During production of florists’ hydrangea, defoliation before cold storage is required for prevention of diseases such as botrytis bud rot (Bailey, 1989). Many hydrangea growers manually remove leaves before natural leaf abscission. This practice is time-consuming, expensive, and can result in damage to stems and buds. An alternative to this practice is chemical defoliation. A wide variety of chemicals have been tested for their effectiveness in defoliation of hydrangeas, but all have some limitations (Bailey, 1989). Some chemicals such as ethephon (2-chloroethylphosphonic acid) adversely influence performance during forcing by retarding growth or reducing inflorescence size (Shanks, 1969; Tjia and Buxton, 1976). Application procedures for some chemicals such as Vapam (sodium N-methyldithiocarbamate) and ethylene require an air-tight storage unit. Some chemicals such as 2-butyne-1,4-diol (BD) are effective defoliants at high concentrations but are poisonous to humans and require special safety regulations for application. The influence of only a few combinations of chemicals on defoliation of hydrangea has been evaluated. Gibberellic acid (GA3) has been found to be able to enhance chemical [BD and TPTA (tributyl phosphorotrithioate)] defoliation of hydrangeas (Bailey, 1990).
During natural leaf senescence, deciduous plants can mobilize nutrients, including nitrogen (N), from leaves into storage tissues (Titus and Kang, 1982). Stored N is important for the initial growth of deciduous plants in spring, and there is a positive relationship between the amount of stored N and spring growth in many species (Cheng and Fuchigami, 2002; Cheng and Xia, 2004; Taylor, 1967; Taylor and May, 1967). The N mobilized from senescing leaves makes an important contribution to whole plant N economy (Chapin and Kedrowski, 1983; Taylor and May, 1967). For example, in apple, N mobilized during leaf senescence constitutes ≈25% of the total plant N (Cheng et al., 2002). Manual or chemical defoliation removes green leaves from plants before any significant N mobilization occurs. In Hydrangea macrophylla ‘Merritt's Supreme’, ≈50% of total plant N was in leaves in early fall (Bi et al., 2008). Early defoliation could potentially decrease N storage and result in poor growth and flower development during forcing.
Foliar sprays of urea in the fall after terminal bud set can increase reserve N in deciduous plants without stimulating new growth late in the season (Cheng et al., 2002; Sanchez et al., 1990; Tagliavini et al., 1998). Increased N from urea sprays can compensate for the N lost from early defoliation and improve plant growth and development during the next growing season (Bi et al., 2005; Guak et al., 2001). Spraying florists’ hydrangea with urea before manual defoliation has the potential to improve growth and increase the number of flowers and flower size during forcing (Bi et al., 2008). At present, there is no available information on whether spraying hydrangea with urea ameliorates the effects of chemical defoliants on growth and flowering performance during forcing.
This article presents the results of two studies assessing the effects of foliar sprays with chemical defoliants and urea on defoliation in the fall and plant performance during forcing in florists’ hydrangea. Our objective was to determine if the combination of urea with defoliant applications promotes early defoliation and maintains or improves growth and flowering performance during forcing.
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