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ARS Home » Pacific West Area » Corvallis, Oregon » Horticultural Crops Research Unit » Research » Publications at this Location » Publication #242106
Title: Effects of Fall Applications of Chemical Defoliants, Urea, and Gibberellic Acid on Defoliation in the Fall and Performance of Hydrangeas during Forcing

Author
item BI, GUIHONG - Mississippi State University
item Scagel, Carolyn

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2009
Publication Date: 10/1/2009
Citation: Bi, G., Scagel, C.F. 2009. Effects of fall applications of chemical defoliants, urea, and gibberellic acid on defoliation in the fall and performance of hydrangeas during forcing. HortScience. 44(6):1604-1607.

Interpretive Summary: Hydrangea macrophylla ‘Merritt’s Supreme’ plants were used to study the effects of different concentrations of chemical defoliants and growth regulators [Def-6, giberellic acid (GA), copper-EDTA (CuEDTA), Florel (F) and urea (U)] on defoliation in the autumn, and growth and flowering performance during forcing. Def-6 was an effective defoliant for use on florists’ hydrangea at different times in the autumn. Increasing the concentration of Def-6 from 2500 up to 10000 mg·L-1 can increase defoliation without visible injury to plants and adverse effects on plant quality during forcing. Our results also suggest adding GA to Def 6 sprays may improve defoliation activity of Def-6 and adding urea to Def-6 sprays has the potential to improve plant performance during forcing. The response of plants to defoliants is affected by many variables such as temperature, humidity and precipitation, species and cultivar, timing, chemical concentrations, soil moisture, nutrition, age of plant, spray pressure and droplet size, adjuvant, water pH, etc.; therefore, it is important for hydrangea producers to conduct proper testing before adopting defoliation methods into production practices.

Technical Abstract: In two separate experiments, Hydrangea macrophylla (Thunb.) Ser. ‘Merritt’s Supreme’ plants were used to study the effects of foliar sprays of Def 6 (tributyl phosphorotrithioate, 2500, 5000, 7500 and 10000 mg·L-1), gibberellic acid, (GA, 50 mg·L-1), copper-EDTA (CuEDTA, 0.5% and 1.0%), Florel (2000 mg·L-1) and urea (3%) on defoliation in the autumn, and growth and flowering performance during forcing. Compared to control (plants sprayed with water only), spraying plants with urea or GA alone had no influence on defoliation or plant performance during forcing, and spraying plants with Florel alone had no influence on defoliation but decreased total flower dry weight during forcing. Combining urea with Florel sprays decreased the adverse effects of Florel on plant quality and combining GA with Florel improved defoliation. Increasing concentrations of Def 6 and CuEDTA increased defoliation. Compared with controls, plants sprayed with CuEDTA exhibited more defoliation, showed bud and leaf necrosis, and produced lower flower dry weight during forcing. Combining urea with CuEDTA sprays decreased the adverse effects of CuEDTA on plant quality. Compared to controls, spraying plants with Def 6 increased defoliation, caused no visible damage to plants, and had no adverse effects on plant quality during forcing. Adding urea to sprays containing Def 6 decreased or had no influence on the efficiency of defoliation and increased total flower dry weight during forcing compared to Def 6 alone. Adding GA to sprays containing Def 6 increased the efficiency of defoliation without adversely influencing plant quality.