QBARS - v14n2 A Comparison of Wounding Methods on the Rooting of Three Rhododendron Varieties

A Comparison of Wounding Methods on the Rooting of Three Rhododendron Varieties
By Robert L. Ticknor and Paul F. Bobula
Department of Landscape Architecture, University of Massachusetts
Contribution No. 1187 Massachusetts Agricultural Experiment Station
Amherst, Massachusetts

Several writers, Bridgers (1), Nearing (3) and Wells (5) have described methods of wounding the stems of rhododendrons to increase the percentage of rooting and to stimulate heavier rooting. Wells described three methods of wounding, while Bridgers described two methods. A previous paper by Ticknor and Bobula (4) dealing with Rhododendron catawbiense Michx. var. 'Roseum Elegans' compared all methods.
R. 'Roseum Elegans' roots comparatively easily, but red flowered varieties of Rhododendron catawbiense Michx. such as 'Edward S. Rand' and 'Dr. H. C. Dresselhuys' are reportedly difficult to root. Therefore, these three varieties were used in the current studies.

Methods
Three to five-inch vegetative cuttings were used in these experiments except for one-half of the R. 'Edward S. Rand' cuttings. Since sufficient vegetative cuttings of this variety were not available, cuttings with flower buds, which were removed at the time of wounding, were used.
Indole butyric acid (IBA) in talc was used as a root inducing hormone at 0.8 per cent in the case of 'Roseum Elegans', and 1.0 per cent for 'Edward S. Rand' and 'Dr. H. C. Dresselhuys'. Sand and sphagnum peat, equal parts by volume, was used as the propagating medium. Electric soil cables maintained a medium temperature of 80° F. Intermittent mist, from 8:00 a.m. to 5:00 p.m. at 4 seconds every minute was applied to R. 'Roseum Elegans', while 6 seconds every 6 minutes was used for the other varieties to provide satisfactory rooting conditions.
The following wounding methods were applied to lots of 100 cuttings:

1. Control - without stem wounds.
2. Slit-vertical cuts through the bark for a distance of 1½ inches at the base of the stem made with the razor blade wounder described by Wells (6).
3. One side wounded-thin slice of bark down to the cambium removed from the basal 1½ inches of the stem as described by Wells (5).
4. Two sides wounded-two slices of bark removed from opposite sides of the stem as in method 3.
5. Sliced-three upward slices, each approximately ⅜ inch in length and ½ inch apart, into the bark as described by Bridgers (1).
6. Stripped-bark removed from the basal 1 inch of the cutting also used by Bridgers (1).

Results
The degree of rooting was evaluated by a method described by Coggeshall (2) in which the cuttings are classified according to the size pot necessary to hold the root ball. Categories used were 4-inch pot, 3-inch pot, 2-inch pot, 1-inch pot, callused, and dead. Fibrous root systems difficult to evaluate by other methods can be compared on a well-known numerical system.
Rooting results obtained with three vegetative lots and one flowering lot of rhododendron cuttings are presented in Tables 1-4. Marked varietal rooting response was noted with R. 'Roseum Elegans' rooting best, followed by 'Edward S. Rand' (vegetative cuttings), R. 'Dr. H. C. Dresselhuys' and R. 'Edward S. Rand' (flowering wood cuttings). Vegetative cuttings of R. 'Edward S. Rand' rooted about twice as well as flowering wood cuttings of this variety. Differences in the percentage of well-rooted cuttings between the check treatment and the most effective wounding treatments were more marked with R. 'Roseum Elegans' than with the red-flowered varieties. However, the number of cuttings in the 3 and 4 inch categories was higher in the wounding treatments than in the check.
Although a high percentage of rooting was obtained with the stripping method when used on R. Roseum Elegans,' it was not an outstanding method when used on R. 'Roseum Dresselhuys' or R. 'Edward S. Rand'. The one and two sides wounded treatments gave the most consistent results, and as shown in Table 5 were less time-consuming to execute. Slitting and slicing did not appear to be markedly superior to the check.
Wounding produces a better balanced root system, with root initiation occurring throughout the wounded area. This is particularly important in rhododendrons where often a single root will develop from the basal callus and a large mass of fibrous roots forms which is easily broken when the cuttings are lifted.

Conclusions
Generally wounding increased the percentage of cuttings with 2, 3, or 4 inch pot balls; however, varietal response was quite marked in this respect. The variety R. 'Roseum Elegans' responded followed by R. 'Edward S. Rand' (vegetative cuttings), R. 'Dr. H. C. Dresselhuys,' and R. 'Edward S. Rand' (flowering wood cuttings).
While the percentage of 2, 3, and 4 inch pot cuttings was not much greater than the check in the wounding treatments, a greater number with 3 and 4 inch pot balls resulted from the one side wounded, two sides wounded, and stripped method.
Cuttings wounded by the stripped method either rooted heavily or not at all.
Wounding cuttings on one or two sides of the stems produced consistently desirable results and were less time-consuming than the sliced or stripped wounding methods.

Literature Cited

1. Bridgers, Bernard, 1953, Propagation of hybrid rhododendrons by stem cuttings, Nat. Hort. Mag., 32: 127-140.
2. Coggeshall, R. G., 1954, More about plastics in propagation, Amer. Nurs., 100 (10): 8-9, 56-7.
3. Nearing, G. G., 1953, How the side sliced cutting came to be, Quar. Bul. Amer. Rhodo. Soc. 7 (2): 95-96.
4. Ticknor, R. L., 1957, Effects of several methods of wounding on the rooting of cuttings of rhododendron, Quar. Bul. Amer. Rhodo. Soc. 11 (2): 94-95.
5. Wells, J. S., 1953, Propagation of rhododendrons from stem cuttings. Amer. Nurs. 97 (9): 11, 70-73.
6. Wells, J. S. 1954. Propagation tools, Amer. Nurs. 100 (7): 14-15, 49-53.