JARS v9n1 - Propagation of Evergreen Rhododendrons By Cuttings of Six Types

Propagation of Evergreen Rhododendrons By Cuttings of Six Types
Mieczyslaw Czekalski
Professor, Department of Ornamental Plants
Agricultural University, Poznan, Poland

Several types of cuttings have been used in propagation of broadleaved evergreen rhododendrons. The most widely used standard cutting consists of a stem 3-6 inches long (depending on species) with 3-5 leaves and a terminal vegetative bud. Often such a cutting has a wound up to 1 inch long made at its base. Sometimes the terminal bud is removed in order to stimulate more intense branching of the rooted cutting. In the 1930s Skinner successfully introduced leaf-bud cuttings (8). Since then, however, leaf-bud cuttings have become somehow forgotten. Several investigators (3, 9) have shown positive results when standard cuttings were prepared in such a way that they had a portion of 2-year-old wood at the base. In recent years, asymmetrical cuttings with all the leaves removed on one side (so called "southern side") and wounding on the opposite side (so called "northern side") have been becoming more and more popular among nurserymen. Despite many publications on rhododendron propagation, very little work has been done comparing different types of cuttings under uniform conditions. This was the objective of the present study.

The experiment was conducted on four cultivars of broadleaved evergreen rhododendrons: 'Catawbiense Grandiflorum', 'Cunningham's White', 'Lee's Dark Purple', and 'Fastuosum Flore Pleno'. The following types of cuttings were compared (see Figure 1):

I.  Standard cuttings with terminal bud present
II.  Standard cuttings with terminal bud removed
III.  Standard cuttings with 1/2 in. terminal removed
IV.  Portions removed from the cuttings of type III
V.  Single node cuttings, i.e., 1 in. long stem segment with a single leaf
VI.  Leaf-bud cuttings

Cuttings were taken on Dec. 8, 1990, from 2-year-old plants growing in containers in Dobrygosc Nursery, Poland. Polish-made rooting stimulator "Ukorzeniacz AB," which contains 0.05% IBA, 0.3% NAA and 0.1% benomyl, was used. Cuttings were placed in a greenhouse on benches under double plastic. Rooting medium consisted of peat, pine bark, perlite (3:1:1, v:v:v). Electric bottom heat on benches maintained temperature of the medium around 70°F. Rooting was evaluated after 29 weeks.

Table 1. Rooting percentage of four cultivars of evergreen rhododendrons propagated by cuttings of six types.
Cultivar	Type of cutting						Average
	I	II	III	IV	V	VI
'Cunningham's White'	86.7	60.0	26.7	93.3	60.0	80.0	69.1 bc
'Lee's Dark Purple'	46.67	43.3	26.7	50.0	60.0	50.0	45.2 a
'Catawbiense Grandiflorum'	60.0	73.3	56.7	86.7	96.7	76.7	74.3 c
'Fastuosum Flore Pleno'	56.7	56.7	50.0	70.0	66.7	60.0	60.9 b
Average*	62.5 bc	58.3 b	40.0 a	75.0 d	70.8 c	66.67 bcd	62.4
* averages followed by the same letter are not significantly different at P=0.01 (t-test

Results are presented in Table 1. Mean values for cultivars ranged from 45.2% for 'Lee's Dark Purple' to 74.3% for 'Catawbiense Grandiflorum'. Mean values for types of cuttings ranged from 40.0% for type III to 75.0% for type IV. There was no interaction between cultivars and cutting types. Leaf cuttings without an axillary bud were also tested (data not shown), but only 5-15% of them rooted. All of these cuttings, however, failed to develop stems and died shortly thereafter. It has been reported that leaves of Rhododendron simsii (2) and R. 'Felicitas' (1) were successfully rooted, but they also did not develop stems.

Unexpectedly, the types of cuttings of minor importance or not used by nurserymen at all, i.e., IV, V, and VI, performed better than standard cuttings of types I, II, and III. There have been previous reports that R. 'Cunningham's White' rooted 100% from cuttings of type V (7). Under conditions in this experiment 60% of these cuttings rooted. The same type of cutting was tested on R. maximum (4, 6) and 60% also rooted. Even though shorter cuttings, as those of type IV, V, and VI, rooted in higher percentage than standard ones, they produced shorter stems during the first season of growth (data not shown). Nevertheless, when there is a need to increase a number of clonally propagated plants, and in vitro technology is not immediately available, as often is the case with new cultivars which have not reached market, short cuttings prepared from segments of stems and even leaf cuttings with a bud can serve as a valuable propagation material.

References
1.  Chaikin, R. W. Rooting rhododendron leaves. J. Amer. Rhod. Soc.45(2): 96; 1991.

2.  Custers, J. B. M. Adventitious shoot formation on leaf cuttings in vivo, a tool in horticulture. Dept. Hort., Agric. Univ. Wageningen, The Netherlands, Publication 528; 1986.

3.  Czekalski, M. Rooting of evergreen rhododendron cuttings, taken from old parent plants, in cold medium (in Polish). Rocznik Dendrologiczny 25: 43-53; 1984.

4.  Czekalski, M.; Pokojowczyk, K. Propagation of Rhododendron maximum L. from stem and single-node cuttings (in Polish). Ogrodnictwo 11: 20-21; 1984.

5.  Czekalski, M. a. Propagation of evergreen rhododendrons by stem cuttings (in Polish). Rocz. AR w Poznaniu 189: 33-42; 1988.

6.  Czekalski, M. b. Propagation of Rhododendron maximum and R. smirnowii by stem cuttings. Acta Horticulturae 226: 573-576; 1988.

7.  Czekalski M.; Anisko, T. Comparison of growth of Rhododendron 'Cunningham's White' plants propagated from cuttings of five types (in Polish). Rocz. AR w Poznaniu 225: 23-27; 1991.

8.  Leach, D. G. Rhododendrons of the World. New York: Charles Scribner's Sons; 1961.

9.  Nakamura, M.; Matsui, S.; Harada, H. Studies on the adventitious root formation of a Japanese native Rhododendron sp. I. Effect of auxin and other treatments on the rooting of stem cuttings from mature trees. J. Jap. Soc. Hort. Sci. 47(2): 227-236; 1978.