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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 22, Number 2
April 1968

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Chemical Control of Rhododendron Growth and Flowering
R. L. Ticknor and C. A. Nance
Oregon State University, North Willamette Experiment Station, Aurora, Oregon

        Considerable information has been published on the use of growth regulating compounds to induce flower bud formation, and to control plant size in pot-grown azaleas and rhododendrons in the greenhouse, by Cathey (1), Criley (2), Crossley (3) and McDowell (4). No published information on the use of these compounds on rhododendrons out-of-doors was found. Growth control may be of as much interest to many rhododendron hobbyists as the increased flowering. Shoot internodes are reduced in length, producing more compact plants as shown in Figures 25 and 26.

'Roseum Elegans' showing normal growth 'Roseum Elegans' showing compact growth induced by foliage sprays of Cycocel
     Fig. 25.  Four-year-old 'Roseum Elegans' showing
                   normal growth
    Fig. 26. Four-year-old 'Roseum Elegans' showing
    compact growth induced by foliage sprays of
    Cycocel at 4 oz./gal.

        The cultivar 'Roseum Elegans' was used as the test variety for foliar applications of growth regulators under field conditions. This cultivar was also used by Cathey (1) and Criley (2). Year-old container-grown plants were planted in June, 1964. The following fertilizer program was used in this experiment: 1964, 40 lbs. nitrogen from 11-28-0; 1965, 60 lbs. nitrogen from 1616-16; and 1966, 80 lbs. nitrogen from 16-20-0.
        Growth regulator treatments were started in 1965 using 139 (N-dimethylsuccinamic acid) at 0.25 and 0.5 per cent or 2500 and 5000 ppm; Cycocel (2-Chloroethyltrimethlammonium chloride) at 1, 2 and 4 oz./gal. or 922, 1844 and 3688 ppm; DMSO (dimethyl sulfoxide) at 0.5 per cent or 5000 ppm; and combination of these chemicals. Ten treatments with 6 replications, each of which contained 5 plants, were used. Foliar sprays directed at the upper leaf surface were applied to the drip point.
        Since young leaves are more active in absorption of foliar-applied chemicals in most plants, applications were made when this type of leaves was present. An application was made as the leaves started to expand on the elongating shoots. A follow-up application was made 2 weeks later to the expanded but immature leaves.
        Three growth flushes in 1965 and 2 in 1966 were induced by pinching all the terminal buds on the plants as the buds matured. In 1965, only the second growth flush was sprayed. In 1966, leaves of both the first and second growth flushes were sprayed.
        Recent research by two graduate students, Dr. David Adams and Mr. Charles Johnson, while working at Oregon State University, has established that the terminal bud-either floral or vegetative-has its complete complement of flowers or leaves before the shoot begins to elongate. Thus, any growth regulator application has to be made one growth flush before the final flush of the season to influence the flower formation on the final flush.
        Flower buds and all the enlarged vegetative buds at the short tips were counted in December, 1965; the results are shown in Table 1. Data were collected in October, 1966, on width, height, and type of terminal buds on each shoot of each plant. The averages of these data are presented in Table 2.

Table 1. Flower and total bud production of Rhododendron 'Roseum Elegans'
following growth regulator treatment in 1965.
tr.
no.
material average no. buds percent
flower to
total buds
flower total
1. Check 24 526 4.56
2. B-9 0.5/0 54 496 10.97
3. B-9 0.5% + DMSO 0.5% 26 402 6.47
4. CCC 2 oz./gal. 53 441 12.01
5. CCC 2 oz./gal. + DMSO 0.5% 41 460 8.90
6. CCC 4 oz./gal. 72 421 17.10
7. CCC 4 oz./gal. + DMSO 0.5%. 63 478 13.17
8. B-9 0.25% + CCC 1 oz./gal. 12 373 3.19
9. B-9 0.50% + CCC 2 oz./gal. 100 590 16.95
10. B-9 0.50% + CCC 2 oz./gal. + DMSO 0.5% 55 449 12.25

Cycocel (11.8% liquid formulation) was provided by the American Cyanamid Company, Agricultural Division, Princeton, New Jersey.
B-Nine (5% liquid formulation) was provided by the Uniroyal Chemical Division, Uniroyal, Inc., Naugatuck, Connecticut.

Table 2. Growth and development of Rhododendron 'Roseum Elegans' in 1966
following growth regulator applications in 1965 and 1966.
tr.
no.
material plant size average
flower
no. buds
vegetative
percent
flower to total buds
height
in inches
width
in inches
1. Check 20.8 28.7 16.0 52.5 23.4
2. B-9 0.5% 21.3 28.3 16.3 50.7 24.3
3. B-9 0.5% + DMSO 0.5% 19.0 26.8 11.3 47.8 19.2
4. CCC 2 oz./gal. 16.3** 26.0* 26.7* 33.9** 44.2
5. CCC 2 oz./gal. + DMSO 0.5% 16.0** 26.0* 26.8* 39.0* 40.8
6. CCC 4 oz./gal. 13.3** 24.5* 24.0 30.8** 43.7
7. CCC 4 oz./gal. + DMSO 0.5% 14.3** 25.3* 26.5" 31.7** 45.6
8. B-9 0.25% + CCC 1 oz./gal. 17.0** 26.2* 13.3 39.0* 25.5
9. B-9 0.5% + CCC 2 oz./gal. 16.3** 26.3 23.6 36.2* 39.6
10. B-9 0.5% + CCC 2 oz./gal. + DMSO 0.5% 17.0** 25.3* 20.3 46.3 30.6
* LSD 5% 2.4 2.5 8.8 12.8  
** LSD 1% 3.5 3.5 12.6 18.3  

Five plants were selected at random from each of the 10 treatments for transplanting on April 3, 1967. Observations on flower development were made in late May, 1967. Data collected in the fall of 1967 on width, height, type of terminal bud, length of shoot growth and plant color are presented in Table 3. A scale of 1 for dark green color, 2 for normal green color and 1 for yellow-green color was used.

