Rhododendrons In Oklahoma
Leonard O. Miller, D. D. S., Grove. Oklahoma
I live in northeastern Oklahoma, near Grand Lake, which has 42,000 acres of water. The lake offers a tempering effect to the weather providing humidity, cooler summer temperatures, and some warming effect in the winter. We live on ten acres of assorted oak and hickory trees. The soil is mostly acid-rock, and that's not music.
We have an average rain fall of 42" per year. Most of the rain is unevenly distributed. Large amounts of rain fall in short periods followed by long periods of drought. The average length of the growing season is approximately 190 days. We have a mean average temperature of 59° in January and 92° in July. Average minimum temperatures are 26° in January and 69° in July. Temperatures of 90° or hotter occur as early as March and as late as October. During an average year, temperatures of 90° or higher are registered on approximately 60 days. The cold temperatures average 80 days of below freezing weather. The relative humidity averages approximately 55 percent during the summer months and 60 percent during the winter months. Colder temperatures are more likely, if you live in a hollow or low place.
Weather stations that record such data locate their thermometers five feet above the ground. A plant at ground level will experience weather several degrees colder. Northeastern Oklahoma's relative mild weather, with high humidity, makes it possible to grow rhododendrons. We are listed as zone 613, with an average record low of -10°. The state's record low is -27°, just 30 miles away. The rhododendrons with a hardiness of -15° seem to fair well, but those with less hardiness, such as "Blue Diamond' and "Unique', were victims of this year's winter. The cold temperature rather than the hot, seems to be more detrimental to my plants which are brought to Oklahoma as yearlings. These young plants are easier to air freight, but you give up some to Old Man Winter.
After several years of planting several hundred hybrid rhododendrons in both spring and fall, I find that spring planting is more suitable for my climate. Just the opposite is true of azaleas, the spring and summer heat doesn't seem to be a factor as long as you can avoid drying winds and have rapid drainage. The average wind speed is 13 miles per hour in April and March.
My first three hybrid rhododendrons died because I failed to protect the plants from the strong March wind and sun. Newly set plants need protection until the leaves come out on the trees in mid-April. The plants that are planted on the north side of a house are the most successful because the sun does not hit them until they are well established. The second best location is on the north side of a Post oak. It does not lose its leaves until spring, therefore, it gives shade to the plants during winter. When planting in the garden, I surround the plant with snow fencing until the winds have died down. As the plant matures, usually after the second year, I take away this protection. Some plants set out in the fall probably die from cold exposure because of over-watering during the dry fall. This phenomena is called "Nursing them to death". This leaves too much water in the leaves and stems, and they lack the hard wood needed to go into winter. With spring planting, we have a long summer for the wood to harden. Root rot will occur when the soil temperature rises, usually as a result of the sun striking the soil near the plant. Both R. catawbiense album and 'Anah Kruschke' have succumb to it.
Five years ago, my first plantings of rhododendrons were R. maximum and R. catawbiense. They were mature plants and the R. catawbiense has not grown over its three feet stature. R. maximum makes eight inches growth per year on the north side of our house.
My plantings include over a hundred different varieties of azaleas, which all do well except some Mollis and some Satsuki which split bark fairly easily. The Mollis hate our Oklahoma heat, but the pink and white 'Gumpo' grow well. I grow Exbury, some Mollis, and natives such as R. speciosum, canescens, austrinum, prunifolium, and serrulatum. Serrulatum does not flower well for me, I think I am a little too far north for it. Rhododendrons that are doing well are 'Holden', 'Janet Blair', 'Spring Dawn', 'Mrs. T.H. Lowinsky', 'Sappho', 'Marchioness of Landsdowne', 'Blue Ensign', 'Gomer Waterer', 'Ice Cube', 'Catawbiense Album', 'Vulcan, 'Mars', 'Lord Roberts', 'Tony', 'Wilsoni', and 'Mrs. Charles Pearson'. The other plantings are too new to tell, especially after the winter of '77.
The proper exposure of plants in N.E. Oklahoma seems to vary according to the parentage of plants. The most ideal, seems to be on the northern side of houses and buildings. For those who want to limit the number of plants, this offers deep shade and holds the plant dormant until the last freeze has past. More importantly, it protects the plant from the drying March winds. Most plants will set buds because of our many cloudless days.
In order to find more suitable locations for rhododendrons in my garden, I have built a cedar fence to protect them from the wind and sun. All the plants behind the fence have prospered. Sometimes though, I think rhododendrons wilt from Oklahoma moonlight.
A planting medium in areas of marginal climate is very important. I have used every medium that can be legally shipped to Oklahoma. They all seem to work as long as you remove the clay subsoil, or plant above the ground. My personal preference is a mixture of peat moss, perlite, pine bark and wood soil. My only problem is making myself dig big enough holes.
I have had some problems with pests. Lace Wing fly has attacked 'Blue Peter', but after spraying with Malathion, the fly was controlled. My worst experience with pests was a problem with garden centipedes. I bought four nicely rooted Exbury Azaleas, but they didn't show much growth and tended to decline during the growing season. In the fall, I dug them up and they had lost one-half of their roots. Close examination of the roots showed many white centipedes. I am afraid they have become a serious problem on my property. I will have to use Lindane on all my azaleas and rhododendrons.
With all the hybridizing going on across the nation, I know that someday even Oklahomans can enjoy the beauty of a garden full of rhododendrons. Until that time, however, the north side of buildings offer the microclimate and protection for the hardier types.