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Journal American Rhododendron Society

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Dr. Glen Jamieson ars.editor@gmail.com


Volume 30, Number 4
Fall 1976

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Residual Effects of Growth Retardants on Rhododendrons
R. L. Ticknor and J. W. Fox
Aurora, Oregon

        It has been possible to induce many varieties of rhododendrons in 6" pots to form flower buds in one growing season using growth retardants; (1, 3, 4) Cycocel (2-chloroethyl-trimethyl ammonium chloride) and Phosphon (Tributyl-2, 4-dichlorobenel phosphonium chloride) applied as soil drenches were most effective. Besides inducing flower buds, Cycocel and Phosphon reduced the elongation of shoots that developed following application.
        To determine if this growth inhibition persisted in subsequent years, four plants which had been treated with Cycocel (200 ml. of 4000 ppm solution per pot) and four plants treated with Phosphon (0.4 gram in 200 ml. of water per pot) in April 1972 were planted in nursery rows in May 1973. These plants had been forced into bloom in the greenhouse in February and March 1973. Only two control plants which had also been forced into bloom were planted since untreated plants with flower buds were in short supply.
        Measurements of the plants were taken in December 1975 after three growing seasons in the field. These measurements are shown in table 1:

Table 1. Average height (H) and width (W) in inches, number of vegetative
(V) and flower (F) buds of several rhododendron cultivars after 3 growing
seasons following growth retardant treatment the previous year.
Cultivar Treatment H W V F
'Blue Ensign' Check 21.0 28.5 23.5 11.0
  Cycocela 17.0 25.0 4.0 22.0
  Phosphonb 16.3 25.0 19.7 9.3
'John Walters' Check 21.0 30.0 11.0 15.0
  Cycocel 17.5 27.8 28.5 5.8
  Phosphon 10.8 21.3 15.5 13.3
'Kluis Sensation' Check 26.5 30.3 34.5 6.5
  Cycocel 19.9 30.1 18.0 15.0
  Phosphon 13.1 25.0 10.3 18.0
'Madame de Bruin' Check 21.0 24.0 3.0 10.0
  Cycocel 21.0 24.5 3.0 15.5
  Phosphon 13.3 19.7 7.7 10.0
'Nova Zembla' Check 24.0 28.5 18.5 11.5
  Cycocel 15.3 28.3 11.0 16.7
  Phosphon 15.0 21.5 7.5 17.5
'Purple Splendour' Check 19.5 32.5 1.0 34.5
  Cycocel 20.3 26.5 3.3 32.5
  Phosphon 15.5 24.0 1.0 32.0
'Roseum Elegans' Check 27.0 32.0 23.0 34.0
  Cycocel 22.8 31.0 28.0 26.3
  Phosphon 13.5 24.0 20.0 19.0
'Sappho' Check 27.5 30.5 16.5 6.5
  Cycocel 26.0 32.0 8.0 20.5
  Phosphon 18.5 24.5 9.5 19.0
'Thor' Check 16.5 26.0 16.5 23.5
  Cycocel 17.5 28.0 9.0 41.5
  Phosphon 8.7 15.7 5.3 28.0
'Unique' Check 15.0 25.0 53.5 11.5
  Cycocel 12.0 14.5 20.0 2.0
  Phosphon 11.0 15.5 22.5 4.5
a Cycocel - 200 ml. of 40000ppm / 6" pot
b Phosphon - 0.4 grams in 200 ml. water/6" pot

Results and Discussion
        Plants treated with Phosphon were markedly smaller after three growing seasons in the field. The residual effect of Cycocel on plant size was not as great as Phosphon. After this trial was started, Dr. Philip Kozel of Ohio State University (2) stated that he had observed persistent effects of Phosphon on rhododendron growth.
        Whether this growth reduction is considered desirable would probably depend on the cultivar treated and the use made of the plant. In the case of a leggy-grower such as 'Sappho', some retardation produces a more attractive plant. Compact cultivars like 'Thor' and "Unique" normally produce relatively short growths and plant appearance isn't improved by shortening these growths further. Plants where growth needs to be restrained, such as in containers, under windows, or other restricted areas, might be benefited by the application of a growth retardant.
        The long term effects of Cycocel and Phosphon on flowering, varied with cultivar and treatment. Cultivars showing a positive flowering response (an average gain of 5 or more blooms per plant) to Cycocel compared to the control were 'Blue Ensign', 'Kluis Sensation', 'Madame De Bruin', 'Nova Zembla', 'Sappho' and 'Thor'. Cultivars showing a negative flowering response (an average loss of 5 or more blooms per plant) were 'John Walters', 'Roseum Elegans', and 'Unique". 'Kluis Sensation', 'Nova Zembla', and 'Sappho' had a positive flowering response to Phospon. A negative flowering response to Phosphon was shown by 'Roseum Elegans' and 'Unique'.
        Growth retardants are useful tools for inducing flowering in one growing season from a rooted cutting on many varieties of rhododendrons. The resulting small plant can make a very attractive pot plant. Residual effects of growth retardants will have to be considered and approval by the E. P. A. will be needed before this method of producing small budded plants can be used commercially.

Literature Cited
1. Cathey, H.M. and R.L. Taylor, 1965, Guide-lines for regulating flowering of rhododendrons - Light and Growth Retardants, Quar. Bul. Amer. Rhodo. Soc., 19(1).
2. Kozel, P.C., 1973, Personal Communication.
3. Myers, S.P. and P.C. Kozel, 1972, Practical procedures for increasing flower bud initiation on rhododendrons, Amer. Nurs., CXXXV(3): 13,114.
4. Ticknor, R.L., 1974, Controlling size and flowering in rhododendrons, Quar. Bul. Amer. Rhodo. Soc., 28(3).


Volume 30, Number 4
Fall 1976

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