The Propagation of Hybrid Rhododendrons from Cuttings
By P. H. Brydon
In the title of this paper the term hybrid has been used purposely to indicate that it is concerned with the increase of cuttings of such varieties as 'Alice', 'Betty Wormald', 'Lady Bligh', 'Jean Marie de Montagu', to mention a few, although the methods outlined might be employed to root the more easily propagated evergreen azaleas and such species as R. racemosum, R. ciliatum, R. crassum, R. hippophaeoides, etc.
Many propagators, consciously or unconsciously, have contributed to the following notes and the degree of success which has been obtained by using the methods outlined is not directly attributable to any one of them but rather due to the techniques which have been evolved by many growers whose successes and failures have been observed through the years. I am particularly grateful to my associates, Mr. John Henny and Mr. Aloys Wennekamp of Brooks for their assistance in the preparation of this article and, what is more important, the opportunity to observe and practice the modifications which they have made on the accepted methods of rooting hybrid Rhododendrons from cuttings.
In 1937, shortly after the publication of a paper on "The Effects of Growth Substances on Rooting Response of Cuttings" by Hitchcock and Zimmerman of the Boyce Thompson Institute I made a series of tests with the following Rhododendrons, R. 'Pink Pearl', R. 'Sappho', R. 'Purple Splendor', R. 'Cornubia', and R. 'Christmas Cheer' using various strengths of indoleacetic, indole-butyric, and naphthalineacetic acid. The results on the treated lots were not spectacular but I was agreeably surprised to find, that, with the exception of R. 'Purple Splendor', the untreated lots rooted on the average of 50% and eventually made excellent plants. By present day standards, I would not consider an average of 50%, as being good, although in 1937 there was no way for me to evaluate the results of the test. Prior to this time I have propagated the larger leaved hybrids in the conventional manner, namely by grafting on R. ponticum and, like many others had assumed that it was not practical to increase rhododendrons by cuttings, the consensus of opinion being that the "strike" would be poor and the resulting plant inferior to a graft. I have since found that neither of these two objections are valid. As a matter of fact, in many instances a bushier and better shaped plant can be obtained from a cutting than from a graft. This is clearly evident in such varieties as R. 'Blue Peter', R. 'May Day', R. 'Fabia', R. 'Mrs. Chas. Pearson', and R. 'White Swan'.
Since 1937 a great deal of research has been done on the use of root inducing substances and according to a recent paper on the "Propagation of Rhododendrons from Cuttings" written by Mr. James S. Wells and published in the American Nurseryman, a commercial preparation called Hormoroot has been found an excellent aid in rooting. It must be stressed however, that the value of these synthetic substances will vary in proportion to the growers ability to provide the optimum external conditions for the initiation of roots on the cuttings. The old quip that "be who holds the hose grows the rose" is especially appropriate in the propagating house! Of course there are many conditions which are beyond the control of the propagator, for example, a sudden hot spell immediately following the taking of the cutting has upset many a grower's calculations. An unexpected frost when the stock plants are making their spring growth will have a considerable effect upon the quality of that year's cuttings. These and other vagaries of the weather are accepted as part of the game, but the selection of the wood, the preparation of the cutting, the correct degree of moisture, temperature, light and the control of insects and fungi are all conditions within the control of the grower and deserve his utmost consideration.
The rooting of rhododendrons on a large scale is essentially a task for the professional nurseryman, although there is no reason why the amateur with a glasshouse at his disposal may not enjoy success in the increase of some of his choicer hybrids. There is always that sense of personal satisfaction in growing your own. While the methods described in the following pages are based upon a commercial production, I am sure that they could he modified by the interested amateur who wishes to try his luck at this fascinating pastime. The fascination is understandable since, after thirty years in horticulture, there still remains with me the temptation to tug just a little at the cutting to "see if it is rooted yet"!
Fig. 1: Typical greenhouse used for the propagation of
rhododendrons in the Pacific Northwest.
Photo Henny and Brydon, Inc.
