Using Hormones To Root Cuttings
Rhododendron ABC's #16
The last issue addressed preparing a compact- heated propagation case, suitable for a rhododendron hobbyist to use. The next step is to go about the seemingly unlikely task of causing a woody stem to produce roots and, ultimately, a new plant.
Until about 40 years or so ago, little was known of the mechanism that plants use to cause certain cells in the plants cambium (inner bark) layer to differentiate and send out roots so that a cutting may take up nutrients and become a new plantindividual. We have known for a long time that some plants are easily duplicated by simply inserting cuttings into almost any well-watered earthy space. While others, including rhododendrons, rarely, if at all, will respond by simple insertion of woody stems.
In the Western World, it was taken for granted that only a few woody plants could be propagated this way. In China, however, it had been discovered covered more than a 1,000 years ago that treating woody stems with the urine of barnyard animals would, in some magical way, causing roots to form and produce new plants. Chinese farmers had no idea of how or why this happened, but they did make good use of this "magic". We scientists finally got around to studying this "magic". Within a relatively short time they found that growing plant cambium cells require, what has come to be called plant hormones or auxins, in order to differentiate from stem cells to root cells. The detailed chemistry of all this is beyond the scope of our discussion but it has been found that auxins, produced abundantly in easy to root plants and sparingly in others (e.g., rhododendrons) are related to substances found in (you guessed it!) barnyard animal urine! The basic chemical is termed indole acetic acid. Related synthetic chemicals that are even more active, such as indole butyric acid, have been discovered. Use of these materials in rooting mixtures has made major improvements in plant propagation techniques. Their use has resulted in the propagation of most woody plants.
Whew! Let's leave all this chemical science and get down to the practical steps of rooting cuttings:
Despite the use of rooting compounds, some rhododendrons resist and will not respond to the cutting treatment. These require grafting or layering which will be addressed in the next ABC.
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