Rhododendron Propagation - Philadelphia Style
Ross B. Davis, Jr. Wayne, PA
Many years ago a stranger rang the front door bell and asked if he might take cuttings from some of my rhododendron. When asked, "Can you root them?", he modestly admitted to having had success with some ironclads and some unknown hybrids from two old estates on the Main Line.
This was a wonderful discovery for me. After working with the then stranger and now old friend, George Capriola from Devon, for two years or so, rooting some of the "new" Gables, 1 decided to build my own greenhouse. It is small, 10' X 17' plus a potting and heating area, and by no means in a commercial classification. Since my production of a few hundred plants per year is a peanut size operation to the commercial grower, it would be presumptuous on my part to appear as an expert. However, in the past twenty-five years, I have probably made most of the possible mistakes in propagation. Perhaps others may avoid the frustration, time, and other penalties of my mistakes.
Twenty-five to thirty years ago the commercial propagators played things close to the vest and jealously guarded their secrets of propagation. James S. Wells' book, Plant Propagation Practices, published in 1959, plus my friend George's access to a few Ph.D.'s in Chemistry opened a new world to me. The USDA, various university publications, and ARS Quarterlies have been other valuable sources of information.
Now I could branch out, pick other people's brains and select the best rhododendron for this area. Thanks to such people as Dr. John Wister, Dr. Franklin West, Charles Herbert, Joseph Gable, Heman Howard, and others, the cream of the crop was now available.
All that is done in a greenhouse may be done in your basement with artificial lights and a polyethylene encasement. There are Nearing Cold Frames and other successful methods which have been reported in the Quarterly and there is no need for repetition, so let's start in the greenhouse.
The first order of business is to strip the house of all old rooting material, soil, or vegetative material and then sanitize thoroughly. Clorox and other similar materials are satisfactory for this purpose, but be careful of the chlorine released as it is a deadly poison. The sanitizing step is vital to successful propagation. During the rooting period any dead leaves or cuttings should be removed and destroyed, thus eliminating a possible source of fungi or disease.
After the clean up, the rooting bed, which is 8" deep, receives a 2" layer of perlite, then the electric heating cable controlled by an accurate thermostat is installed, and a 6" layer of 60% coarse peat and 40% agricultural grade perlite is added. The bed is treated with Captan and with Aqua-Gro, a non-ionic wetting agent. Aqua-Gro was used originally to wet the peat used in the rooting bed and is also used for all potted and flatted plants as insurance against over watering. Captan, besides being a fungicide, is reputed to be a rooting aid.
If I had my "druthers" in timing the taking of cuttings, I'd take cuttings in July, using the first growth which, by then, is usually sufficiently hardened off. Small leafed varieties may often be taken sooner. The old commercial propagator usually preferred one or two stinging frosts, especially on the red varieties, before taking cuttings, but this has not proven out for me in the Philadelphia area.
A sharp pruning shear, not of the anvil type, is used to take the cutting and to reduce the cutting to three or four inches long. All but three to five leaves are then stripped off. On very large-leafed cuttings do not hesitate to remove approximately one-half the leaf with shears or scissors, thus reducing or eliminating excessive overlap of leaves in the cutting bed.
The cutting is wounded by removing a 1" slice on each side (double wounding) being careful not to wound to the bottom. This method of wounding appears to help reduce stem rot. The cuttings are then dipped in 2-3% indolebutyric acid powder and tapped to remove excess powder. The cuttings are then ready for sticking.
To interject an opinion at this time about the "hard or impossible" to root varieties, I believe that almost all can be rooted. (See Note A) in these cases it seems obvious that a vigorously growing plant stock is necessary - preferably a young plant (juvenility). On past occasions, cuttings from old plants resulted in a 10% or less success rate and from younger, vigorous plants a 60% or better success rate. With any given strength hormone, or rooting ester, perhaps earlier or later timing would provide the key to success. Sometimes 2-4-TP in weakened strength has proven successful. There are other rooting materials that should be explored.
However, there is one word of caution in this approach. There are dip materials in the trade that call for a few seconds of soaking. Beware of exceeding directions or of using too strong a hormone which may produce excellent rooting percentages but the resulting plants may not break into growth for one or two years, or may die within a year.
After the bed and cuttings are prepared, a lead hole is punched with a pencil, 2" separation in the row and a 6" separation between rows. If you wish to be efficient, a series of nails in a piece of 1" x 2" wood strip may be used to punch a whole row of "pencil punching" at one time.
An hour or so after sticking, the cuttings are watered-in with a standard rose spray to settle the rooting medium tight to the cuttings. The heating cable (bottom heat) is set at 70° F and air temperature on the greenhouse heater is set at 60° F. Theory says the 10° F differential produces optimum growth at the root, not the tip. Of course, until the weather cools down in late September, the temperature differential may be rather moot, but saving on fuel.
Sometimes, when cuttings are slow to root, it is helpful to push the bottom temperature to 75-80° F for one to three days and then reduce to the normal 70° F. This process may be repeated in a week or so. Perhaps further inquiry could be made in this facet of propagation.
Misting at all times is set at 10 sec./10 min. during strong sun hours by means of a time clock, and the greenhouse glass has been painted to reduce the sunlight that would otherwise cook the cuttings.
Rooting usually takes three to six months, depending on many factors, but when the root mass is walnut size or lager, the cuttings are potted or flatted on a spacing of 4" or more each way. The potting medium is approximately 2/3 peat, 1/3 perlite. My use of any soil for potting was abandoned years ago. Contrary to popular belief, recent research very strongly indicates that the no-soil medium does not inhibit the roots from expanding into the surrounding soil when planted outside. After approximately 200 plants are flatted and located in the side of the greenhouse opposite the rooting bed, they are subjected to flash lighting. In my case, flash lighting consists of 4 seconds of 20 foot candle incandescent light per minute from 10 P.M. to 2 A.M.
The USDA once recommended 1 sec./min. but now says 10 sec./min. is necessary. Since my Rube Goldberg timing device produces 4 sec./min. and seems successful, let everyone, as they say in Brooklyn, "Pay yer money and take yer chance."
If you use fluorescent light, flash lighting requires a special ballast and the problem of the red spectrum must be accommodated. With the flash lighting, fertilization is done every 10-14 days and, since I'm careless, sometimes less frequently. The non-scientific approach is to use one level tbsp. of Rapid-Gro or other foliar fertilizer per 12 qt. watering can and just water the flats and pots with this dilute solution.
Using this method with July or August cuttings it is not unusual to have 18" plants with 2-4 breaks by the middle of the following May, when frost is unlikely here. Then all plants are planted outside in slat shaded areas for one year.
After all my ramblings, please do not abandon any method that has been successful for you, since one cannot argue with success. However, a new method may be tried and evaluated against your existing method and whichever proves best for you should be followed.
A successful method for hard to root clones has been to take a cutting of an easy to root clone and to it, cleft graft the hard to root clone cutting. An example is 'Acclaim' (hard), the scion, grafted to 'County of York' (easy), the stock. Neither is yet rooted. Stick the 'County of York' in the rooting bed, barely covering the graft. This oddity has rooted for me in as little as six weeks, but count on 9-12 weeks. After rooting, the plant is potted about ½-1" deeper than when in rooting bed. Any suckers are removed and the 'County of York' leaves gradually cut away. When planted outside, it is again "buried" ½-1" deeper. The result is an own-root plant to all intents and purposes. With this method the peat perlite mix seems imperative.