The Control of Azalea Flowering with Day length,
Temperature and Gibberellic Acid
Roy A. Larson
Department of Horticultural Science
North Carolina State University, Raleigh
There is adequate information available to enable one to bring azaleas into flower at any time of the year (4, 7, 9, 10, 11). Two very important environmental factors that must be considered are day length and temperature. Long days are favorable for the initiation and development of vegetative shoots, and flower bud initiation in most cultivars is enhanced with short days. Temperatures of 65-70° F are considered satisfactory for the development of vegetative shoots, and flower buds also will be formed at these temperatures. Flowering will not occur, however, unless the "budded up" plants are exposed to temperatures of 35-50° F for 4 to 6 weeks, reportedly to break flower bud dormancy. Again, knowledge does exist on how gibberellic acid (GA3) can be used as a substitute for the cool temperature treatments, enabling one to get azaleas into flower without cooling facilities (1, 2, 3, 5,6, 8). Very little use has been made of this knowledge.
N. C. State research on the use of GA3 as a substitute for cool temperatures to break dormancy started six years ago. Many studies have been highly successful. Uniform flowering has been achieved on several cultivars when the minimum temperature was 65° F, eliminating completely the need for artificial or natural cooling. Larger flowers and faster flowering were other beneficial results of the GA3 treatments (6). 'Red Wing' flowers have been at least one inch larger in diameter on treated versus non-treated plants. 'California Sunset' plants flowered two weeks earlier when treated with 5 applications of GA3 at 500 ppm, than plants given 6 weeks of storage at 48° F, and 3 to 4 weeks sooner than plants subjected to 6 weeks at 35°F. Another promising prospect for azalea growers is a combination of cool temperature and GA3 applications to break flower bud dormancy. The plants can be placed at cool temperatures for 3 weeks, then moved to a forcing temperature of 60 to 65° and given 3 applications of GA3 (6,9). Refrigeration facilities are necessary in this program but twice as many plants can be handled this way then when a 6-week storage period is used alone.
The azalea variety 'Dogwood' was used in an experiment in the spring and summer of 1971, to show the feasibility of manipulating growth and flowering with day length, temperature and GA3.
The major day length treatments were 4 to 6 weeks of long days (16 hours) after the final pinch, followed by 6, 8 or 10 weeks of short days (9 hours) prior to start of dormancy breaking treatments. The dormancy breaking treatments were 6 weeks at a continuous 48° F temperature, and 3 weeks at 48° F followed by 3 applications of GA3 at 250 ppm after the plants had been moved to a greenhouse with a minimum night temperature of 62° F. The first GA3 application was made 4 days after removal from the cooler, and the 2 remaining applications were made 7 and 14 days after the first application.
Small plants deliberately were chosen for this study because of limited cooler space and because previous research had shown that meaningful results could be obtained from such plants. The plants had been pinched January 8, and were pinched again March 8 at the start of the study. There were 10 plants in each treatment.
Data recorded were dates of flowering (25% of flower buds open), number of flowers, number of by-passing vegetative shoots surrounding the flower bud, and quality of the plant at time of flowering. A rating of 10 was excellent, and 1 was poor. Flower size was not measured because a preliminary experiment on the var. 'Dogwood' had shown no GA3 effect on flower diameter.
Flowering was accelerated by about 3 weeks when the plants were given 4 weeks of long days, 6 or 8 weeks of short days, and then 3 weeks of 48° F and 3 applications of GA3 (Table 1). The 3 weeks would be equivalent to the head start such plants had in forcing in the greenhouse. (Previous work (6) had shown that 3 weeks of cool temperature without GA3 applications after storage resulted in delayed, uneven flowering and the plants would not have been acceptable commercially.) The most flowers per plant were always obtained in the 6 weeks cooler treatment, compared to 3 weeks at 48° and GA3 applications, regardless of day length treatments. The differences ranged from 2 to 8 more flowers per plant. More by-passing vegetative shoots generally were produced on the plants exposed to 48° for 6 weeks. (All of our studies have shown an increase in the number of bypass shoots produced as the time required for flowering increased.)
Very good to excellent quality plants were produced in all treatments. No advantage was realized by following a prolonged schedule such as 4 or 6 weeks of long days, followed by 8 or 10 weeks of short days. Flower number and plant quality were not improved and flowering was delayed by as much as 7 weeks. This study was conducted during a naturally bright time of the year in North Carolina beginning in March and continuing through the summer, so the short period of time from pinch to the start of dormancy-breaking treatments (16 to 20 weeks) was satisfactory. Previous research (4) indicated that longer periods of long and short days were required at darker periods of the year.
Some commercial azalea growers have used GA3, with apparent success and desire to make it a definite part of their cultural program. Time of application is crucial, and the 3 weeks in the cooler reduce the chances for premature application. The need for artificial cooling is not eliminated, however, and that is a fundamental objective of our azalea research.
In summary, a combination of 3 weeks at 48° and 3 applications of GA3, at 250 ppm successfully broke azalea flower bud dormancy in the var. 'Dogwood.' Sixteen to 20 weeks from pinch to the start of dormancy-breaking treatments resulted in acceptable plants in this study initiated in early March. Flower size was not affected by GA3, applications but in other experiments 'Red Wing' flowers were as much as 1" larger in diameter on GA3 treated plants, compared to untreated plants.
Table 1. Azalea flowering as affected by day length, temperature and GA3 treatments. The study was conducted from March through August, 1971, on the cv. 'Dogwood.' Weeks of long days Weeks of short days Weeks at 48°F GA3 treatment Days1 to flower Flowers per plant By-pass shoots plant Quality2 4 6 6 - 163 30 3 9.0 4 6 3 + 140 28 1 9.3 4 8 6 - 167 35 6 9.3 4 8 3 + 147 29 2 8.8 4 10 6 - 176 33 8 9.0 4 10 3 + 176 29 7 8.7 6 6 6 - 167 37 7 9.7 6 6 3 + 148 29 2 9.2 6 8 6 - 173 39 4 9.3 6 8 3 + 175 33 7 9.0 6 10 6 - 192 36 10 9.6 6 10 3 + 192 34 9 8.3 From date of final pinch (March 8, 1971) to date when most plants in the treatment had 25 percent of the flower buds open. 2Quality 10 = Excellent 1 = Poor.
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