QBARS - v23n3 Hybrid Rhododendron Culture in the Southeast

Hybrid Rhododendron Culture in the Southeast
William Garren, Travelers Rest, S.C.

Planning a garden is somewhat like planning a dinner party. The analogy here may seem irrelevant until you reflect upon your experiences as a gardener or as a host. When you put together a group, be they rhododendrons or Homo sapiens, there are basic fundamentals you observe, and if you're successful in your endeavors it is the result of concentrated effort, not haphazard luck.

When planning the dinner party, you consider your guests personalities and preferences. You try to put together a group whose interests are similar and personalities are compatible. So it is with planning a garden, especially a rhododendron garden. You acquire as much knowledge as possible about your "guests of honor"; their requirements, their companions, and their environment.

The environment in this case is all encompassing - it includes the area from the subsurface of the soil to the atmosphere surrounding the plant. Therefore, of first importance is one's selection of a planting site. One of the major considerations in selecting the site is the prevailing weather; for the weather in the Piedmont and mountainous section of the Southeast is not unique; but is something to be reckoned with if one doesn't consider its vagaries. We have four definite seasons. Spring makes a hopeful attempt in February to encroach on winters reign. Unfortunately, it is usually rebuffed by winter's last and heaviest snowfall. March is the month in which Spring officially arrives throughout the country. In the Southeast March brings days which are predictably warm and nights which are unpredictably frosty. The very last traces of winter occur in April, as the earth awakens to the warmth of the season.

Summer is usually bright, hot and humid, but seldom dry. Those hot, humid days can be expected to bring a refreshingly cool shower by nightfall.

Fall arrives gradually, creeping across the mountain tops and settling into the valleys by November. This season is pleasant and without surprises, weather wise. The most notable part of Fall aside from its beauty is its tenacity. Reluctantly, it yields to winter's siege and frigid onslaught in early December.

Once Winter pervades, it seems bent on revenge and ferociously sweeps in with frigid northerly blasts and alternately dry southwesterly winds. There is little or no snow cover to protect the plants from the devastating winds. So, in choosing the site where rhododendrons are to be planted, remember that the Southeast's climate is far less congenial than that of the Pacific Northwest since we lack the mild winters and cool summers which favor rhododendron growth. It is the well advised grower who keeps in mind these vagaries of weather when selecting the site for rhododendrons.

Summarily, we must provide adequate winter protection as well as summer protection against heat and direct sunlight. Locating the plant beds with a northern exposure is preferred in almost every case inasmuch as the plants are afforded protection from the winter's drying southwestern winds and the fluctuating spring temperatures. The soil and air cool steadily in Fall preparing the plants for winter and, once frozen, plants remain dormant until Spring. Another advantage of northern exposure is the bright light but lack of intense summer sun which can play havoc with foliage, especially the large leaf hybrids. In order to take advantage of the ideal northern exposures, fences, buildings or hedges can be utilized.

Assuming that the ideal site was for some reason unavailable, a second choice for locating beds would be under high shade provided by such trees as oaks and pines. In such a situation, the lower fifteen feet of tree trunk should be devoid of branches so that the slanting rays of early morning sun and late afternoon sun can reach the plants. The limbs remaining overhead shield against mid-day heat.

Only where no other choice is available should hybrid rhododendrons be placed in an eastern exposure. The hybrids which would tolerate early morning to midday sun in such a location are few indeed. The early flowering sorts would be a lost cause, since the cells of the flower buds would be ruptured by too rapid thawing in early morning, resulting in bud blasting.

Under no circumstances should rhododendrons be used as foundation planting when the exposure faces South or West. Both exposures are inhospitably severe. A wall facing south or west absorbs and retains quantities of heat during the day and releases its stored energy after sundown preventing soft growth from hardening sufficiently for winter. And, too, it is from the southwest that the devastatingly dry winds of winter originate.

Having selected the planting site, the next step is the preparation of soil. In the Southeast the soils are predominantly acid and clay types. The acidity is generally found to be in the desirable range for good rhododendron growth, but the addition of acid peat moss is necessary regardless of the pH reading to assure proper aeration and drainage. In many areas the topsoil is shallow and almost invariably has an underlying layer of compacted clay or "hardpan". When digging beds it is vital to remove and retain the topsoil and break through the "hardpan" with pick or mattock and discard as much of the impervious clay as practical. Then add copious quantities of peat moss - at least 50% by volume and sand where topsoil is a clay loam rather than sandy loam. Mix these additives with the topsoil and incorporate five pounds each of superphosphate and cotton seed meal in an area 10'x10'.

