QBARS - v18n2 Growth Retardants

Growth Retardants

Much interest has been shown recently in the use of chemical growth retardants on various ornamental plants in order to get better bud set and facilitate forcing. Following are abstracts of two papers recently delivered before the American Society for Horticultural Science. We understand azalea growers are quite interested in these materials for forcing types and reports from the industry will be coming along. It will be interesting if they prove effective in putting flower buds on some of the hard-to-bud rhododendron species and varieties.
Stuart, Neil W., U. S. Department of Agriculture, Plant Industry Station, Beltsville, Md., Controlling the Flowering of Azaleas with Growth Retardants, Photoperiod, and Storage Temperature . Several azalea cultivars respond to the growth retardants Cycocel, (2-chloroethyl) trimethyl-ammonium chloride, Phosfon-D, tributyl-2, 4dichloro benzyl phosponium chloride, and B995, N-dimethylaminosuccinamic acid, by stopping shoot growth and initiating flower buds whenever appropriate concentrations are supplied to growing plants as foliar sprays and soil drenches. Phosfon-D and -L must be applied as soil drenches to avoid spray-induced foliar blotching but the new structure, Phosfon-S, is effective as a foliar spray at low concentration without injury. Concentrations of Cycocel greater than 0.4 per cent cause yellowing of leaf tips when applied to the foliage. Repeated spray applications of B995 at 0.25 per cent cause initiation of excessive numbers of flowers and delay flowering. Flower buds form on treated intact or pruned shoots without further vegetative growth. Flower bud formation on untreated shoots is delayed by long and promoted by short (8-hour) photoperiods. Effects of long photoperiods can be offset and flower bud formation induced by the chemical growth retardants. Flower buds develop sufficiently in two to three months after treatment for normal flowering after exposure of the plants to dormancy-breaking cool storage for four to eight weeks. Application of growth retardants for bud initiation suppresses or delays vegetative shoot development during forcing.
Cathey, H. M., U. S. Department of Agriculture Plant Industry Station, Beltsville, Maryland, Initiation of Flower Buds on Rhododendron Following Application of Growth Retardants and Artificial Light . Liners of Rhododendron cv. 'Roseum Elegans' responded to soil applications of Phosfon-D and foliar applications of Phosfon-S and B995 by slowing growth and initiating flower buds 3 to 5 months after treatment. The plants were grown in the greenhouse on natural day lengths with 20 foot-candles of incandescent light from 10 PM to 2 AM, at a minimum night temperature of 65° F. Flower initiation was prevented by 8-hour days and reduced light intensity immediately following application of the growth retardants. Flower initiation was delayed on plants exposed to continuous incandescent light and on plants treated later than the middle of July growing on natural day lengths. After flower bud initiation, the plants were transferred to natural short days and were held 2 to 4 months to promote flower bud development. The plants were exposed to cool storage (50° F.) with 12 hours of incandescent light daily for 8 weeks, and were then returned to the greenhouse to bloom 6 to 8 weeks later. Flower buds on treated plants tended to be more difficult to force and bore flowers that were smaller than those typical of the variety.