QBARS - v19n1 Guidelines for Regulating Flowering of Rhododendrons - Light and Growth Retardants

Guidelines for Regulating Flowering of Rhododendrons -
Light and Growth Retardants

Henry M. Cathey and R. L. Taylor
Horticulturist and Agricultural Research Technician, respectively
Crops Research Division, Agricultural Research Service
U. S. Department of Agriculture, Beltsville, Maryland.

Released for publication under the terms of the Memorandum of Agreement between the Society of American Florists, the American Association of Nurserymen, and the U. S. Department of Agriculture.
A grant from the New York Florists' Club helped finance the air-cooling of greenhouses.

Stuart (11-14) reported that the growth retardants Phosfon, Cycocel, and B-Nine regulated flower initiation in evergreen azalea cultivars. These responses did not depend on minimum age or size of the plant or a specific photoperiod. Treated plants still required several months for flower buds to develop, and for their exposure to temperatures below 55°F for 6 to 8 weeks, and the final forcing into flower in a greenhouse maintained at a night temperature of 60°. Stuart suggested that the chemicals regulated in some degree the time of initiation and the number of flower buds produced; prevented shoot growth while flower buds were developing; and delayed it during flowering. Subsequently, applications of growth retardants accelerated the flowering of Camellia (7,13), llex (9), Malus (1), Pyrus (1), and Prunus (1).
Skinner (10) and Weiser and Blaney (15) reported the effects of light in regulating growth of rhododendrons. Long days accelerated the growth of liners. Davidson and Hamner (5) reported that 18-inch plants of Rhododendron catawbiense Michx. initiated flower buds under long days but abnormal flowers developed when exposure was prolonged. Davidson and Watson suggested that Rhododendrons require long days for initiation and short days for flower development (6).
Rhododendrons were reported to be responsive to Phosfon, Cycocel, and B-Nine (2,3,4). This report describes some of the observations from rhododendron hybrids treated with the growth retardants and grown under various photoperiods and it also suggests guidelines with which to test rhododendron cultivars and seedlings for regulated flowering.

Materials and Methods
Rooted or started cuttings (liners) were prepared in the greenhouses at Beltsville, Md., or obtained from commercial sources. 4 They were planted in standard 5- or 6-inch clay pots with a mixture of equal parts composted soil, peat moss, and sand. The medium was drenched to saturation with a 0.1% solution of a non-ionic wetting agent, 5 to promote drainage and ease in watering. Rhododendron plants grown without the precautionary treatment of wetting agent were easily over-watered during the winter months, resulting in severe damage to the plants. Fertilizer (½ lb. of an ammonium-urea source 25-10-10 dissolved in 1 gal. of water) was applied with an injector in the proportion of 1:100. The plants were fertilized once a week while in active growth; they were not fertilized during low-temperature storage or during forcing.

4 Plants were supplied by Koster Nursery, Seabrook, N.J., Wells Nursery, Red Bank, N.J., Henry F. Mitchell Co., King of Prussia Pa.; George J. Ball, Inc., West Chicago, Ill. and William Germain, Horsham, Pa.
5 Blend of several surface active agents supplied as a concentrate by Aquatrols Co. of America, Camden, N. J.

The recently transplanted plants were grown in a greenhouse on the natural day lengths (from 10.2 to 15.7 hours) with an interruption of the dark period, from 10 P.M. to 2 A.M. nightly, with at least 20-foot candles (ft-c) of incandescent light. The shoots were topped once and 3 to 4 shoots were allowed to develop on each plant, unless otherwise noted. The plants were grown in a greenhouse maintained at a minimum night temperature of 65°F.
Solutions of Phosfon (2, 4-dichlorobenzyltributyl-phosphonium chloride), 6 Cycocel [ (2-chloroethyl) trimethylammonium chloride], 7 and B-Nine (N-dimethylaminosuccinamic acid), 8 were applied to 250 ml to a 5-inch pot, 500 ml to a 6-inch pot, and 1000 ml to an 8-inch pot. This volume of liquid was sufficient to moisten the whole soil volume thoroughly and to allow a slight excess. The plants were sprayed with aqueous mixtures of the growth retardants with 0.1% Tween20. The growing point and expanding leaves were sprayed with a plastic sprayer until the excess began to run off.

6 Phosfon was provided as the technical material by V-C Chemical Corp., Richmond, VA.
7 Cycocel was provided by the American Cyanamid Company, Princeton, N.J., as a 50% liquid.
8 B-Nine was provided by the Naugatuck Chemical Division, United States Rubber Co., Naugatuck, Conn.
Mention of these companies, their products and trade names is for information only with the understanding that  no discrimination is intended and that no guarantee of reliability is implied by the U. S. Department of Agriculture.

