JARS v51n4 - Rhododendron Cutting-Grafting

Rhododendron Cutting-Grafting
Dr. Henri Galibert
Saint Geniez d'Olt France

Cutting-grafting is a method of vegetative propagation where the graft of the plant to be propagated is made at the same time as the cutting of the understock. Below you will find a description of the technique that I use, which is my own and does not constitute a lecture on propagation by cuttings or grafting.

1. GENERAL TECHNIQUE. The aim is to reproduce species, clones and hybrid rhododendrons which are difficult to propagate by classical means.

Condition:

To ensure success you must possess understock stems of different sizes and diameters which can be matched to the stem measurements of the plant for grafting; as in any graft, scion and understock sections must permit an optimal superposition of their cambium.

The vegetation of the scion must be later than or at the same stage as the understock.

In practice, after choosing and sectioning the stems of the plant to be grafted, you must choose the stems on the understock which will be used. The time used for this must be as short as possible in order to avoid desiccation of the vegetal tissues.

Means:

(a) Cutting . This is a classical cutting with a final stem of 10cm, mature, whose terminal bud has been removed. Once spare leaves have been taken away, the two to four which are kept must be divided to avoid bulkiness and excess transpiration of moisture.

The lower end must be beveled with a grafting knife, or you can wound it in order to increase the surface of cambium in direct contact with the rooting medium to produce a greater number of roots.

The upper extremity must be split to a length of around 4cm which will receive the scion in a wedge graft.

(b) Scion . The scion is also prepared by removal of surplus leaves and trimming the rest. The lower part is bevel-edged with a knife, as neatly as possible, in two faces of around 3cm (which in fact corresponds to two-thirds of the length of the notch made in the cutting).

(c) Cutting-grafting . The scion must be gently introduced into the notch in the understock, avoiding rubbing which could damage the scion. A rubber strip is used to keep the two parts in contact as closely as possible. I do not use grafting wax to coat the callusing zone. Then the cutting can be treated with hormone powder (if necessary) and put into the propagation medium. This is a classical medium composed of sphagnum peat moss, perlite and coarse sand. Each cutting is planted in a 6cm plastic pot and plunged in sand in the bottom of the propagation case.

I use a propagation case with automatic watering at pre-set intervals by a programmer, bottom heated by cables to 20-24°C.

2. PARTICULAR DATA. I have been practicing this technique for about 10 years. As we have just said, the aim is to propagate rhododendrons which are difficult to root by cutting, and whose small number of branches cannot provide sufficient propagation stock to permit a comparative test. So for each propagated species, the number of cutting-grafting which have been made are limited and the success rate is not significant from a statistical viewpoint. Nevertheless, I consider that doing this you increase the success rate of propagation compared to classical cuttings (you will see why below).

(a) Scions . The species of rhododendrons that I have used as scions are: R. faberi , R. wightii , R. rex , R. coelicum , R. eclecteum , R. phaeopeplum , R. lochiae , R. eudoxum , R. flavorufum , R. fulvum , R. macabeanum , R. wardii , R. pachysanthum , R. basilicum , R. adenogynum , R. montroseanum , R protistum , R. hylaeum , R. pseudochrysanthum , R. auriculatum , R. diaprepes , R. bureavii , R insigne , R. arboreum and species from the Fortunei series.

I have not noticed any incompatibility as a general rule; it is with this perspective that I have tried cutting-graftings in the vireya series with favourable results.

I have not tried cutting-grafting with hybrids, but this should not pose any particular problem.

(b) Understock . The understock I use is an old R. catawbiense hybrid. I have enough at my disposal to get a suitable choice of stems with adequate sizes and diameters. Cuttings root rather easily, as soon as they are a little mature or in their period of complete vegetative rest, so it is available for cutting-grafting nearly eight months out of twelve.

(c) Graft . I essentially use cleft graft or, more rarely, saddle graft.

(d) Aftercare . You must regulate moisture and bottom heat in order to get a temperature around 24°C and a constantly wet rooting medium.

The observation of the cutting-grafting is essential.

