JARS v43n4 - Rhododendrons At The Missouri Botanical Garden

Rhododendrons At The Missouri Botanical Garden
Walter R. Behrendt
St. Louis, Missouri

Dedicated to Alan P. Codlewski who gave me the opportunity to work with rhododendrons.

One does not usually think of St. Louis as a place to grow rhododendrons and azaleas. We don't have the cool moist climates of the west coast, the warm winters of the south, or the snow of the eastern mountains. What we do have are hot and humid summers with temperatures sometimes climbing above 100°F. Then, to make things more challenging, our winter temperatures go below zero with little or no snow cover and wind chill factors that plunge to -30°F. It's not uncommon, in the winter, for temperatures to be in the 60's one day and drop into the teens or lower the next day.
The Azalea and Rhododendron Garden is located at the north end of the Missouri Botanical Garden and is one of the first gardens a visitor sees. This approximately one-acre garden is situated in part on the site of a previous large perennial garden. This perennial garden was enclosed by three-foot-thick limestone walls for background.
Another part of this site contained 20 to 25-foot tall specimens of Magnolia grandiflora and American holly. A small collection of Glenn Dale hybrid azaleas, some of which had obtained a height of five feet, was also located here. Several years before construction started, the garden experienced a series of very hot, dry summers and "hundred year" winters. This devastating combination killed many of the large azaleas and drove the Magnolia grandiflora trees into decline.
The south half of this site contained very large specimens of pin oak, red oak, burr oak, sycamore, Amur cork tree and a mature grove of Magnolia x soulangiana . These trees gave us the high shade over half of the garden which we needed to grow rhododendrons in St. Louis.
Much of this site was for two years used as a staging area for the construction of the Ridgway Center. This building serves as the main visitors' entrance and houses our Education and Public Relations Departments, the Gardenview Restaurant, the Orthwein Floral Hall, our Gift Shop, and exhibit facilities. This construction left the soil in appalling condition. In some areas as much as eight feet of fill was needed to raise the grade to the required level before we could start putting in the Azalea and Rhododendron Garden.
The soil was of heavy brown clay mixed with limestone rock, concrete wash from cement trucks, asphalt from old paths, bricks from years past, and construction debris of all kinds. Add to this two years of compaction by construction equipment of all kinds and you have the soil base for much of the Azalea and Rhododendron Garden at the Missouri Botanical Garden.
We started working on the garden in the fall of 1983 by adding tons of compost in thick layers. A backhoe was then used to loosen the compacted soil and work in the compost. During this process, tons of debris were removed, including several old road beds that we discovered had been buried. The site was then tilled, graded, and planted with K-31 tall fescue grass to eliminate winter mud. Early the following spring a crane was used to plant five 30-foot tall Eastern white pines on the northwest side of the garden. The west side was then planted with large specimens of Canadian hemlock, red pine, American holly, and Hinoki cypress. These plantings provided protection from the north and west winter winds and gave us a pleasing dark background for the azaleas and rhododendrons while screening nearby buildings and greenhouses.
In late spring a tree spade was used to bring in several large red oaks and white oaks. However, due to the lateness of planting and our summer heat, most of these trees died the following summer. These trees were subsequently replaced the following fall and winter with red oaks, again using a tree spade. This time all of the trees survived. Smaller balled and burlapped specimens of swamp white oak, burr oak, tulip tree, and yellow-wood were also planted to complete the planting of overstory trees and provide future shade within the garden.

To separate and define the beds and lawn areas, a double brick edge was installed. This also provided a walkway so people could better view the plantings during wet weather.
We then installed two separate irrigation systems, the first one being an automatic irrigation system equipped with an injector. This injector allows us to acidify our water, which in St. Louis normally has a pH of 9.0 to 9.5. This system also allows fertilization of the garden as needed. The second system is a frost-free manual system which allows us to water if needed during the winter.
During the summer of 1984 we began the final soil preparation of the planting beds. We began by tilling in the lawn planted the previous fall. We then tilled in about a six-inch layer of Canadian peat moss, using over four hundred bales. This was followed by the addition of large amounts of sulfur to lower the pH of our soil. Finally, a three to four-inch layer of Turface was tilled into the soil to improve drainage.
Turface is a baked montmorillonite clay which has very good water-holding capacity while allowing exceptional aeration and friability of the soil. Plants, in particular azaleas and rhododendrons, grown in soil where Turface has been added have shown a tremendous increase in root growth resulting in a much more vigorous plant. Turface looks like ground-up flower pots or cat litter. In the process of preparing the planting beds we used over fifty tons of this material.
After each bed was amended and tilled, it was rough-graded. As grass from the lawns and other weeds grew, the beds were re-tilled. It was not until the following spring that all the beds had been prepared in this manner.
While these activities were taking place we were growing, from cuttings taken at the garden, over 600 evergreen azaleas that we knew would thrive in the St. Louis area. These included 'Atlanta', 'Boudoir', 'Cascade', 'Corsage', 'Fedora', 'Gable's Tall Lavender', 'Hino Crimson', 'Palestrina', 'L. C. Fisher', 'Purple Splendor', 'Sherwood Orchid', and Rhododendron obtusum 'Amoenum.' We had also grown several hundred plants from seed and from tissue culture. These included R. mucronulatum , R. mucronulatum 'Cama', R. mucronulatum 'Nana', R. schlippenbachii , R. calendulaceum , and our one Missouri native, R. prinophyllum ( R. roseum ). These groups of plants formed the foundation for the garden. We planted these in drifts to give us large areas of color with many small pockets in which to plant new varieties later.
Planting azaleas and rhododendrons was finally started in the spring of 1985, over a year and a half after we started. Due to the heavy clay soil, we planted in raised beds. Depending on the size of the plant's root ball, we either planted the root ball halfway out of the ground or right on the surface. A prepared soil mix, which consisted of four parts Canadian peat, four parts Turface, three parts garden soil, plus sulfur to adjust the pH, was mounded around the root ball. Finally a two to three-inch layer of shredded pine mulch was put down. The garden was fertilized once that first year and has been fertilized twice a year after that.

