Plant Growth Regulators Increase Branching of Hydrangea paniculata 'Limelight' and 'Jane' (Little Lime™)

Pruning is commonly performed during production to produce symmetrical, compact plants that are pleasing to the consumer’s eye.  Pruning can also allow for closer spacing and reduce breakage during production and shipping.  To achieve desired branch architecture and crop uniformity, nursery growers typically hand prune or, less commonly, apply plant growth regulators.  However, hand pruning is expensive and is not always effective, and efficacy of plant growth regulators can depend on cultural practices, environmental conditions, irrigation, cultivar and rate.  Therefore, the objectives of these experiments were to evaluate Hydrangea paniculata 'Limelight' and 'Jane' (Little Lime™) hardy hydrangea in response to single foliar applications of three plant growth regulators (PGRs) at two rates: dikegulac sodium (Augeo®) at 800 or 1600 ppm, benzyladenine (Configure®) at 300 or 600 ppm or ethephon (Florel®) at 500 or 1000 ppm.  There were two additional treatments: a hand-pruned control leaving three nodes and an unpruned water control (untreated) applied the same day as PGRs.  Plants were potted in 3-gallon containers with 85% pine bark:15% peat, topdressed with Harrells 19–1.7–6.6 (N–P—K), 5–6 month control release fertilizer (64 g per container).  Vegetative growth, floral attributes, plant quality and phytotoxicity were assessed.  Experiments were conducted using a completely randomized design with 12 (Limelight) and 10 (Little Lime) single pot replications.  Limelight and Little Lime had similar branching response to dikegulac sodium. For example, Limelight treated with dikegulac sodium (800 and 1600 ppm) had 74.4 (279%) and 76.7 (287%) more branches than hand-pruned (26.7) plants and 75.7 (298%) and 78 (307%) more branches than untreated (25.4) plants.  Moreover, Little Lime treated with dikegulac sodium (800 and 1600 ppm) had 35.8 (163%) and 27.4 (125%) more branches than hand-pruned (21.9) plants and 44.7 (344%) and 36.3 (279%) more branches than untreated (13.0) plants.  Hand pruning Limelight decreased flower number compared to untreated and PGR treated plants, with the exception of plants treated with 1600 ppm dikegulac sodium, which had flower numbers not different from hand-pruned and untreated plants; whereas, hand pruning Little Lime resulted in fewer flowers than all other treatments.  Initial bleaching and interveinal chlorosis was observed on new growth of both cultivars; however, 6 weeks after treatment neither cultivar had ratings different from the untreated.  These results suggest dikegulac sodium (800 and 1600 ppm) may be a viable option to achieve more branching and crop uniformity without reducing flower number of Limelight or Little Lime.