The accompanying illustration shows a type of glasshouse used by several growers in the Portland area Its dimensions are as follows: length 50 feet, width 17 feet, height at eves 6 feet, height at ridge 9 feet. The construction is of reinforced concrete walls, six inches thick and three feet in height and the superstructure is of kiln dried cedar with tubular steel framework. There are two side benches 3 feet wide and one middle bench 6 feet wide which allows for two paths 30 inches wide, sufficiently roomy for most operations. Our native Oregon cedar, 2 inches thick, was used throughout in the construction of the propagating benches. The overall dimensions of the two side benches is three feet wide and fourteen inches deep. The center bench is 6 feet wide but only eight inches deep and is used primarily for the rooted cuttings when being grown on preparatory to planting out in the lath house. The sashes covering the side frames are six feet long and three feet wide and made from one by four cedar and covered with the heavy grade of a transparent plastic, which is wire reinforced and sold under the trade name of Celloglass. It is imperative that the sash fit closely to the frame top and also that all joints on the sides of the frame be calked or stripped to prevent undue loss of moisture or heat. A space of 1/4 inch between the bottom planks will be sufficient to allow surplus water to drain. Lack of attention to such detail may very well be the difference between a 20% and a 90% strike. There have been instances where a row of cuttings inserted directly below a poorly jointed frame have been killed due to the loss of moisture and heat at a critical stage of growth. Once the roots have formed it does not seem to be so important but during the early stages of rooting and particularly during a hot spell, improperly closed frames can be disastrous. A great deal has been said about that mystical appendage called a "green thumb", but if there is not a close regard for cleanliness in the propagating house then all of a man's digits can be as green as Paddy's coat and still his rooting percentages will be below par. It is excellent insurance to paint the woodwork of the frames and also the interior of the benches with a copper naphthalene stain. This will prevent the growth of harmful fungi and add several years to the life of the lumber. This stain does not seem to have any harmful effect upon the cuttings even on those which lie next to the side of the frame. Before bringing in the peat and sand for rooting the cuttings, all growing material ought to be removed and the house cleaned thoroughly from top to bottom. Brush out the cob-webs, wash down the glass and interior woodwork, damp down the paths and then fumigate with cyanide. After the house has aired for twenty four hours then the heating cables can be placed in position and the rooting medium filled into the benches.
The heat within the propagating frames is provided by thermostatically controlled lead resistance cables and in the house by two 4 kilowatt space heaters. The two space heaters are in use only during the colder parts of the year. Better control would be affected with a circulatory hot water system and there is no doubt but that such a system would be mandatory in a climate more severe than ours. However, it is a question of whether or not an investment of several thousand dollars would be justified in order to provide a more efficient heating system for a matter of 4 to 6 weeks, particularly in view of the percentages obtained without the installation of hot water heat. The winter of 1950 was one of the most severe experienced by Oregon growers, yet the losses due to cold within the propagating houses at Henny & Brydon Inc. were nil for the simple reason that while the temperature within the house was below freezing, there was sufficient insulation provided by the enclosed sash frames to hold the heat given off by the lead resistance cables. (For the benefit of those readers who reside out side Oregon, the lowest recorded temperature at Brooks in January 1950 was 12 degrees below zero.) The lead resistance cables used in the frames is a General Electric product and may be purchased in sets of 60 feet in length which are ready to plug into any 110 volt outlet. It may also be purchased in bulk and cut to length depending upon the service available. Where 220 volts is available then the cable may be used in lengths of 120 feet. A thermostat assembly for providing automatic temperature control is essential and the better ones are accurate to within 3 or 4 degrees. The cables are placed in the bottom of the frames so that they are about six inches apart, which means that on a 220 volt service. a length of 120 feet will be sufficient for approximately 20 feet of a 3 foot wide bench. Once the cables are installed the frames are ready for the propagating medium.
Some of the finest cooks can compound a recipe by using a pinch of this and a handful of that and the resulting cake or pie is just out of this world, yet if they were asked to state the specific amounts used in the recipe they would be at a loss. Mixing a rooting medium is not quite so empirical but the consistency is often a matter of judgment on the part of the propagator. There are no two batches of peat moss exactly alike and the proportion of sand to peat may vary from time to time. Generally speaking, about 60% by bulk of peat moss to 40% by bulk of sand is about right. The peat moss is usually quite dry and is best broken up in the frames and, after adding the sand, the whole mixed thoroughly and spread evenly over the cables. The grade of peat moss use is quite important. If the grind is too fine then the rooting is noticeably poor. The brands known as Green Fingers and Magic Touch have been found excellent and about the right consistency. Washed sand from the Columbia River and obtainable from Portland sand and gravel contractors has been found very satisfactory although the washed sand from the Salem area has caused several growers quite a little grief since it is apparently conducive to the growth of injurious fungi. After mixing the two ingredients the next step is to firm the mixture to the proper level in the bench. The depth of the mixture is a critical factor for if it is too shallow then it dries out too rapidly and the base of the cutting is too close to the heating cable. For most rhododendron cuttings the optimum depth of the propagating medium would be seven inches after watering. This allows for sufficient distance from the cable to prevent drying out and also provides enough peat and sand below the base of the cutting for a good root ball when cutting out time comes. Firm packing is very important otherwise, after two or three watering, the base of the cutting is not in close contact with the medium and poor rooting results. Therefore, after leveling off the mixture, soak thoroughly until the water runs out of the bottom and before inserting the cuttings, tamp the medium by using a two by four laid flat on the surface and pound to the proper level with a hammer. Some growers have found to their chagrin that leaving the peat and sand in the benches for two or more years is not as economical as it appears. After one year in the benches, there is a great deal less air space in the medium caused by the deterioration of the peat moss with a consequent loss of drainage and slowing down of root development. Furthermore there is greater chance of a carry over of injurious insects and fungi. Therefore clean out the frames each year and start afresh with clean peat and sand.
To be continued