In Vitro Propagation, Not Plant Growth Regulators or Pruning, Increases Branch Number, Canopy Density, and Visual Plant Quality in Clethra alnifolia 'hummingbird'

Woody ornamental plants are more marketable when plants have symmetrical, well-branched canopies.  Pruning or plant growth regulators (PGRs) are often used to modify growth and increase branching and canopy density.  However, pruning is time consuming, labor-intensive, and expensive and PGRs do not always perform consistently.  Some evidence supports that in vitro propagation (IVP) produces plants with denser canopies than cutting-propagated (CP) plants.  The purpose of this study was to determine if pruning, PGRs or IVP produced the most well-branched, symmetrical canopies. Augeo® (dikegulac sodium) at 800-ppm, Configure® (benzyladenine) at 600 ppm, Topflor® (flurprimidol) at 150 ppm, water or a pruning treatment were applied to both CP and IVP Clethra alnifolia 'Hummingbird'.  Symmetry, branch number, density (branch number per unit of height), phytotoxicity, and quality were assessed. Experiments were conducted using a completely randomized design with 10 single plant replications.  No treatment improved symmetry of any plant species.  Pruning and, in general, PGRs were ineffective at increasing branch number, density, or plant quality. IVP clethra had 92% and 78% more branches and were 100% and 81% denser than CP plants at the initiation of the study in 2012 and 2013, respectively.  At 12 WAT, IVP plants continued to have more branches by 51% and 39% and were denser by 58% and 41% in 2012 and 2013, respectively.  Phytotoxicity symptoms were observed on Augeo-treated plants at 2 weeks after treatment (WAT), but by 4 WAT, phytotoxicity symptoms were not detected. IVP plants had a higher quality than CP plants by 74% and 18% in 2012 and 2013, respectively.  Growers who purchase IVP clethra instead of cuttings, may acquire dense, well-branched, high quality plants with no inputs, and thus less cost. IVP, not PGRs or pruning, increases branch number, canopy density, and visual plant quality in Clethra alnifolia 'Hummingbird'.