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Journal American Rhododendron Society

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Volume 21, Number 2
April 1967

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"Out-of-Season" Flowering of Quality Azaleas
Roy A. Larson and Martin L. McIntyre
Department of Horticultural Science
North Carolina State University at Raleigh

        The possibility of producing flowering azalea plants in the greenhouse throughout the year was given a tremendous impetus by Neil Stuart at Beltsville. His work in the early 1960's showed that flowers could be uniformly produced on some varieties during periods of the year which previously had been considered as "out-of-season."
        Research on azalea flowering has been in progress at N. C. State since 1962. Day length, temperature, and growth regulators have all been studied in conjunction with azalea culture and flower production. Most attention has been focused on the influences of day length on flower initiation and early development, and on the influences of temperature on later flower development.
        By 1966 the authors felt sufficient information had been gathered to make scheduled azalea flowering a distinct reality, at least for the greenhouse forcing variety 'Red Wing'.
        The plants in this study were propagated from greenhouse-grown stock plants, rooted under intermittent mist, and grown in the greenhouse for the entire life of the plant. No diseases or insect pests were ever detected. The plants were finally potted in 6-inch clay pots, in 100% acid peat moss. The fertilization program consisted of weekly applications of 21-7-7 or KNO3 at 20 ounces/100 gallons of water during the summer, and biweekly applications at the same concentration during the winter.
        Earlier experiments had indicated that long days immediately following the final pinch, followed by short days previous to the pre-cooling treatment, resulted in uniform flower initiation and early development. In this study 'Red Wing' plants were pinched at 2week intervals, given 4 weeks of 16-hour days immediately after the pinch, followed by 6 weeks of 9-hour day lengths. The long days were achieved with normal day lengths plus incandescent light until midnight. The short days were obtained by covering plants with black sateen cloth at 5PM, and removing the cloth at 8 AM. The minimum night temperature of 60F was maintained for the 10 weeks following the final pinch.
        Upon completion of the short day treatments the plants were placed in a cooler kept at a temperature ranging from 46-48F. The plants were lighted with incandescent lights for 12 hours each day. Every effort was made to maintain a high relative humidity in the cooler, to prevent leaf abscission. The low temperature treatment, used to break dormancy and to further develop the flower buds, was continued for a period of 6 weeks. The plants were then moved back to the greenhouse, for forcing. A forcing night temperature of 60F was desired, but could not be achieved during 
July, August and September.
        Date of first appearance of flower bud color, date of first fully opened flower, and number of flowers per plant were recorded.  Twelve plants were placed in the study every 2 weeks.  The schedule of treatments and the effects on flowering are shown in Table 1.

Table 1. Schedule of treatments and results obtained with 'Red Wing' azalea
plants in 1966-1967 "out-of-season" study.
  Dates various treatments started Date of
first
flower
No. of days
pinch to
1st flower
Ave. No. of
flowers/
plant
Pinch and
long days1
Short
days2
48F Forced
1. March 16, 1966 April 13 May 25 July 6 July 29 135 147
2. March 30 April 27 June 8 July 20 Aug. 11 134 170
3. April 13 May 11 June 22 Aug. 3 Aug. 24 133 182
4. April 27 May 25 July 6 Aug. 17 Sept. 8 134 113
5. May 11 June 8 July 20 Aug. 31 Sept. 27 139 176
6. May 25 June 22 Aug. 3 Sept. 14 Oct. 13 141 186
7. June 8 July 6 Aug. 17 Sept. 28 Nov. 1 146 98
8. June 22 July 20 Aug. 31 Oct. 12 Nov. 11 142 142
9. July 6 Aug. 3 Sept. 14 Oct. 26 Dec. 31 50 219
10. July 20 Aug. 17 Sept. 28 Nov. 9 Dec. 9 142 198
11. Aug. 3 Aug. 31 Oct. 12 Nov. 23 Dec. 27 146 267
12. Aug. 17 Sept. 14 Oct. 26 Dec. 7 Jan. 7 143 306
13. Aug. 31 Sept. 28 Nov. 9 Dec. 21 Jan. 22 143 299
14. Sept. 14 Oct. 12 Nov. 23 Jan. 4 Feb. 4 143 228
15. Sept. 28 Oct. 26 Dec. 7 Jan. 18 Feb. 18 143 291
16. Oct. 12 Nov. 9 Dec. 21 Feb. 1 Mar. 7 146 197
1Long days were 16 hours.
2Short days were 9 hours.
Twelve plants were inserted in the study every 2 weeks.
     
'Red Wing' azalea plant
     Fig. 9.  'Red Wing' azalea plant, given final pinch August
     17,  forced December 7, and photographed January 25.
     Plants in this phase averaged 306 flowers/plant.

        Plants which were given the final pinches during the period of March 16 to April 27 flowered faster than plants pinched after this period. This hastened flowering was primarily caused by the high forcing temperatures prevalent in the greenhouse from July 6 to early September.
        Flower number was much larger in the later phases of the study. An effort was made to use uniform-sized plants throughout the study, but this was not always possible. The authors did note, however, that shoots which were produced during the early phases of the experiment frequently had only one or 2 flowers. Clusters of multiple flowers were frequently noted on shoots produced in the later phases. It was also observed that the flowers were not true 'hose-in-hose' during the early phases, while normal flowers were produced during the later phases. General quality of the plants in the early phases was not impaired, however.
        Vegetative by-passing of the flower buds was very uncommon throughout the study. The highest average recorded was 3 by-passing shoots/plant, for phases 7 and 10 (plants pinched June 8 and July 20).
        The plant shown in Figure 9, was pinched August 17 (phase 12), forced December 7, and photographed January 25. Plants in this phase averaged 306 flowers/plant.


Volume 21, Number 2
April 1967

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