A common practice in the Southeast is the use of raised planting beds where drainage is a problem or where the soil is too alkaline to cope with. The advantages of using raised beds far outweigh the additional cost of such a practice. However, the grower must pay closer attention to watering in raised beds until the capillary attraction has been reestablished.

With the beds now prepared to receive the plants, it is advisable after removing the burlap or container from the earth ball to set the plant approximately one inch above the bed surface so that as the organic matter decays and the soil settles the plant will not sink too deeply. Our experience has shown that Spring planting is preferred to Fall planting. Winter in our area is a bit more rigorous than further south where Fall planting is often advised and newly set plants would be put to the severest test to survive the drying winds without being well established. We also suggest that the roots of plants to be set be exposed by gently washing away the soil in which they are growing. This practice generally assures that developing roots will penetrate the new soil.

Cultural practices following the establishment of plants are extremely important to success. For example, fertilizing, watering and mulching are yearly maintenance chores that require rather close observation and generous amounts of logic.

There seems to be a lot of controversy about fertilizing rhododendron. Those favoring the "lean diet" contend that since rhododendrons occurring in nature are usually found on soils of low fertility, it is unnecessary to add more than an annual mulch, which when broken down by soil bacterial supplies the plants with necessary nutrients. Unfortunately, this theory doesn't take into account the rapid deterioration of mulch under our southern sun nor the tie up of available nitrogen by soil organisms during the decay process. We have always made it a practice to apply an organic fertilizer annually. Cottonseed meal or the commercial fertilizers formulated specially for acid tolerant plants are recommended at the rate of five pounds per 100 square feet of bed area. We apply dry fertilizer twice yearly; first in early Spring prior to emergence of new growth and again about 8 weeks later prior to the occurrence of the second flush of growth. If additional fertilizer seems necessary after July 1 it is in the form of a completely soluble liquid which is absorbed primarily through the leaves. It is at this time we add soluble iron chelates, if needed. All fertilizer applications cease by August 1.

The frequency of watering should be determined by logic and not by habit. It is a poor cultural practice to water every three days or once a week without regard to the plants' requirements. Weekly soakings are not necessary if the soil has been properly prepared and the site well chosen. Of course, the exception would be during extended drought. New growth is apt to wilt or flag during extremely hot weather. This is especially noticeable in the afternoon and should not be alarming unless the plant fails to regain full turgidity by morning. If the foliage is wilted in the morning, then water and water thoroughly. One thorough watering is far more valuable than frequent sprinkling. Watering too frequently merely leaches the soil of nutrients and sets up ideal conditions for invasion by root rot diseases. No matter what watering practice has been followed during the growing season, the routine is gradually reduced as Fall approaches. By mid-September routine watering ceases so that the plants' tissues will begin to mature and harden for the coming winter. Watering is resumed around mid-November if natural rainfall is lacking. It is vital that the plants do not go into winter in a dry condition.

There are a number of mulches used commonly in the Southeast. Among them are peat moss, pine needles, oak leaves, ground tree bark and sawdust. All are useful. They serve the purpose of retaining moisture, providing a blanket in winter and a shield in summer against heat and weed invasion. Each grower determines which mulch he will use by its cost, availability and the appearance he wishes to create in the garden. Each type has advantages and disadvantages distinctly its own. Having learned to use sawdust, it is now our mainstay for mulching. Learn to use material of this sort can be costly to the beginner so it is with trepidation that I even mention it. The more commonly used materials such as pine needles and ground oak leaves are far more practical for the home garden. Only on extensive plantings would some other materials seem practicable. A layer of loose mulch from 6 to 8 inches thick should be applied following fertilizing in the Spring. By Fall, the layer will be only an inch or so thick, having undergone deterioration during the summer. This thin layer has very little insulating value, but adding more mulch should be delayed until after the ground begins to freeze. This time occurs shortly after Thanksgiving in the Piedmont. Applying a winter mulch is valuable only if plants are properly hardened for winter. Mulches are not beneficial if water and fertilizer have been improperly handled during the summer and fall months.

