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Journal American Rhododendron Society

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


Volume 26, Number 2
April 1972

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Practical Procedures For Increasing
Flower Bud Initiation on Rhododendrons

S. P. Myers and P. C. Kozel
Department of Horticulture, Ohio State University

        Research in the field of bioaesthetics at The Ohio State University during the past few years has yielded much information on the use of plant growth regulators. One of our major areas of emphasis has been the use of plant growth regulators on rhododendrons to enhance flower bud production. More specifically, our goal was to develop techniques whereby rhododendrons could be cultured from rooted cuttings to salable budded plants in one year.
        Research results from previous years combined with current year data suggest the following procedures should be followed in order to achieve maximum benefits from chemical treatments:

Obtaining budded plants from rooted cuttings in one year.
        A) Initiate supplementary light treatments (from 10 a.m. until 2 p.m.) amounting to 20 foot candles of light at the plant surface beginning in February. This light intensity can be provided by 100 watt incandescent lights 4 feet apart and 4 feet above the plants.
        B) Apply Phosfon to the cuttings during the first week in May. Use this material as a soil drench (0.4 gm. per 1 gal. container). This rate is equivalent to using a 1:50 Phosfon-water dilution and applying 7 oz. of this solution per gallon container. Water the plants well prior to treatment to assure good distribution of the Phosfon throughout the soil mass.
        C) When the danger of frost is past, transfer the plants out of doors maintaining the supplemental light treatments.
        D) Terminate the extended photoperiod treatments on September 1.

Obtaining flower buds on plants that usually will not initiate them until a later growing season.
        A) Apply Phosfon as a soil drench and at concentrations previously described. The application is most effective if applied before or during the first flush of spring growth. It is advised to apply Phosfon during the first week in May. Phosfon will enhance flower bud set if applied at those times without utilizing 20 foot candle supplemental light treatments. In all tests to date, however, the supplemental light treatments (10 a.m. to 2 p.m.) from June 1 to September 1 enhance the flower bud count in Phosfon treated plants above those plants treated but not so lighted. Hence the utilization of supplemental light is strongly advised to maximize the flower bud set.
        B) One can, if earlier treatment was not possible, achieve reasonably good results from treatment no later than the end of June. Treatments applied at this date must be accompanied by supplemental light treatments until September 1 to be effective. The rhododendron cultivars, 'Roseum Elegans', 'Nova Zembla', 'English Roseum' and 'Boule De Neige' have been in our trials and have been successfully treated with Phosfon to enhance flower bud development. The rhododendrons must be cultured in good fashion to achieve maximum effects from the Phosfon treatment. Our cultural treatments were as follows:

Fertilizer - applied in liquid form every three weeks.

Per 80 gallons of water or per 5 gallon container using a 1:16 proportioner.

Fungicide Treatment - applied once after potting

Per 80 gallons of water or per 5 gallon container using a 1:15 proportioner.

Additional Recommendations

  1. The rooted cutting used to produce one-year-old budded plants was propagated in the fall. The rooted cutting was then maintained at low temperatures 40 degrees or below until the time of placement in one-gallon containers in March. Once transplanted the temperature in the greenhouse from March till movement out of doors in late May was maintained at 70 degrees F. day and 60 degrees, night.
  2. It is extremely important to use a well drained media in the production of all container grown plants. The mixture used at O. S. U. for example is Canadian Peat Moss, course grade perlite and soil (2:2:1).
  3. Rhododendrons developed best flower bud set when they were grown in full sun. Plants should be misted from early morning through late afternoon using approximately 15 seconds mist every 15 minutes. This amount can be adjusted as situations dictate. The mist results in increased photosynthesis by the plants and it also decreases the temperature and increases relative humidity around the plants. The misting procedure does not replace normal watering practices. Misting should have wide application for many container grown plants.
  4. The amount of Phosfon essential to enhance flower bud initiation is the same regardless if the plant is in a one or two gallon container. If a two gallon container is used merely double the amount of water (7 oz. Phosfon solution + 7 oz. of water) in order to achieve adequate distribution of the Phosfon in the soil.
  5. Generally six weeks of 40 degrees F. temperature or below is essential to overcome the cold requirement of the flower bud. Plants can be forced at 70 degrees F in 3 to 5 weeks depending on variety grown.
  6. Rooted cuttings pinched at the time of potting and again at the beginning of the plants second flush of growth produced compact salable plants. Maximum numbers of breaks per pinch were realized when the new shoot was allowed to elongate about one inch in length before it was pinched. Plants pinched earlier by rolling out the terminal bud resulted in the development of fewer lateral branches.

Volume 26, Number 2
April 1972

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