Ancymidol and uniconazole are plant growth regulators that slow plant growth by inhibiting sterol and gibberellin biosynthesis (Henry, 1985; Shive and Sisler, 1976). These growth regulators have been shown to restrict height of several floricultural crops, including poinsettia [Euphorbia pulcherrima (Holcomb et al., 1983)], chrysanthemum [Dendranthema × grandiflorum (Barrett, 1982)], lily [Lilium speciosum (Bailey and Miller, 1989)], and several bedding plant species (Barrett and Nell, 1992).
Use of plant growth regulators during woody ornamental plant production is less common than in floriculture crop production. Several studies have tested nursery crop response to uniconazole (Frymire and Cole, 1992; Frymire and Henderson-Cole, 1992; Henderson and Nichols, 1991; Norcini and Knox, 1989; Warren, 1990; Warren et al., 1991). Woody plant response to growth regulators varies with environment, application rate and method, and species being treated. Recommendations for rate and timing of effective growth regulators for a variety of woody species are needed (Warren et al., 1991).
Oakleaf hydrangea is an important container crop for many nurseries in the southern and midwestern United States. It is used in the landscape as a border or as a solitary specimen plant. In commercial practice, shearing is required at least once or twice during the production cycle to produce a salable plant. After shearing, however, oakleaf hydrangea is susceptible to dieback, especially when excess moisture is present (P. Havenar, personal communication). A search of the literature revealed no reports regarding research on control of plant size of oakleaf hydrangea with growth regulators; however, uniconazole has been shown to reduce shoot length, stem dry weight, leaf area, and inflorescence dry weight of florists’ hydrangea [Hydrangea macrophylla (Bailey, 1989)]. The objective of this research was to determine whether foliar or substrate surface applications of ancymidol or uniconazole, or pinching reduces plant growth of oakleaf hydrangea.
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