Propagation of Rhododendrons
Alfred J. Fordham
Arnold Arboretum, Harvard University
At the Arnold Arboretum the propagation of broadleaved rhododendrons has been carried out under polyethylene plastic for many years. The peninsular type benches which we use are 6 feet long by 5 feet wide and 6 inches deep. They are constructed of 3/4 inch transite. To prepare them we first line the inside with 2 mil polyethylene plastic. One and one-half inches of medium is then placed in the bottom and heating cables are installed at that level. A medium consisting of equal parts sphagnum peat moss and horticultural grade perlite is then used to fill the bench. Welded joint wire of 2 x 4 inch mesh is used to support the 2 it polyethylene covering. It is known as turkey or utility wire and can be purchased by the roll. It may be cut and bent to any desired shape. Our frames are fashioned to hold the plastic about 10 inches above the rooting medium. Bottom heat is maintained at 75 degrees.
In autumn, after the summer propagation program is completed, we shut down our mist units. Winter propagation of cuttings is done either under polyethylene or on open greenhouse benches.
After cuttings are inserted, the medium is watered so that it will settle around the cuttings. The transite bench sides are wetted and the polyethylene film is placed over the framework and down over the wet transite. It clings tightly, thereby making the unit vapor-proof. On cloudy, humid days the coverings are removed and the cases are checked for fallen leaves and dead cuttings. These are removed as a sanitary measure. An inspection at this time reveals whether the medium is drying or if infection is occurring. Before replacing the polyethylene, a spray application of 50% Captan wettable powder at the rate of 2 teaspoons to the gallon is applied as a precautionary measure. (This would only be necessary once in several weeks.)
Polyethylene plastic has the property of being air permeable yet vapor proof. Temperatures within the propagating structures are warmer than the surrounding atmosphere and water continually condenses on the inner surface. It accumulates in droplets which become too heavy to remain and fall back onto the cuttings and into the medium. The supporting framework should be formed in a manner whereby the polyethylene lies flat at the top. Having it parallel to the medium leads to even distribution of the falling drops. This constant circulation of moisture plus the air permeability of the plastic, no doubt accounts for the fact that we have never experienced stagnation despite the warm humid atmosphere.
The use of polyethylene chambers has some distinct advantages. Nutrients do not leach from the cuttings as can happen under mist. It is carefree and can be left for weeks on end without attention. There is little chance of loss through mechanical or human failure. In areas where hard water presents a problem, there is no build-up of substances on the cuttings. Many subjects considered difficult such as mountain-laurel (Kalmia latifolia) and Pieris phillyraefolius can be rooted in high percentages.
When to take rhododendron cuttings
Broadleaved rhododendron cuttings are made from current years' wood and can be taken as soon as the growth ripens. Ripeness is indicated by a change in color from light green of new growth to the dark green of the mature growth. In the Boston area, the first flush of growth reaches this stage about mid-July. At this season rhododendron propagation is not convenient to our work schedule so it is delayed until October and November. Cuttings taken at this time have rooted well while those taken later in winter have not been as successful. Vegetative growth is preferred and cuttings bearing flower buds are avoided when possible for it has been shown that they may not root as well. However, it is sometimes difficult to get cuttings which do not bear flower buds, particularly from plants which are in poor condition. We are sometimes confronted with this situation for one of our responsibilities is the propagation of plants which are failing. In such cases we break off the flower buds when the cuttings are being made and rooting usually follows.
Wounding of rhododendron cuttings
In order to induce the cutting to initiate a well distributed root system, wounding is performed. This is accomplished by removing two 1 to 1½ inch slices from opposite sides of the cutting's lower stem. By this procedure more surface is exposed to the action of root inducing substances and large numbers of roots develop from the wounded areas. Wounding on two sides is important for cutting only one side leads to a lop-sided root ball which is difficult to manage. We favor deep wounds which cut through the rind and take part of the wood. When rhododendron cuttings are wounded lightly the wound can heal over quickly and lead to what is essentially an unwounded cutting.
A few years ago a local nurseryman was perplexed by the fact that in some years his rhododendron cuttings had such poorly attached root systems that they fell away when the cuttings were handled. We noticed that the wounds had healed over and rooting occurred only from the bases of his cuttings. In order to verify our suspicions we prepared cuttings using a light wound and the next few slides demonstrate the outcome. The rooted cuttings appears to have a satisfactory root ball but such is not the case for the entire mass stems from a solitary root which arose from the base. The cutting is now raised slightly to show a wound which has healed and from which roots have not originated. When the cutting was lifted the root system promptly separated. The nurseryman's comment that this occurred only in certain years could well be explained by the fact that different people had done the work. In years when root balls were satisfactory the propagator may have wounded heavily, while in these years when they were unsatisfactory, another worker may have made light wounds.
