abc logo 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:

  • 1. Select a relatively straight stem from the current year's new growth while it is still green and somewhat soft. Not too soft but before it becomes very woody and stiff. The best time for taking cuttings is July through September.
  • 2. Use clean and sterilized nippers to cut the stem and get a cutting with its leaf whorl. Preferably, the cutting should be around two to three inches long. If the new growth is shorter, the cuttings can be shorter, but this may make rooting more difficult .
  • 3. This step is important and may be the trickiest part of the whole procedure. Using a sharp and sterilized knife, carefully make a slice in the top bark to expose the lower cambium layer near the bottom end of the cutting. The slice should be from 1/4 to, 1/2-inch long and may be made on one side or both sides of the cutting.
  • 4. Dip the cutting into clean water and shake off excess moisture. Various preparations contain auxins and are available in nursery supply stores. For the hobbyist, one of the most convenient of these rooting aids to use is called Hormodin and contains about 0.8% of the active ingredient in talc powder. The wounded and pre-cutting is dipped into the powder and the excess is carefully tapped off. It is then gently inserted into the propagation medium described in previous ABC's.
  • 5. With the case closed to conserve moisture and the temperature held at about 70 degrees F, the treated cuttings will callus and then produce a root ball, generally within a period of 8 to 10 weeks.
  • 6. At this point, the rooted cuttings are removed from the propagation bed and transplanted to a growing medium similar to that used for seedlings. Using these simple procedures, it is possible to propagate most rhododendrons.

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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Dave Goheen