After around one or two months, a callus appears at the cutting-scion union that you can observe in free spaces left by the binding: this is a yellowish formation, a little verruculous, which seems to move the understock away from the scion. If you remove the binding, the cutting-scion join should be effective and you have to use force to separate the two elements. Usually the cutting is still unrooted. Consequently you must leave the cutting-grafting in the propagation case. Later, suckers can appear at the understock level, and they will have to be removed.

Several possibilities arise:

If you get a scion-understock join with sufficient rooting, your aim is reached and you will treat the cutting-grafting like any classical cutting or grafting: hardening, removing binding and suckers, transplanting, etc.

The scion can rot, which is a failure. The understock can rot while the scion does not; since callus isolates the two elements, the rot does not spread rapidly to the scion. Here lies the point of careful watching; if the understock rots whereas the scion is healthy, you must cut the binding and remove the understock. The scion is plunged again into the propagation medium; once a callus is made you just have to get the growth of roots and you are back to a type of simple cutting. However, you have gained a stage of development by the production of a callus, the stage preceding the appearance of roots on a classical cutting. An own-rooted plant is obtained in this way.

Thanks to this device, you increase the chances of success for the propagation of rhododendrons difficult to root by classical cuttings which often rot before the cambium heals.

The "secondary" cutting can be treated once again to increase the chances of rooting:

- modification of callus with a grafting knife

- use of hormone powder again

- use of new propagation medium.

3. DISCUSSION. Cutting-grafting is a means of mixed propagation that I use for rhododendrons which are difficult to propagate by classical cuttings or, for rare rhododendrons with few branches, to obtain new plants in case of loss of the parent and also to perpetuate plants attacked by certain diseases (later success depends on their nature). I cannot make an objective comparison among the different ways of propagation (seedlings, cuttings, grafting, cutting-grafting, layering) for a given species of rhododendron, but several points must be underlined:

Controlled pollination is required to produce seeds from which to grow seedlings of a given species; the method will not reproduce hybrids. Moreover, many years are necessary to get a plant to flower from a seedling.

In a classical graft you must grow seedlings in a special way to obtain suitable understocks; they must be in an advanced vegetative state compared to the scion. This implies preparation and obviously entails a period which will be favourable for grafting, short and planned for in advance. For cutting-grafting there is no particular preparation for the rootstock which must, however, be in vegetative rest. By using two rootstocks of different vegetative period, you could propagate by cutting-grafting all through the year, as the understock can only serve to protect the scion. Timing depends only on the vegetative stage of the scion.

Layering requires the use of the low branches only or the preparation of special plants which will be put in soil. Air layering only allows a limited amount of reproduction.

The technique of cutting-grafting does not require any particular skill, except making very neat sections. Classical cuttings are simpler and quicker but can be disappointing according to the rhododendrons to be propagated. Classical grafts require a certain skill and delicate aftercare.

The understock I use obviously produces suckers, even many years later. This can be countered by the use of understocks which do not sucker: 'Cunningham's White', R. fortunei , R. decorum .

An advantage to cutting-grafting is the secondary cutting which can be systematically used by removing the understock, rotten or not, in order to obtain own-root plants. The use of an overly soft understock will certainly lead to rot, but watching the cutting-grafting will permit the transformation of it to a secondary cutting in time.

4. FUTURE. Many ways of exploration or improvement are imaginable, including under glass propagation which is simpler than the propagation case and which can be more easily used by the amateur.

5. CONCLUSION. The cutting-grafting technique for the rhododendron, not previously described in its entirety, can be used for species or hybrids which are difficult to propagate: sterile varieties, those that root badly, those whose classical graft produces poor vegetation. The secondary cutting is an interesting option allowing one to obtain own-root plants.

Amateurs can take advantage of its long period of operation, the absence of special plant preparation and the economy in vegetal material for the plant to propagate.

References

1.  Davis, Ross B., Jr. 1980. Quarterly Bulletin, American Rhododendron Society, 34:3, pp. 160-162.
2.  Ayers, Jack. 1985. Quarterly Bulletin, American Rhododendron Society, 39:2, pp. 91-92.

Acknowledgments
Thanks to the translator, Miss Stephanie Burliga.