Root inducing substances
During the 1962 meetings of the International Plant Propagator's Society Dr. Charles Hess described a discovery made at the Boskoop Experiment Station in Holland. They found that by adding 'Captan' to root inducing substances the rooting of cuttings was greatly improved. After hearing his remarks we prepared mixtures as follows: 50 grams of 'Homodin #2 and #3' - were each combined with 10 grams of 'Captan 50% wettable powder'. During the 1963 summer season the wide variety of subjects that we routinely propagate by cuttings were treated with these combinations on a comparative basis. The results were so strikingly favorable that all our root inducing powder formulations now contain a fungicide. However, we no longer prepare our own mixtures for IBA formulations with the fungicide Thiraet added for they have become commercially available. Such a preparation with IBA at the rate of 8 m. to a gram of talc have worked well with both broadleaved rhododendrons and azaleas.
First winter survival of azalea cuttings
Many deciduous azaleas which propagate from softwood cuttings during summer present a survival problem during the following winter. They go into dormancies from which they never recover. One means of overcoming the problem is by inducing the cuttings to make new growth after they have rooted. This is done by providing supplementary lighting.
First winter loss can also be averted if the cuttings are not disturbed after they have rooted. We do this rooting of the cuttings in plastic flats under mist. When rooting has taken place the cuttings are left undisturbed in the flats, given a light feeding and hardened off under polyethylene plastic. In our work we deal with a large variety of taxa but with small numbers of each. Cuttings are started at varying times and some root sooner than others. This makes it impossible to use a weaning procedure after cuttings have rooted. Therefore, we place them under polyethylene plastic for hardening off, and it works well. In November the trays of undisturbed cuttings are transferred to our cold storage unit where the temperature is maintained at approximately 34°.
In February or March the trays of rooted cuttings are returned to a warm greenhouse where new growth soon appears. The time of return to the greenhouse is based on convenience to the work program rather than a specific time schedule. As soon as growth begins the cuttings are moved to peat pots or plastic containers depending on how they will be carried on. Those to be planted in our Saran Shade house would be moved to peat pots, while those to be grown in our can section would be placed in containers.
This slide shows a tray of rooted Rhododendron prunifolium cuttings which were treated as previously described. One hundred and thirty cuttings were originally inserted and 121 are now growing and ready to be processed. Enkianthus cernuus rubens, a relative of the azalea, which has also presented first winter survival problems is now shown. Thirty rooted cuttings were left undisturbed through winter and all survived. Twenty-four cuttings of Enkianthus perulatus were treated similarly and 21 came through in excellent condition. In contrast, 25 heavily rooted cuttings of E. perulatus were moved after rooting, and in spring none revived.
Through the years the Arnold Arboretum has imported many azaleas. Most have been from Europe while a few came from Japan. In 1965 a shipment comprising 'Knap Hill', 'Exbury' and 'Ghent' azaleas was received from a well known European nursery. It consisted of 35 cultivars with 2 plants of each. Thirty-one kinds were grafted and 4 were layered. The plants arrived in mid-March with washed roots and after having passed through the quarantine procedure at Port of Entry, they were put in containers and placed in a warm greenhouse. The night temperature was 65 degrees while on sunny days it rose much higher. We use warm environments when dealing with imported plants that have had their roots washed and it has been highly successful. Of the 70 plants 68 came into growth and grew well. Unfortunately, the two which failed were of the same cultivar.
As the plants became active, suckers from the understocks also developed and in many cases became a serious nuisance. We make it a practice to propagate imported plants as quickly as is feasible for sometimes the propagants survive when the originals fail.
The plants were kept in the greenhouse until danger of frost had passed. In the third week of May they were planted out in our Saran Shade House. Softwood cuttings were taken in mid June. In the first attempt at propagation by cuttings, 31 of the 34 cultivars rooted readily and most did so in high percentages. Grafting azaleas such as these is not only unnecessary, but it is unjustified. It is as bad as the practice of propagating lilacs by grafting on lilac understocks. In either case the insidious take-over understocks can lead to trashy plants rather than the named cultivars which the purchaser had chosen.
The next few slides show the outcome of work done a few years ago in the propagation of a rhododendron relative, Elliottia racemosa. This subject has remained rare since its discovery over 160 years ago because of propagation difficulties. Root sections about Vs of an inch in diameter and about 4 inches long were taken when the plant was dormant. They were placed horizontally, one half inch deep, in flats of sandy soil. This was done on March 24th and by May 19th multiple shoots began to appear. As they became large enough, they were removed and inserted as cuttings using a medium suitable for rhododendrons or azaleas. The root pieces were left in place and continued to produce crops of rootable shoots for more than a year. All cuttings taken rooted quickly and presented no first winter survival problem for they were physiologically juvenile.
Mr. Henry Hohman of Kingsville Nurseries has informed me that he has successfully propagated every native azalea from root cuttings. Shoots that arise from azalea root pieces would behave as did the Elliottia and root readily, no matter how difficult stem cuttings from the same plants might be. It should be added that shoots which originate from the bases of plants are also juvenile and therefore more rootable than would be growth taken from higher levels. Root inducing substances are of benefit when dealing with juvenile shoots - they lead to faster rooting and more extensive root systems.