worst root-rot disease of all, caused by the water mold type of fungus, Phytophthora cinnamomi. A quick-draining planting medium will be of no help to a rhododendron that is already heavily infected with Phytophthora cinnamomi but the quick-draining planting bed may help a lightly infected plant to recover. We treat the roots of the root rot-infected rhododendron with a drench of Banrot. This treatment is followed the following day with a root stimulator such as Superthrive (vitamin B-1) and Peters 20-20-20 fertilizer as a planting medium drench.
We never worry about root rots caused by Phytophthora cinnamomi when planting the selected rhododendron varieties listed hereafter. In fact, 'Hino-Crimson' azalea had been very difficult for us to grow here in the lower South because of Phytophthora cinnamomi. We now give this azalea the sharp drainage required for the broad leaf rhododendrons and never lose any of them.
Rhododendrons may be attacked in the lower South by several leaf blights. The only air-borne species of the Phytophthora group, Phytophthora cactorum, affects the leaves and the stems of some rhododendrons. Too much shade and poor air drainage during hat and excessive humid weather is usually the cause for this pathogen attack on a rhododendron. The disease starts first on the leaves and travels to the stems and forms a brown canker on the stem. This blight must be stopped at once otherwise the disease will continue down the stem and kill the plant. The stem should be cut to green wood below the canker. The entire plant must be sprayed every ten days with 2½ tablespoons of Ferbmate per gallon of water plus sticker-spreader or until there is no sign of the blight on the plant.
|SELECTED RHODODENDRON VARIETIES FOR THE SOUTH|
|R.||A. Bedford||U. S.||Shade||Lavender|
|R.||Album Elegans||U. S. & L. S.*||Part Shade||White|
|R.||America||U. S. & L. S.||Full Shade||Dark Red|
|R.||Anna Rose Whitney||U. S. & L. S.||Part Shade||Deep Pink|
|R.||Antoon Van Welie||U. S. & L. S.||Part Shade||Pink|
|R.||Betty Wormald||U. S. & L. S.||Part Shade||Pink|
|R.||Blue Ensign||U. S. & L. S.||Part Shade||Lavender Blue|
|R.||Blue Peter||U. S. & L. S.||Part Shade||Lavender Blue|
|R.||Catawbiense Album||U. S.||Part Shade||White|
|R.||Catawbiense Grandiflorum||U. S. & L. S.||Part Shade||Lilac Lavender|
|R.||Chionoides||U. S. & L. S.||Part Shade||White|
|R.||Cynthia||U. S. & L. S.||Shade or Sun||Carmine Red|
|R.||Dr. A. Blok||U. S. & L. S.||Part Shade||Rose|
|R.||Fabia Roman Pottery||U. S. & L. S.||Part Shade||Coppery Orange|
|R.||Goldsworth Orange||U. S.||Part Shade||Orange Tinted Pink|
|R.||Gomer Waterer||U. S. & L. S.||Part Shade||Flushed White|
|R.||Hyperion||U. S. & L. S.||Sun or Shade||Lilac|
|R.||Jean Marie de Montague||U. S. & L. S.||Part Shade||Brightest Scarlet|
|R.||Kate Waterer||U. S. & L. S.||Part Shade||Pink, Yellow Center|
|R.||Lady Clementine Mitford||U. S. & L. S.||Part Shade||Pink|
|R.||Margaret Dunn Golden Bells||U. S.||Part Shade||Yellow Flushed Pink|
|R.||Mrs. Charles Pearson||U. S. & L. S.||Part Shade||Blush White|
|R.||Mrs. E. C. Stirling||U. S. & L. S.||Part Shade||Blush Pink|
|R.||Purple Spendour||U. S. & L. S.||Shade||Purple|
|R.||Roseum Elegans||U. S. & L. S.||Part Shade||Pink|
|R.||Vulcan||U. S. & L. S.||Shade||Bright Red|
|*U. S. - Upper South; L. S. - Lower South|
Other fungicides such as Benlate, one tablespoon per two gallons of water, are effective. Our regular spray for rhododendrons is two tablespoons of Zineb mixed with 2 tablespoons of Ferbmate per gallon of water plus Sticker-Spreader. Phaltan is also used to control some rhododendron leaf spots. However, we prefer the Zineb Ferbmate treatment, alternated with Benlate, for possible better control.
The same insects which bother the azalea series of rhododendrons may under certain conditions also bother the broad-leaf rhododendrons. However, our selected group of rhododendrons, when growing under our conditions, have not been bothered by leaf-feeding insects with one exception: R. 'Peter Koster' is bothered by leaf-chewing worms.
Our standard spray treatment, Diazinon plus sticker-spreader, is sprayed on both sides of the leaves which will take care of about all insect problems. We are more concerned about the welfare of the rhododendron roots. Treat the planting medium and mulch twice a year with a mixture of four teaspoons of Lindane and one tablespoon of Clordane per gallon of water. The planting bed is drenched thoroughly with this mixture. We also scatter lightly over the mulch, 15 per cent granular Di-Syston and water in. This holds down the nematode population and discourages leaf sucking insects. Care must be used when treating any plant with Di-Syston. Too much will kill a plant. Use according to directions and never use on a prize plant without testing this material on others of the same variety.