Hydrangeas (Hydrangea sp.) are extremely popular and account for an estimated $73 million in U.S. sales annually [U.S. Department of Agriculture (USDA), 2009]. With an estimated 23 Hydrangea species worldwide (U.S. National Arboretum, 2005), there are only five species [florist hydrangea (H. macrophylla), hardy hydrangea (H. paniculata), smooth hydrangea (H. arborescens), oakleaf hydrangea (H. quercifolia), climbing hydrangea (H. anomala ssp. petiolaris)] generally available in the United States. The popularity of hydrangeas among consumers can be linked to the large, showy inflorescences, remontant blooming of some selections, and range of bloom colors, including blue.
Pruning is a common practice in nursery production to improve overall shape and reduce plant size, encourage branching, and influence flower development, all of which have been reported by retail consumers as important characteristics of plant quality (Glasgow, 1999). However, hand pinching or pruning does not always yield optimal branching (Hester et al., 2013; Starman, 1991). Starman (1991) reported fewer primary branches on manually pinched ‘Aurore’, ‘Celerio’, ‘Eurema’, ‘Isopa’, ‘Marumba’, ‘Morio’, ‘Patula’, ‘Phoebis’, ‘Selenia’, and ‘Sesia’ impatiens (Impatiens hybrids) compared with no pinching. Hand pinching also affects reproductive development. For instance, manually pruned Little Lime™ hardy hydrangea had reduced flower number compared with unpruned plants (Cochran and Fulcher, 2013). Still, growers continue to rely on hand pinching or pruning to control growth or modify plant architecture during production.
Hand pinching is not only inefficient, but it can be labor intensive and represents a significant cost of production (Holland et al., 2007). Therefore, the use of PGRs to control height, promote symmetrical branching, and/or increase flower production could be a better alternative to manual pruning. PGRs have proven to be effective in improving compactness (Hammond et al., 2007), increasing branch number (Holland et al., 2007), and increasing flower number (Abdelgadir et al., 2010). However, since the introduction of chemical pinching agents in the 1960s (Cathey et al., 1966), there has been an ongoing lack of consistency across plant species. Cohen (1978) reported 4400 ppm dikegulac sodium increased mean number of shoots of ‘Formosa’ indica azalea (Rhododendron indicum) compared with manually pinching yet there was no difference in shoot number of ‘Hexe’ indica azalea (Rhododendron indicum) 10 WAT. Auer et al. (1992) concluded that two petunia hybrid (Petunia ×hybrida) lines differed in absorption of benzyladenine by leaf explants. Bailey and Clark (1992) reported one application of daminozide increased inflorescence size of ‘Schenkenburg’ florist hydrangea, but had no effect on six other florist hydrangea cultivars (Böttstein, Enziandom, Kasteln, Mathilde Cütges, Merritt’s Supreme, and Red Star). Norcini et al. (1994) evaluated ‘Mauna Kea White’ bougainvillea (Bougainvillea glabra) and ‘Raspberry Ice’, ‘Royal Purple’, ‘Summer Snow’, and ‘Temple Fire’ bougainvillea (Bougainvillea buttiana), but only ‘Temple Fire’ had more open inflorescences per branch following two applications (at 0 and 4 weeks) of dikegulac sodium. With these varied responses, PGRs should be tested on cultivars to determine their efficacy. Therefore, the objective of these experiments was to evaluate the effect of dikegulac sodium applied to pruned and unpruned ‘Limelight’ hydrangea.
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