Preliminary Experiments
Plants of 'Roseum Elegans' rhododendron were grown on natural photoperiods plus a 4-hour period of incandescent filament light in the middle of the night. Separate lots of plants were treated in early June with 0.1, 0.2, and 0.5 g of Phosfon, and 2 and 5g of Cycocel, as soil drenches per 5-inch pot. The plants continued to grow on long photoperiods for 4 months. At that time flower buds were visible on the plants treated with Phosfon and Cycocel, while the untreated plants were vegetative. All plants were transferred to natural short photoperiods for the months of October, November, and December. For the months of January and February the plants were exposed to low temperature (50°). They were lighted 12 hours daily with 100-w incandescent lamps, spaced 2 ft. apart and 2 ft. above the plants. The plants were forced without supplemental light in a greenhouse maintained at a night temperature of 65°. The budded plants came into flower 5 to 6 weeks after the start of forcing (Fig. 10). The untreated plants resumed growth by producing multiple vegetative growths. The plants treated with 0.1 g of Phosfon produced flowers on some branches, vegetative growth on others. The plants treated with 0.2 and 0.5g of Phosfon were much more compact than the untreated plants, produced 3 growths, and initiated flower buds. The growths subtending the terminal flowers developed subsequent to the time that the flowers developed. The flowers, though, were typical but were somewhat smaller and paler than those known for the cultivar. The stems of the Phosfon-treated plants were limber and yellow.
The plants treated with 2 and 5 g of Cycocel were much more compact than the untreated. They produced 3 growths, and initiated flower buds. The vegetative growths subtending the terminal flowers developed and obscured the flowers (Fig. 10). The foliage of the Cycocel-treated plants developed brown margins during the forcing procedure.
The plants were allowed to remain in the greenhouse on natural days from April to October. Untreated plants produced 2 additional growths during the summer, but remained in vegetative growth. Phosfon-treated plants immediately initiated flower buds on every active meristem. The growth of the shoots subtending the terminal flowers was 4 to 8 cm in length. Plants were exposed to 50°F for 2 months and flowered the following January (Fig. 10). Plants treated with Cycocel during the previous season resumed vegetative growth.
The plants grew for a third season on natural days. All plants formed flower buds. The untreated plants initiated flowers on the ninth flush of growth whereas the Phosfon-treated plants had flowered on the third, fourth, and fifth flushes of growth made in three successive years. The Cycocel-treated plants initiated flower buds on the third and seventh flushes of growth; the flower buds on the seventh flush developed into normal inflorescences.
This experiment proved that growth retardants, by limiting growth, accelerated the initiation of flower buds of rhododendrons. The effects of Phosfon persisted for several years. The effects of Cycocel were transitory with abnormal inflorescences. Detailed experiments (reported elsewhere) conducted on 'Roseum Elegans' were concerned with the following: action of the various growth retardants; photoperiod for flower initiation; photoperiod for flower development, temperature and light for breaking dormancy: and forcing treatments for flowering. A standard photoperiod growth-retardant technique was established as a model to test various cultivars and seedlings.

Fig. 10
Fig. 10.  Plants of 'Roseum Elegans'
rhododendron grown on natural days at
65°F. minimum night temperature; receive
the following treatments: upper photo,
10 months after treatment; lower photo, 20
months from treatment. Left to right:
0.2 g of Phosfon, untreated, 2g of Cycocel.
Fig.11.  Response of 3 rhododendrons
cultivars to regulation with light and growth
retardant. Left, untreated, right, soil
treated with 0.4g of Phosfon. Upper,
'Alice', middle 'Cynthia', and lower
'Rosa Mundi'
Photos by author

Action of Growth Retardants
A standard photoperiod-growing technique was used to evaluate the action of the growth retardants on 'Roseum Elegans' rhododendrons. The chemical treatments began with the supplemental lighting.

When applied only once, a dosage of 0.05 or 0.1 g of Phosfon sufficed to retard growth of plants in 5-inch pots. The plants remained in vegetative growth. A total of 0.2 or 0.4 g of Phosfon applied once, in 2 or 4 equal parts, at monthly intervals, retarded growth; flower buds were initiated. A total of 0.8 g of Phosfon suppressed growth completely. The plants remained in vegetative growth.
The amount of Phosfon required to retard growth and to promote initiation of flower buds varied with the volume of soil. A dosage of 0.2 g of Phosfon, applied to a 5-inch pot, was near optimum for the desired growth response. Twice and 4 times the amount of Phosfon, 0.4 g and 0.8 g, respectively, were required to obtain the same percentage of stem reduction on plants growing in 6-inch or 8-inch pots.