Finally, we come to the selection of hybrids for the Southeast. Fortunately for the beginner, he has an extensive list of hybrids from which he can select. Not long ago the beginner had little choice but to start with the "iron clads" or those which were proven for their ability to withstand both heat and cold. Today, however, he can choose freely from a list of well over one hundred hybrids which perform well in southern gardens. The list is growing rapidly as new, modern hybrids are being evaluated for their performance under southern skies. No doubt, in several years there will be a sufficient number of rhododendron clones to rival the omnifarious azalea for a place in the southern garden.

Because of their heat tolerance and ease of culture, the "iron clad" hybrids will undoubtedly remain on the most popular lists until superior hybrids come along which are even more resistant to heat and disease. The best in the group are:

  • 'Roseum Elegans' - probably the easiest of all the hybrids to grow. The confusion of names in this hybrid group is sometimes frustrating but the best is the lavender-pink form. It is likewise the best landscape plant and will grow with little or no protection from the sun.
  • 'Nova Zembla' - Undoubtedly, the best red of the "iron clad" in overall garden habit. The red color of its flowers carries a slight tendency towards blue, but its compact growth habit more than offsets this slight imperfection.
  • 'America' - The ungainly growth habit of this hybrid sometimes relegates it to the rear of the plant border, but the pure red color of it's flowers still commend attention no matter where it is planted. 'Everestianum' - The best iron-clad in the lavender to lilac hues. Its growth habit is neat and compact and what it lacks in flower size it inordinately surpasses in floriferousness.
  • 'Gomer Waterer' - A completely hardy white in the Southeast. The buds are a faint blush pink prior to opening. This is another hybrid which will tolerate considerable heat.

The H-1 and H-2 hybrids which we've just discussed are the best of each color group.  Based on popularity, general performance, plant habit, tolerance to heat and resistance to disease, the following group I consider the top ten varieties for southern gardens:

  1. 'Mrs. Charles Pearson' - A vigorous shrub to seven feet, flowering in mid-May, probably the finest performer of all in our area. The large truss is composed of creamy white florets edged with blush mauve and burnt sienna spotting on the upper lobe.
  2. 'Roseum Elegans' - Extremely easy to grow and very tolerant to heat, this is the most widely grown hybrid in the Southeast.
  3. 'Jean Marie de Montague' - Plant habit alone makes this an excellent choice. It is by far the top red in this list. Rather dwarfish, with dull green foliage, it flowers in mid-May with brilliant scarlet blooms in a neatly rounded truss.
  4. 'Cynthia' - A consistently good performer, its only disadvantage is its rangy habit. if grown where light is abundant, it will maintain a more compact habit, however. The enormous trusses are rosy-crimson.
  5. 'Anah Kruschke' - This is a hybrid from Oregon: It resulted from a cross of 'Purple Splendor' x ponticum . The tightly formed conical truss is lavender-blue. The stamens are quite prominent. Extremely compact grower with luxuriant dark green foliage. Flowers in early June.
  6. 'Belle Heller' - The best of the whites. Introduced by Shamarello Nursery, it is a cross of catawbiense alba x white catawbiense seedling. It is a vigorous grower. becoming tall but always well proportioned to width. Trusses are large globes of glistening white with a golden blotch.
  7. 'Mrs. Furnival' - A very free flowering hybrid, it introduced a distinctive color change in the garden, by virtue of the prominent burnt sienna blotch set upon the warm peach pink of its flower. It grows rather slowly and tightly, ultimately attaining a height of five feet.
  8. 'Damozel' - A striking fiery red which flowers freely from an early age. The only disparaging feature of this hybrid is its tendency toward weak stems. Otherwise, it is a top ranked plant. Flowers mid-May to June.
  9. 'Vulcan' - Very brilliant, glowing red. Of medium stature it will ultimately reach four feet. It flowers consistently, year after year.
  10. 'Caroline' - A Gable hybrid of excellent quality for the Southeast. The parentage is unknown. This is the only hybrid in this list that introduces fragrance of flowers as an asset. Aside from the fragrance, it offers exception al foliage on a vigorous, upright plant. The flowers are pale lavender, almost orchid.