Many of the late Dr. J.R. Schroeder's evergreen azaleas were obtained in the spring of 1986. We planted one-gallon size plants in small pockets during that summer. These cultivars are reported to be hardy to -15°F. The following winter had temperatures which stayed above 0°F and had little snow cover. The following spring the performance of these cultivars was mixed. 'Dr. Henry Schroeder', 'Mrs. Nancy Dippel', 'Schroeder's Lavender Rose', 'George Hyatt', and 'Margaret Hyatt' held most of their foliage and bloomed well. On the other hand, 'Robert Hyatt', 'Carrie Amanda', 'Schroeder's Snowflake', 'Dr. James Dippel', and 'Moby Dick' all lost most of their foliage and most of the flower buds blasted.
However, the second spring we had better results. Following a winter with temperatures as low as -3°F with little snow cover and fluctuating temperatures, nearly all cultivars bloomed very well with little foliage damage. The one exception being that of 'Dr. James Dippel' which has not done well for us from the beginning.

The south end of the garden with its high shade was planted with both lepidote and elepidote rhododendrons. We obtained large specimens of 'Roseum Elegans', 'English Roseum', 'Lee's Dark Purple', 'Nova Zembla', 'Catawbiense Album', 'Catawbiense Boursault', and 'America' from a west coast nursery. These cultivars were expected to thrive in St. Louis and give us the needed protection from winds and background for new cultivars. However, we have had problems with these large plants adjusting to the rigors of our Midwest climate. The first year we had trouble keeping enough water on them; as a result, these plants wilted often in our warm spring and summer weather. It took two years for these plants to establish root systems large enough to prevent excessive wilting.

Due to the frequent irrigation, our high summer temperatures, and humidity, we have had problems with Phytophthora root rot. Many of the larger rhododendrons have shown signs of this disease, gradual dieback of the plants and general lack of vigor. Some of these plants died completely. However, we found that by using the fungicide Subdue as a soil drench and in some cases severely pruning the plants back to six to ten inches above the ground, many of these plants returned to vigorous growth. We have had much better results buying small plants and growing them in pots in our soil mix for a year or two before planting out.
Several of the lepidote rhododendron hybrids have thus far proven to be very good growers in the St. Louis area. We have planted many of these cultivars in small groups. Among them are 'Alice Swift', 'Yellow Eyes', Spring Delight', 'Epoch', 'Caroline Rose', 'Jenny', and 'Ethel Mae'.
Many of the lepidotes that we have are the result of the generosity of a man I met while attending the American Rhododendron Society convention in Atlanta, Georgia. He was John Neal, then of Kirkwood, Missouri, who was very enthusiastic about our starting a rhododendron and azalea garden in St. Louis. He was so enthusiastic that he donated many of the lepidotes and R. yakushimanum hybrids that have added so much to this garden.
A short path leads a visitor to a bench in the middle of the rhododendron area. It's along this path that one finds the R. yakushimanum hybrids. These cultivars, which include 'Yaku Queen', 'Yaku King', 'Yaku Duchess', 'General Schmidt', 'Pink Sherbet', and 'Ken Janeck', have all done well for us.
The white blooms of 'Dora Amateis' are among my favorites, while 'Ginny Gee' has been a delight to grow with its pink and white blooms that last for weeks. But it's R. dauricum , the first to bloom, that signals the starting of the blooming season. It has bloomed as early as February, but is usually hit by spring frosts. 'PJM' and the cultivars of R. mucronulatum then follow with their early spring show. This progression of bloom lasts into May and is a joy to behold.
An azalea and rhododendron garden would not be complete without some companion plantings. Scattered throughout this garden are many Japanese maples and magnolia hybrids. The yellow magnolia 'Elizabeth' is outstanding, as are the hybrids 'Nimbus', 'Pristine', 'Spectrum', and 'Leonard Messel'. We have also planted several Magnolia stellata and Magnolia denudata . These, along with two Styrax japonica , have added a great deal to the beauty of the garden.
So far we have been blessed with relatively mild winters. This has allowed the garden to mature and hopefully will better able the plants to withstand a harsher winter. The summer heat, has, so far, been more of a problem. Some cultivars and species that we thought would grow in St. Louis have suddenly died in the summer heat. Others have struggled and not lived up to expectations so they were removed. But we will keep trying new cultivars and species to learn what will do well in this area. Many plants are presently being grown in our cold frames and as they become large enough they will be planted out and evaluated.

Walter Behrendt, Horticultural Superintendent at the Missouri Botanical Garden, enjoys working with rhododendrons and has attended many ARS National Conventions. The original suggestion for this article came from George Ring, Past President of the ARS, who visited the garden in 1987.