Some dosages of Cycocel suppressed growth. Others gave a slight retardation of growth. None of the treatments gave satisfactory results. Cycocel was discontinued in subsequent experiments.

B-Nine either had little effect on growth or suppressed stem elongation completely when applied to the soil. Lots of plants sprayed only once remained in vegetative growth. A dosage of 0.25% and 0.5%, given twice or 4 times one month apart, retarded stem elongation and promoted initiation of flower buds. A dosage of 1.0% B-Nine retarded growth and promoted initiation of flower buds, but greatly delayed the development of the small flowers. A dosage of 0.1% B-Nine was ineffective when applied twice, but effective when applied 4 times at monthly intervals.

Response of Rhododendron Cultivars
The standard growth retardant photoperiod technique was used to evaluate the usefulness of 24 rhododendron hybrids. Plants propagated the previous winter, were grown on natural photoperiods of summer or on natural days with 4 hr of incandescent light from 10 P.M. to 2 A.M. from June through September, transferred to natural short days from October to December, and exposed for 2 months to 50° F and returned to the greenhouse for forcing. The plants were drenched in early June with 0.2 and 0.4g of Phosfon, once or twice or sprayed with 0.25 and 0.5% B-Nine once or twice a month for 4 months. The response to photoperiod, sensitivity to growth retardant, and forcing performance of some of the cultivars are reported in Table 1.

Table 1. Response of some rhododendron cultivars to light and growth retardants.


Parentage 1
Requirement 2
retardant 3

Forcing 4
'America' 'Parsons Grandiflorum' x Long days XX Excellent 7 weeks
R. catawbiense x Natural long days XX Excellent 6 weeks
'Chionoides' R. ponticum Long days XX Excellent 7 weeks
'Jean Marie
de Montague'
R. griffithianum x Long days XX By-passing of flower buds
'Madame Masson' R. catawbiense x R. ponticum Long days XXX Excellent 8 weeks
'Nova Zembla' 'Parsons Gloriosum' x Natural long days X Excellent 5 weeks
'Rosa Mundi' R. caucasicum x Natural long days X Excellent 4 weeks
'Roseum Elegans' R. catawbiense x Long days XXX Excellent 5 weeks
1 Parentage from "Rhododendrons of the World" by David G. Leach, published by Charles Scribner's Sons,  New York.
2 Photoperiod requirement: Daylength required to allow growth to form flower buds. Natural days of summer or long days (4 hr., 10 PM to 2 AM).
3 Growth retardant: Dosage response from X (least response) to XXX (most responsive).
4 Forcing. Response to low temperature storage and the approximate number of weeks to flower in a greenhouse held at a minimum of 65°F.

The cultivars varied greatly in their response to photoperiod and growth retardant, and in forcing performance. 'America', 'Catawbiense Grandiflorum', 'Chionoides', 'Madame Masson', 'Nova Zembla', 'Rosa Mundi', were useful cultivars for the promotion of early flowering (Fig. 11). The named selections of 'Roseum Elegans' were all responsive to regulation with light and chemicals. Most cultivars initiated flower buds in response to the growth retardants on long days. R. catawbiense f. album (Glass) Rehder formed abnormal flower buds. During forcing cultivars such as 'Alice', 'Charles Bagley', 'Crimson Glory', 'Pink Pearl', 'President Lincoln', and 'Trilby' dropped their foliage. The flower buds on the cultivars 'Caractacus', 'Jean Marie de Montague', and 'Madame de Bruin' were by-passed with vegetative growth; and 'Dr. Rutgers' and 'Edward S. Rand' did not develop their flower buds. There were no apparent relationships of parentage, natural growth habit, and responses to light and growth retardants. Adjustments in dosage of growth retardants and photoperiod sequence were necessary for use on different cultivars.

Response of Rhododendron Seedlings
The same standard growth retardant photoperiod technique as described in the cultivar section was used to evaluate the response of seedlings from plants of 'Pink Pearl', 'Dr. Dresselhuys', Rhododendron fortunei Lindl. and R. catawbiense . The seedlings were grown on natural days, with a 4 hr interruption of incandescent light from 10 P.M. to 2 A.M. The seedlings, for the first 4 to 7 months, produced a series of small leaves. Eventually the leaf-size increased. The plants were treated with 0.1, 0.2, and 0.4 g of Phosfon per 4-inch pot. Eleven months later, 20 percent of the seedlings of 'Dr. Dresselhuys' flowered; this occurred 18 months from planting of the seed (Fig. 12). The growth of more than half the seedlings was suppressed; no flowers were formed; the remaining 30 percent continued to grow without forming flower buds. Apart from determination of flower color, the typical characteristics of the seedlings were distorted by the treatments. Because seedlings varied greatly in response to light and chemicals, a simple screening technique was impossible. The authors doubt that this technique will help to accelerate the breeding of rhododendrons. The use of artificial light to accelerate flowering of rhododendron seedlings was reported by Doorenbos (8).

