Propagation By Grafting and Layering
Rhododendron ABC's #17
While the majority of rhododendrons can be successfully propagated by cuttings as described in the two previous ABC's, a few rhododendrons do not respond very well to the cutting procedure. In such case, other methods which are more cumbersome and tedious than propagating via cuttings must be used.
The two most widely available alternatives are grafting and layering. Both of these methods have been known and used in horticulture for a long time. In fact, before the advent of hormone preparations to help induce root formation in cuttings, grafting was the main method of increasing stocks ofdesirable rhododendrons in the nursery trade. The techniques of grafting, while theoretically simple, require a certain amount of skill and experience and, in these days of high labor costs, are used commercially only when cuttings just will not root.
For the hobbyist, grafting can be interesting and useful. It can be used
to save scions from desirable plants obtained at times of the year when
cuttings do not root well. Details of the various grafting modes are beyond
our ABC's discussions. Information on grafting is available from the County
Extension Services and libraries. Basic, requirements include:
1. A sharp sterilized knife
2. A scion of green wood from the plant to be propagated
3. A root stock to receive the scion
4. Some way to protect the union of the scion with the rooting stock
5. Conditions that result in active growth of the root stock, i.e., warmth and light (particularly if the graft is made in the winter).
There are a number of techniques. They all require that the growing cambium layers of the scions and the root stocks must touch and be held together securely. The basic procedure is called a cleft graf.t In this case, the scion is carefully sliced to create a wedge-shaped end from I to 2 inches long. The root stock, which must be in an active growing condition, is then carefully severed and split with a grafting knife. The split must be long enough to receive the wedge-shaped scion end. Now comes the most important part of the process - insert the scion into the split root stock so that the cambium layers of the two parts are closely touching. They must touch and be held together or the graft will faill. Secure the union with rubber binding and cover with a grafting compound to exclude infection and preserve moisture. With proper care, the joining will occur within a few weeks, and the scion will come into new growth producing a new plant
From this overview, one can see that grafting is much more tedious than the propagation of cuttings. Thus, it is now relatively hard to find grafted plants in the nursery trade. As mentioned, it was once quite common.
Another propagation technique that can be used is called layering. In this case, a plant limb is carefully bent so that a portion of the limb can be held underground or 'in the growing medium. A wound can also be made in the buried part of the limb to help rooting. Sometimes treating the wound with hormones will also help. After a rather considerable period, roots will form, and the limb can then be severed to give a self-sustaining new plant. An alternative to ground layers in called air layering. In this case, the selected limb is wounded and treated with hormone preparation. The selected area is then packed with damp sphagnum or peat moss, and covered with a plastic to form a pouch that will not dry out After a considerably time, roots can form (can but may not!) and the limb can be severed to produce a new plant.
Layering is now used only very sparingly and in exceptional cases. It is interesting, however, and the hobbyist probably should try it a few times to see that it can work.
Mention should be made of a commercial propagating method called tissue culture. This method produces many new individual plants from bits of tissue taken from a plant to be propagated. The procedure is very exacting and requires sterile laboratory conditions. It is of interest, but the method is really not available for the hobbyist and is only successful in specialized commercial operations.
© All Portland Chapter content copyrighted 2011. Content, including photos, may be reproduced only with permission of the Portland chapter of the ARS.