Vegetative Propagation By Cuttings
Rhododendron ABC's #15
We have focused on growing new rhododendrons from seed to produce plants which, in most cases, are quite unlike the parent plants used to create the seeds. The only exception is if species plants of relatively uniform genetic composition are treated with either their own pollen or pollen from plants of the same species. In this case, the seeds will produce plants very much like their parents.
In order to propagate hybrid plants containing diverse genetic compositions, it is necessary to use vegetative procedures (asexual propagation) to create offspring exactly like the parents. There are several ways to do this: Grafting, layering, or rooting cuttings or slips.
The first two procedures are now mostly used in special cases and will be taken up in later issues. Rooting cuttings is by far the most widely used method, and, I believe, the easiest and most rapid way for hobbyists to increase stocks. Both hybrid and species plants may be duplicated by this method.
There are methods that can be used by almost anyone to produce rooted cuttings. Remember, rooted cuttings produce plants with exactly the same traits as the plant from which the cutting was taken. The scale that I am suggesting is limited, say, to 100 cuttings or less. Commercial growers use similar techniques, but have developed much more sophisticated and larger equipment that allow efficient rooting of thousands of cuttings annually.
The first item to consider is the propagating case. All that is needed is a rather shallow box that can contain 4-5 inches of a propagating mixture. Two things are necessary for the box to be a success: Relative humidity over the cuttings must be held near 100% and the medium in the box must be held at about 70° F. Rooting can be done at lower temperatures but it is very slow and there is greater probability of failure due to fungal infections.
Humidity can be maintained by fogging with water sprays. A much simpler method is to erect a canopy of wood or wire over the box with a couple of feet of head space and drape polyethylene plastic over the canopy to keep e evaporation. Warmth can be maintained in the cutting bed by placing the box over any convenient low heat source. Cables, with easy to regulate thermostats, are most convenient and are relatively inexpensive.
The next step is to prepare therooting medium There are many mixtures that have been successfully used. I don't want to imply that the mixture I use is the only one, or the best. I have, however, found that the same medium used for seedlings works well for cuttings. Thus one should prepare a well-mixed, most medium of the following composition (percentages are by volume): 40% Canadian peat, 40% agricultural perlite, 20% sharp, mason's sand.
The mixture in the box should be at least four inches deep. You are now ready to insert the slips or cuttings. The selection and subsequent treatments of appropriate specimens to produce rooted cuttings capable of maturing into plants will be outlined in the next ABC's issue. The steps are relatively simple and production of new plants from old ones can be a very satisfying experience for the hobbyist. I can still remember the thrill of seeing the root balls on my first cuttings!
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