Year-around Flowering
The standard growth retardant photoperiod technique was used to flower separate lots of plants in January, March, May, July, September, and November for 2 consecutive years. The time from start of treatment with growth retardants and long days to flowering, required 9½ to 11 months. The sequence for flower initiation, flower development, low temperature breaking of dormancy, and forcing required 4, 2 to 4, and 1½ months, respectively, for completion. Adjustments in the duration required to initiate a flower bud varied with the time of year. Plants growing on long days required only 6 months for flower initiation and development during summer, but up to 8 months during winter. Flower buds exposed to low temperature too early did not force normally. Sufficient time was essential for the development of the flower buds prior to exposure to low temperature storage. Flower buds were responsive to low temperature storage when the scales around the flower primordia were fully extended and a gelatinous coating appeared on them.

The rhododendron hybrids exhibited a more complex response to photoperiod and growth retardants than did the azaleas (11-14). Two or 3 flushes of growth began after the start of long days and application of growth retardants. The size of the plants increased during the time of flower-bud initiation. For most rhododendron cultivars the natural long days of summer were of insufficient length and duration to promote 3 flushes of growth during the first season after propagation. Response to the growth retardants was exhibited only after 2 or 3 flushes of growth. Dosages and frequency of application were timed to retard growth long enough to promote early initiation of flower buds.
Four cultivars, 'Catawbiense Grandiflorum', 'Madame de Bruin', 'Nova Zembla', and 'Rosa Mundi', initiated flower buds on the natural long photoperiods during the first summer of growth. Rooted cuttings, propagated the previous winter, grew on long photoperiods during the months of February through May. The flower buds were initiated on the fourth or fifth flushes of growth. The other 20 cultivars, remained in vegetative growth as liners. The second or third year following propagation, the untreated plants initiated flower buds; the plants had developed 8 or 9 flushes of growth by that time. Treatment with growth retardants reduced the number of flushes at which flower buds were initiated from the eighth or ninth to the fourth or fifth flush. Limiting growth by exposure of the plants to 8-hr photo periods, or by reduced light intensity, negated flower-bud initiation by the retardants. Reducing day length was essential to prepare the flower bud for exposure to low temperature and thus break dormancy. Continuous artificial light during, or exposure to, long days after initiation of the flower buds promoted development of the vegetative shoots subtending the flower buds and abnormal flowers. No flower buds expanded when the vegetative shoots developed.
The phasing of the treatments with the various growth retardants denoted persistence in soil and plant. Phosfon was effective from a single soil drench applied at the time when a flush of growth was maturing. Evidence of the same dosage of Phosfon on plants 2 years later were their compact growth, multiple flower-bud formation, and yellow stems. Unlike other cultivars the 'Roseum Elegans' treated with Phosfon developed very limber stems.
When Phosfon-treated plants were replanted in untreated soil, they resumed normal growth.
Spray applications of B-Nine had to be timed to coincide with the shooting of 3 flushes of growth. One or many spray applications made within a 1week period had little effect on growth, thus did not promote formation of flower buds. Spray applications spaced at monthly intervals allowed for its presence in the plant for at least 3 to 4 months until flower buds were initiated. Over-treatment with B-Nine was evidenced at forcing by greatly delayed flower development; it drastically reduced flower-size and color intensity. B-Nine, as a soil drench, was extremely toxic to rhododendrons. The ineffectiveness of Cycocel was all the more perplexing in the light of its known useful effect on azaleas (14).


