The Forcing of Rhododendrons
James S. Wells, Red Bank, N. J.
It seems that we are approaching a period of intense interest in rhododendrons. Looking back over the past twenty years, there have been phases in the horticultural activities of this country, associated with the large scale use of cheap Conifers, followed by the equally large scale use of superior Taxus, and now, rhododendrons seem to be coming to the fore. Wherever the soil conditions are suitable (and even where they are not) people are using rhododendrons more and more. Associated with this rise in general popularity can be the use of the rhododendron as a florist's plant, similar in every way to the Azalea. We have been interested in this possibility for the past two years and have endeavored to find as much information as possible on the subject. There is very little. Some information is available from Holland, and a resume of this, giving varieties and temperatures for forcing, is quoted in David Leach's excellent book, "Rhododendrons of the World."
This report, interesting as it is, is confined to some of the older varieties and it indicates that while many of these are quite suitable for forcing, the minimum time required is about eight weeks and the best variety suggested for this purpose is 'Catawbiense Boursault'. We felt that this was rather a long time, for it should be possible to force a rhododendron more rapidly than this. So, for the past two years we have been carrying out very modest experiments and we think we have found out a few simple facts which may be of interest and value.
As might be expected, the most important item is variety. We have found a wide difference in the response of different varieties, and we have found that the varieties which bloom earliest in the Spring are the most responsive.
A typical example is 'Cunningham's White'. Here is a plant with the required round, compact shape which all growers of this plant know, and which may bloom in the Fall, if conditions are right. This would indicate that it is highly responsive to variations of temperature, and it's natural dormancy must be practically over as the plants enter the Winter. This variety responds very readily to forcing conditions and can be in bloom in about six weeks. There are a number of new hybrids, using 'Cunningham's White' as a parent, especially among the group of Shammarello plants, which are much better in color, equally good in plant habit and which are equally responsive to forcing conditions. The Shammarello variety, 'Cheer', is especially to be recommended, because this produces a fine, large truss and responds to the forcing conditions very well indeed. I believe that this variety, which has a most interesting foliage pattern as well, is destined to become a first class florist's plant. There are others in this Shammarello group with a similar parentage, which we have not tested, but it would seem reasonable to suppose that any of the 'Cunningham White' group may be as valuable.
We have discovered a second variety which is even better than this, strange to relate, and this is one of the Dexters. It is, I believe, a cross between R. fortunei and R. haematodes. It was introduced first from the Morris Arboretum, Philadelphia and was known as Morris #3. It has since been named by John Wister, 'Wissahickon'. This plant is quite easy to propagate and has only one disadvantage. This is, unless it is pinched back, severely, as a small plant, it tends to grow tall and rangy. However, with a little careful attention to pinching in the early stages, a well-shaped plant, 15 inches high and budded, can be "constructed." This plant buds very heavily and the color of the flower is a deep rose red, HCC 24/2 Tyrian Rose, on the Royal Horticultural Society Color Chart. When forcing, of course, the color is not quite as deep, but still a bright attractive shade of pink. The real value of 'Wissahickon' is that it is the most responsive variety we have found, to forcing conditions. If the plants are allowed to come to dormancy in the open ground and are subjected to one or two frosts, we can lift the plants about the middle of November, pot them, and obtain a little root action by about the first of December. Then we bring them in, to develop in gentle heat. Plants maintained for one week at 65 degrees, and then kept for three to three and a half weeks at 70 degrees, will show color in four weeks and should be in full bloom in five weeks. We believe that we can get plants in bloom for Christmas, particularly if they were lifted earlier and pre-chilled. We have no facilities for the careful evaluation of the varieties, similar to that applied to azaleas, but from the response we have seen to our methods, we believe that 'Wissahickon' is a variety which can produce bloom from Christmas until the normal flowering date, which is about May tenth, in New Jersey.
The method which we used to force these plants is very simple. The plants are lifted from the open ground toward the end of October. They are potted into a suitable mixture and held in a cool greenhouse to re-root. They are placed, then, on a bench at 60 to 65 degrees, for about a week. Then the temperature is raised to about 70 degrees until color is seen. The plants are syringed as frequently as possible, usually three or four times per day. We have had equally good results by placing them in a bench in which we have a mist propagation system.
We are now attempting to gather varieties which we normally would not grow because they are not hardy here, but which the records of the Society indicate are early bloomers, plants such as 'Christmas Cheer', 'Rosa Mundi', etc. We believe that any of the varieties listed as early bloomers should respond in a similar manner to forcing conditions and this is what we hope to test in the future.
We believe that there is a tremendous potential here, which has not been tapped at all. Plants of the right kind will have to be specially grown to produce the compact, well-filled plant. low to the ground, which is essential for retail sales. But, varieties are available with this natural tendency and I am quite sure that growers will, in the not too distant future, produce and sell to garden lovers all through the country, prepared stock of this kind.