QBARS - v33n4 Growing Vigorous Rhododendrons

Growing Vigorous Rhododendrons
Henry Burdette, Clemson, S.C.
Adapted by B. D. Barnett from a speech delivered by Henry Burdette on May 10, 1979 before the William Bartram Chapter in Greenville, S. C.

The most common reason we fail with rhododendrons in our gardens is because we don't duplicate what they are accustomed to in their native habitats. Failure in this piedmont region of the Carolinas and Georgia is not due to heat as we have been led to believe.
In experiments that I have performed over the past few years, using a rock base, I have become convinced that superior drainage is the key to success in growing healthy, vigorous plants. This rock base consists of three to four inches of coarse crushed rock. Directly over this I place about eighteen inches of planting media. This planting mixture consists of one-fourth peat- one-fourth ground pine bark and one-half leaf mold and mountain loam. If the mountain loam from an area where rhododendrons are growing well is unavailable, you can use one-fourth leaf mold and one-fourth washed creek sand.
When rhododendrons or azaleas are set in this mixture and supplied with one-half inch of moisture in the afternoon, along with appropriate fertilizer, they grow better at 90 degrees in strong light than at cooler temperatures. These experiments were performed during June, July and August.
I have confirmed the fact that superior growth occurs in hot weather with several commercial growers on the west coast. They report that they don't start getting good growth until the weather gets hot.
Most of us try to grow rhododendrons in the soil we have in our gardens. If they would grow there without any further help from us, nature would
have them growing there already. Even in areas where the climate is cooler, nurserymen have learned the value of good drainage for rhododendrons. Mr. Tony Shammarello of Cleveland, Ohio tells me that he has a rock base under his nursery. Ted van Veen uses drain the under his plants along with one foot of fir bark and sawdust incorporated into the soil.
I feel it is time the people who spend their money for these plants are told how to grow them. I realize some nurserymen won't be selling as many plants because the buyer's survival rate will increase to better than 95%. Ninety percent of all rhododendrons in the trade today will grow here in South Carolina if a suitable rock bed is used. The reasons this rock bed is so important are:

  1. It serves as a barrier to roots to prevent their entering the tight soil which almost surely contains Phytophthora.
  2. It gives drainage which these plants must have in order for air to reach the roots.
  3. It holds the root zone at a cooler temperature much longer than ordinary soil.
  4. It prevents surrounding surface water from entering your planting media which can be disastrous after the soil temperature reaches 70 degrees F.

My conclusions about vigorous plants are based in part on my own experiments, but also from studying the growth of plants in nature. The unhealthy and dying plants of minus and maximum in the wild have the tell-tale pinkish-red band around the cambium layer where the plant enters the soil. Digging the plant will usually reveal that the roots have entered tight soil. This is really what made me change my horticultural practices.
Since I have been following these procedures I have had less than 0.5% mortality rate and the plants have become unbelievably vigorous. What I'm trying to tell you, in essence, is that we shouldn't fight nature but rather duplicate it and make it work for us.
While the rock bed, media and water described above will promote healthy and vigorous growth of rhododendrons we need to go a little beyond what nature provides for maximum growth. Nature is in less of a hurry than most of us. We like to see plants reach a large size quickly so we can market them if we are commercial nurserymen, or to see what our crosses have produced if we are plant breeders, or see our landscape plan develop if we are homeowners. Fertilization is necessary for this very rapid and vigorous growth. For this I use a 20-40-12 soluble fertilizer with the addition of eight soluble trace elements plus ferrous sulfate, magnesium sulfate and gypsum.
Some of you may have some doubts about what I have said about duplicating nature. Get your party together and I'll be glad to spend a few days with you in the mountains if it takes that long to enlighten you on the culture rhododendrons and azaleas are accustomed to in the wild. I only wish I had had this information eight or nine years ago. I wouldn't have wasted so much time and money. I further challenge any horticulturist or botanist, with Ph.D. or not, to prove me wrong in any of this.
If you have doubts about the success of my system in the hard clay region of South Carolina with its hot summers you are invited to come to my nursery in Clemson and see the "Proof of the Pudding."