Richard W. Bosley, Mentor, Ohio
In the 25 years I have been in the business of growing hybrid rhododendrons I have been exposed to many expert opinions both in person and through programs and publications such as the Plant Propagators Society. I have appreciated these ideas as they have been useful in expanding my knowledge and often enabling me to produce a better product. Two years ago I left the nursery I had been associated with for most of my life to start my own nursery which is called "Plant Systems". I would like to share some revelations I have had regarding one hybrid Rhododendron, 'Nova Zembla', during these two years and how these experiences compare with the myths I have been exposed to over the years.
I do not wish to imply in any way that the experiences that others have had were not valid because I am sure they were. I would also like to suggest that what worked for me may not work for another person or for another plant.
To give a little background regarding the crop environment, they were potted into ½ bushel baskets in a medium composed of 70% ground bark, 15% course sand and 15% peat. They were subject to constant feed through the whole growing season. Soil and leaf samples were taken every three weeks during active growth in order to keep the nutrition at a near optimum level at all times.
Irrigation was supplied as needed and 1½ inches was supplied at each watering. There was no manipulation of day length or night temperatures.
Multiple Vegetative Bud Breaks
For years I had understood that if the rhododendron were not given a "rest" that the number of vegetative breaks would be few. My experience is that the rhododendron will probably grow continuously for several years with generous breaks. In the middle of this past growing season I found that the common number of breaks was three to seven and some were as high as ten to twelve. This was not following a rest but after two months of growing and so I suspect that multiple breaks are more a function of nutrition, temperature, moisture and humidity.
Rhododendrons normally grow in flushes rather than continuous, as an azalea will, but under the near optimum conditions this past spring I observed these rhododendrons cycling into continuous growth instead of flushes.
I was assured that my crop was placed much too close (the baskets were touching but not nested). Experience showed that the interior plants were superior to the edge plants. Again I would like to stress that this may not be true at all under different conditions or other cultivars.
All the nutrition these plants received, from me, was in the inorganic form. Some would suggest that if you don't use this or that organic form of feed the plant will not do well. I used: ammonium nitrate, potassium nitrate, di-ammonium phosphate, magnesium chloride, and iron. All were applied through the electronic injector.
Soluble Salt Levels
It is generally agreed that rhododendrons are a "low feed" crop, and I would agree in principle but I would like to site some extremes I have experienced. You must use caution in taking raw data from one person and using it in your own situation. This past summer I sent identical samples to two different laboratories and found nearly a 300% difference on the reporting on two elements. If you hear figures, check them with your laboratory and under your conditions before committing a crop.
This past year I had some rhododendron 'Nova Zembla' growing in poly bags in pure peat. I was starting to get some foliage burn indicating high salts and therefore sent soil samples in and found the approximate soluble salts to be: 8.000 (ECe 3.6). This is a level far above that which I would have believed the plants would have survived but they did once I had applied four feet of water. There has been little recognition of the fact that soils vary greatly in nutrition retention and what might be an optimum level in pure peat would certainly kill a plant in Lake County, Ohio soil. Any laboratory should relate their soil report data to the exchange capacity of that medium.
Most of us growing rhododendrons have played with growth retardants during the past few years. One of the materials that has been mentioned often is Phosphon. It has been suggested that once the plant has been transplanted that as new roots penetrate the new soil that the plant growth will return to normal. This is not true, at least for me, and any person buying a Phosphon treated plant may find the dwarfing persisting for a number of years. This may not be bad but I prefer less persistent chemicals for growth regulation.
The greenhouse industry mentions night temperatures as separate from day temperatures in the culture of most of their crops. It is seldom mentioned in the nursery industry. I have found that the rhododendron is much more responsive to 65 degree minimum night temperature than it is to night lighting and with much less fuss and expense. I am not knocking night lighting because I use it too, but when it comes to growth stimulation don't overlook the night temperatures.
Root Rot Disease
There has been stress put on root rot (water mold) disease control in nursery crops. It was probably first recognized in connection with rhododendrons but it effects a wide list of nursery crops. We had been having the problem for as long as there were nurseries but it was less of a problem because the much better drainage of the field soils was putting the disease organisms at a disadvantage. As soon as we shifted to container growing and set them on black plastic they at last had the free moisture they needed to survive and spread. The shortness of the soil column in a pot tends to cause a wet zone at the bottom which large or multiple pot holes will not correct. Shoot for a well drained mix and never let free water stand around the base of a pot - even during irrigation. This is a much better root rot control than all the chemicals.
There has been a lot published about how to encourage bloom bud formation on rhododendrons and I suppose the surest method is wait until it is seven years old' Little is still known about the budding mechanism and not much more is known about why some things work - sometimes. I would like to know a lot more about why the plant forms a vegetative or a bloom bud and when this decision is made by the plant. I find that many of the techniques that growers have used in the past either don't work all the time or are very dangerous in regard to the possibility of root injury. If you injure the roots it is the path into the plant that the root rot needs.
The throttling of plant growth should be possible with chemical growth regulators rather than the usual method of water withdrawal. This would maximize food production while allowing cell size regulation.
The plant breeder in the future will be the one responsible for at least a portion of production cost reduction for the grower. At the present time I screen rhododendron varieties that show potential to see if they will respond to the accelerated growing methods. If they do not meet the growth criteria they will be rejected no matter how well they may look as an adult. This approach is common in the pot mum industry and is coming to the nursery business.