Table 3. Growth and development of Rhododendron 'Roseum Elegans' during 1967
following two years of growth regulator applications.
tr.
no.
material plant size average no. buds avg length of shoots in inches
height width flower veg. veg. from veg. veg. from flower flower from veg. flower from flower
1. Check 26.8 30.0 37.0 26.0 2.3 1.6 3.6 2.8
2. B-9 0.5% 24.4 27.4 32.4 32.0 1.7 2.8 3.5 3.0
3. B-9 0.5% + DMSO 0.5% 24.4 27.4 41.4 20.8 2.0 1.7 3.3 2.7
4. CCC 2 oz./gal. 19.8 22.0 41.2 19.4 1.6 1.8 3.9 3.6
5. CCC 2 oz./gal. + DMSO 0.5% 21.2 23.4 22.8 41.9 2.1 1.9 4.1 3.5
6. CCC 4 oz./gal. 16.0 21.4 10.0 34.4 2.7 2.1 4.5* 3.9*
7. CCC 4 oz./gal. + DMSO 0.5% 18.2 20.2 22.2 30.8 2.4 2.3* 4.2 4.1*
8. B-9 0.25% + CCC 1 oz./gal. 22.8 27.0 38.4 22.6 1.7 1.4 4.2 3.0
9. B-9 0.5% + CCC 2 oz./gal. 19.2 22.2 19.0 26.0 2.5 2.1 4.0 3.9*
10. B-9 0.5% + CCC 2 oz./gal. + DMSO 0.5% 19.4 23.0 27.4 34.4 2.4 1.7 4.2 3.0
* LSD 5%           N.S. .54 .75
** LSD 1%           N.S. .78 1.07

        The percentage of terminals producing flowers in 1965 was increased by both B-9 and Cycocel applications and the combinations of the two, except for Treatment S. The highest percentage of flower buds followed the application of Cycocel at 4 oz./gal. and B-9 at 0.5% + Cycocel at 2 oz./gal. There was a consistent decrease in the percentage of flower buds when DMSO at 0.5% was added to the growth regulating chemicals.
        Cycocel applications markedly reduced stem elongation during the 1966 growing season, resulting in plants which were shorter by statistically significant amounts. Plant spread was also affected, but to a lesser degree. No significant effect of B-9 on growth was observed.
        Flower bud formation in 1966, as shown by percentage of flower buds to total buds, was significantly increased by the application of Cycocel. Leaves of plants treated with this chemical were a darker green than those of the check or B-9 treatments. In 3 or 4 cases, the percentage of flower buds was lower for treatments which included DMSO.
        In this experiment, B-9 was not so effective in reducing growth or increasing the percentage of flower buds as Cycocel. In his work in the greenhouse, Cathey found B-9 and Phosfon effective while results with Cycocel were not consistent.
        Malformed flowers following application of growth regulators have been observed (3). In this trial there was observed no alteration in the size and shape of the flowers. A slight advance in the blooming date of Cycocel-treated plants was observed. These plants were in full bloom when the other treatments still had many partially opened buds.
        Measurements of the shoot growth produced in 1967, a year after the last growth regulator application, showed no statistically significant growth reduction. Four types of shoots were measured: Vegetative shoots originating from a vegetative shoot, vegetative shoots originating from a flowering shoot, flowering shoots originating from a vegetative shoot, and flowering shoots originating from a flowering shoot. A few cases were found of statistically significant growth increase as compared to the check. Percentage of flower buds was high in the treatments which had a low percentage in 1966 and low on treatments which had a high percentage in 1966. An exception to this was the Cycocel 2 oz./gal. treatment, which had a high percentage of flower buds in both the 1966 and 1967 counts.
        Cycocel at the rate of 2 oz./gal. (11.8% liquid formulation) appears effective in field trials in increasing flowering and dwarfing Rhododendron 'Roseum Elegans'. Other varieties showing these responses to this treatment in limited pot plant trials include 'Blue Ensign,' 'Doubloons,' 'Evening Glow,' 'Lackamas Spice', 'Olympic Lady' and 'Thor'.
        If Cycocel, which is presently registered for use on azaleas and poinsettias, becomes available to home owners, it could help keep rhododendrons from outgrowing their spot in the garden or under a window. It might also be possible to grow some of the beautiful large-growing varieties on the average size home grounds by reducing their rate of growth.

Literature Cited

  1. Cathey, Henry M., and R. L. Taylor, 1965, Guidelines for regulating flowering of Rhododendrons-light and growth retardants, Quar. Bul. Amer. Rhodo. Soc., 19(1):26-35.
  2. Criley, Richard A., and J. W. Mastalerz, 1966, Responses of hybrid Rhododendrons to long days and growth retardants, Penns. Flower Grower Bul., 182.
  3. Crossley, J. H., 1965, 1966 and 1967, Personal communications.
  4. McDowell, T. C., and R. A. Larson, 1966, Effects of (2-chloroethyl) trimethyl ammonium chloride (Cycocel), N-dimethyl succinamic acid (B-Nine), and photoperiod on flower bud initiation and development in Azaleas, Proc. Amer. Soc. Hort. Sci., 88:600-605.
  5. Myhre, Arthur S., and W. P. Mortensen, 1964, The effect of phosphorus on Rhododendron flower-bud formation, Quar. Bul. Amer. Rhodo. Soc., 18(2):66-71.

Volume 22, Number 2
April 1968

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