Pre-treatment . Select uniform plants. Pinch to promote development of 3 lateral shoots. Treat soil (1 part compost; 2 parts peat; 1 part coarse sand) with non-ionic wetting agent at time of repotting. (Drench soil with 0.1% solution). Pot in 5- or 6-inch clay pots.
Lighting. Grow plants on long days (natural days plus incandescent light, 20-ft.-c from 10 P.M. to 2 A.M.). Continue lighting for 2 months in summer (June, July or August) and for 4 months in winter (November, December, January, February).
Temperature. Maintain a minimum temperature of 65°F at all times.
Chemical treatment. When growth is starting, treat separate lots of plants as follows:
1. Untreated.
2. Soil drench, Phosfon 0.2 g/plant. 3. Spray B-Nine 0.25% 3 times at monthly intervals.
Preparation of solutions:
0.2 g/pot. Add 1 oz. 10% Phosfon-D liquid in 15 pints water; treat each pot with 1 pint of diluted chemical.
B-Nine: 0.25% spray. Dilute 5% concentrate, 1 part concentrate to 19 parts water.
Post-treatment. Flower buds should appear on treated plants 3 to 4 months after treatment with chemicals. To allow flower buds to develop transfer the plants to natural day lengths.
Cold storage. Store budded plants at temperature below 55°F for a minimum of 8-10 weeks to break the dormancy of the flower buds.
Forcing. Return plants to greenhouse for forcing. A variety will require 4-8 weeks to force at 65°F.

Timetable for Rhododendrons
Time from start
(measured in months)
Preparation of liners Pot plants. Grow on long days (natural days plus 4 hrs., 20 ft-c,
10 PM to 2 AM). Pinch.
0 Growth starts on plants with desired framework: Continued on long days; treat plants with growth retardant, 0.2 g Phosfon per 5-inch pot; use 0.25% B-Nine as a foliar spray, 4 times at monthly intervals.
2 months Transfer summer-treated plants to natural days.
4 months Transfer winter-treated plants to natural days.
4 months Flower buds should be visible.
4-6 months& Allow flower buds to develop on natural days.
6-8 months, or
6-8½ months
Place plants at temperatures below 55°F plus continuous 10 ft-c incandescent light.
8½ months Bring plants into 65°F for forcing.
10-11 months; Plants should flower.
Fig 12
Fig.  12.  Seedlings of 'Dr. Dresselhuys' grown on long days
and the plant on the right treated with 0.2 g of Phosfon.
Photographed 18 months after planting seed.
Photos by author

Literature Cited

1. Batjer, L. P., M. W. Williams, and G. C. Martin, 1964, Effects of a growth retardant on physiological responses of deciduous fruit trees, Proc. Amer. Soc. Hort. Sci., 85 (In press).
2. Cathay, H. M., 1963, Horticulturists study effects of growth retardants on plants, Flor. Rev., 132 (3426) 35, 81-84.
3. 1964 Physiology of growth retarding chemicals. Ann. Rev. Plant Physiol., 15: 271-302.
4. N. W. Stuart, 1961, Comparative plant growth-retarding activity of Amo-1618, Phosfon, and CCC, Bot. Gaz., 123: 51-57.
5. Davidson, 11, and C. L. Hamner, 1957, Photoperiodic responses of selected woody ornamental shrubs, Mich. Agr. Exp. Sta. Quart, Bul., 40 (2) : 327-313.
6. Davidson, H. and D. P. Watson, 1959, Teratological effects of photoperiod on Rhododendron catawbiense Michx., Proc. Amer. Soc. Hort. Sci., 73: 490-494.
7. Gill, D. L. and N. W. Stuart, 1961, Stimulation of camellia flower bud initiation by application of growth retardants, A preliminary report, The Amer. Camellia Yrbk., 1961: 129-135.
8. Doorenbos, J., 1955, Shortening the breeding cycle of rhododendron, Euphytica, 4: 141-146.
9. Marth, P. C., 1961, Effect of growth retardants on flowering, fruiting, and vegetative growth of holly (Ilex), Proc. Amer. Soc. Hort. Sci., 83: 777-781.
10. Skinner, H. T., 1940, Factors affecting shoot growth and flower bud formation in rhododendron and azaleas, Proc. Amer. Soc. Hort. Sci., 37: 1007-1011.
11. Stuart, N. W., 1961, Initiation of flower buds in rhododendron after application of growth retardants, Science, 134: 50-52.
12. Stuart, N. W., 1962, Azalea growth rate regulated by chemicals, Flor. Rev., 130 (3373) : 35-36.
13. Stuart, N. W., 1962, Stimulation of flowering in azaleas and camellias, Proc. XVIth Int. Hort. Cong., Vol. 5: 58-64, Brussels, Belgium.
14. Stuart, N. W., 1964, Report of cooperative trial on controlling flowering of greenhouse azaleas with growth retardants, Flor. Rev., 134 (3477) : 37-39, 74-76.
15. Weiser, C. J. and L. T. Blaney, 1963, Rooting and night lighting trials with deciduous azaleas and dwarf rhododendrons, Amer. Hort. Mag., 42: 95-100.