Effects of Several Methods of Wounding on the Rooting of Cuttings of Rhododendron
By Robert L. Ticknor
Department of Landscape Architecture
Several writers, Bridgers (1), Nearing (3), and Wells (4) have stated that wounding the stems of rhododendron cuttings increases the per cent of cuttings that root and stimulates heavier rooting.
Five different methods of wounding have been described: three by Wells (4) and two by Bridgers (1). The response to these methods as compared with an unwounded control by cuttings of Rhododendron catawbiense 'Roseum Elegans' was measured in this experiment.
Three- to five-inch vegetative cuttings were dipped in Hormodin No. 3 powder, which contains 0.8 per cent indolebutyric acid, hereafter described as 0.8 per cent IBA. These cuttings were inserted in a 1:1 volume mixture of sand and peat held at a temperature of 80° F. by electric soil cables. Interrupted mist for four seconds of each minute from 8:00 A.M. to 5:00 P.M. provided conditions satisfactory for room initiation.
One hundred cuttings were inserted on October 14, 1955, for each of the following wounding methods:
- Control-without stem wounds.
- 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 (5).
- One side wounded-thin slice of bark down to the cambium removed from the basal 1½ inches of the stem as described by Wells (4).
- Two sides wounded-two slices of bark removed from opposite sides of the stem as in the previous treatment.
- Sliced-three upward slices, each approximately ½ inch in length and ½ inch apart, into the bark as described by Bridgers (1).
- Stripped-bark removed from the basal inch of the cutting also used by Bridgers (1).
TABLE 1-Effect of wound treatments upon rooting response of Rhododendron
catawbiense 'Roseum Elegans' cuttings treated with 0.8 per cent IAA.
NUMBER OF CUTTINGS POT SIZE CALLUSED NOT
DEAD 4" 3" 2" 1" Control 19 23 34 20 4 Slit 2 29 27 33 5 4 One Side Wounded 12 57 19 11 Two Sides Wounded 24 49 18 7 2 Sliced 1 19 25 43 23 Stripped 20 64 8 3 3 2 Contribution No. 1065 of the University of Massachusetts, College of Agriculture Experiment Station, Amherst, Mass.
Degree of rooting was determined on December 15, 1955, 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, 3inch pot, 2-inch pot, 1-inch pot, callused, not calused, and dead. The results tabulated according to this system are shown in Table 1.
Three methods of wounding, one side wounded, two sides wounded and stripped, clearly produced a higher percentage of heavier rooted cuttings. The percentage of three- and four-inch pot size cuttings was 69, 73 and 84 respectively. However, the time necessary to wound 100 cuttings by the different methods varies considerably as is indicated in Table 2.
METHOD TIME IN MINUTES Slit 4.0 One Side Wounded 4.9 Two Sides Wounded 8.7 Sliced 12.2 Stripped 23.4
Root initiation over the entire stripped area is the advantage of the stripped method since the entire root mass of unwounded rhododendron stem cuttings is often attached to the stem by one easily broken root. Root initiation in the other methods of wounding was not as well distributed along the stem as in the stripped method. Although some roots did originate from the slit wounds, most roots arose in the basal callus area. A similar situation prevailed in the one side wounded method. The two sides wounded method produced many roots along the stem, although the greater number appeared to arise from the basal callus. Basal origin was the primary method of rooting in the sliced method, but some roots did come from the sliced areas.
Wounding of cuttings of Rhododendron catawbiense 'Roseum Elegans' increased the percent of cuttings that root and stimulated heavier rooting.
The response in heaviness of rooting appears to be directly related to the severity of the wounding. The more severe the wounding, the heavier the root system produced.
The per cent of cuttings that rooted in the acceptable 2-, 3-, and 4-inch pot sizes by the three most effective methods were: one side wounded, 88; two sides wounded, 91; and "stripped," 92. In comparison, the control, slit, and sliced treatments yielded 42, 48, and 45 per cent of these sizes respectively.
Because of the time necessary to prepare the cuttings, either the one side wounded or two sides wounded will probably be more commercially accepted than the stripped method, which requires approximately three times as long to prepare.
1. Bridgers, Bernard, Propagation of hybrid rhododendrons by stem cuttings, Nat. Hort. Mag., 32: 127-140, 1953.
2. Coggeshall, K. G., More about plastics in propagation, Amer. Nurs., 100 (10): 8-9, 56-7, November 15, 1954.
3. Nearing, G. G., How the side sliced cutting came to be, Amer. Rhodo. Soc. Quar. But., 17 (2), April 15, 1953.
4. Wells, J. S., Propagation of rhododendrons from stem cuttings, Amen Nurs., 97 (9): 11, 70-73, 1953.
5. Wells, J. S., Propagating tools, Amer. Nurs., 100 (71-74, 49-5, October 1, 1914.