JARS v45n2 - Chemical Pruning of Rhododendrons

Chemical Pruning of Rhododendrons
R.L. Ticknor and Mary Skinner
OSU North Willamette Research and Extension Center
Aurora, Oregon

Profuse branching in evergreen florist azaleas is induced by chemical pruning using Off-Shoot-0 (methyl esters of fatty acids C 6 - C 12 45%) or Atrimec, Atrinal (Dikegulac - Sodium) 1.67 lbs. per gallon. Branching results from killing soft growing branch tips (Off-Shoot-O) or by inhibiting the growing terminals of shoots (Atrinal) so axillary buds along the stems grow. Off-Shoot-0 can kill mature leaves as well as growing points if it doesn't dry quickly or is not washed off the foliage under slow drying conditions.
Previous trials of North Willamette Research and Extension Center and elsewhere have shown that lepidote Rhododendrons such as 'P.J.M.' respond to these treatments while elepidote Rhododendrons such as 'Vulcan' do not. An exception to this was found when newly established tissue cultured elepidote Rhododendron were treated at NWREC in 1980 with very high rates 8, 10, and 12,000 PPM. Axillary buds started to develop, but with limited leaf area failed to develop into shoots. Lower rates of Atrinal did not cause the axillary buds to start developing.
Summer student interns at NWREC have one or more projects which they help plan, execute, and collect data. Of necessity, these have to be of short term nature. When we learned that some of the florist azalea producers were applying a combination of Atrinal and Off-Shoot-0, it appeared to be a good project, as we had the chemicals and plants from five sections of the genus Rhododendron: lepidote, elepidote, Vireya, deciduous and evergreen azaleas on hand. Fortunately Atrinal and Off-Shoot-0 have long shelf lives, as another requirement of a student project is that is has to be low budget.
The number of plants available were limited so four replications with varied numbers per type of Rhododendron were used with each treatment. The treatments applied with a compressed air sprayer on June 28, 1990, were: 1) water 2) Atrinal 4,000 PPM, and 3) Atrinal 2,000 PPM in a 1:10 dilution of Off-Shoot-0 with water.

Table 1: Branch Number of Five Types of Rhododendrons as Influenced by Chemical Pruning Treatments Applied June 28, 1990. Data Recorded August 14, 1990. Four Replications with Different Numbers of Plants Per Type.
Type of Rhododendron Control Atrinal (4000 PPM) Atrinal (2000 PPM) in Off-Shoot-0
1:10 Dilution
LSD 0.5%
Elipidote 'Vulcan' (5) 3.6 4.6 4.2 N.S.
Lepidote 'PJM Compact' (3) 4.6 5.3 11.2 2.7
Vireya R. macgegoriae (1) 2.3 1.8 2.0 N.S.
Deciduous Azalea R. occidentale (4) 9.6 17.7 21.7 7.9
Evergreen Azalea
'Girard's Variegated Hot Shot' (5)
39.2 47.5 59.9 18.9

The total number of branches per plant were counted on August 14, 1990, and are shown in Table 1 as well as the number of each type of Rhododendron used per treatment. The elipidote 'Vulcan' and the Vireya R. macgregoriae showed no response. The lepidote 'P.J.M. Compact', deciduous azalea R. occidentale , and evergreen azalea 'Girard's Variegated Hot Shot' all had a significant response to the combination treatment. Only R. occidentale had a significant increase in branch number from Atrinal at 4,000 PPM.
Cost per gallon of spray based on the price of one gallon containers of Atrimec (Atrinal) and Off-Shoot-0 in 1990 wholesale supply catalogs was $5.04 for Atrimec and $8.87 for the combination spray. Since the plant is sprayed to wet the foliage, the amount used and cost per plant varies with plant size.
This trial was done with a limited number of cultivars and quantities of plants so further trials should be done before large numbers of plants are treated. The effect of the combination spray under slow drying conditions also needs further evaluation.

R.L. Ticknor is a professor of horticulture and Mary Skinner is a student intern at Portland Community College, Portland, Oregon.