Red firespike is a vigorous ornamental shrub growing to ≈1.8 m in height (Daniel and McDade, 1995). Red firespike has an upright growth habit and is commonly cultivated as an ornamental plant in the garden due to its attractive tubular red flowers. The plant has the potential to become a popular flowering potted plant because of its attractive red flower spikes and dark green foliage. Since the plant tends to grow upright, enhanced branching has the potential to improve plant architecture, increase number of inflorescences, control plant height, produce uniform plants, and increase available propagules on stock plants (Latimer and Freeborn, 2010). Controlling the plant’s architecture will meet marketing requirements and provide more uniform and well-shaped, symmetrical plants.
To achieve better uniformity, growers often use cultural practices including hand pinching and PGRs to release apical dominance. This allows dormant, lateral buds to grow resulting in well-branched plants (Grossman et al., 2013). Pinching promotes lateral branch development and more synchronous flowering (Larson, 1985). Pinching controls plant height, enhances plant width, and increases the number of branches (Beniwal et al., 2003; Rakesh et al., 2003). The number of branches per pot in purple firespike increased significantly as number of cuttings and pinches increased (Rezazadeh and Harkess, 2015). However, hand pinching is labor intensive and does not always result in optimal branching (Hester et al., 2013), thereby chemical pinching has been used to enhance lateral branching. Application of PGRs is generally less labor intensive than hand pinching; however, PGRs may cause phytotoxicity (Meijón et al., 2009).
Plant growth regulators with different modes of action have been developed to suppress apical dominance and enhance branching. Dikegulac sodium is translocated in the phloem to the apical meristem, inhibiting DNA synthesis resulting in a chemical pinching effect. Benzyladenine, a synthetic cytokinin, is commonly used to promote branching in ornamental plants (Latimer and Freeborn, 2008). A previous study showed a foliar spray of BA at 300 or 600 ppm doubled the number of branches in ‘White Swan’ and ‘Double Decker’ purple coneflower (Echinacea purpurea) 2 weeks after transplanting (Latimer and Freeborn, 2008). Multiple BA applications increased branching without any impact on growth index (GI) in nandina [Nandina domestica (Keever and Morrison, 2003)]. Two applications of 80 or 160 ppm BA, with the second application 3 weeks after the first, increased branching and flowering of petunia [Petunia (Carey et al., 2008)]. Grossman et al. (2012) reported an increase in branching in four of five herbaceous perennials treated with BA before transplanting.
Dikegulac sodium and BA have been used to effectively enhance branching in bigleaf hydrangea [Hydrangea macrophylla (Hester et al., 2013)]. Foliar sprays of 400 or 800 ppm DS increased branching of ‘Gallo Yellow’ blanket flower [Gaillardia aristata (Latimer and Freeborn, 2010)]. Dikegulac sodium at 800 or 1600 ppm increased branch number of pruned and unpruned ‘Limelight’ panicle hydrangea [Hydrangea paniculata (Cochran et al., 2013)]. The foliar application of DS between 4000 and 6000 ppm suppressed dominant shoots and enhanced uniform regrowth without causing marked chlorosis or shoot distortion in orange jessamine [Murraya paniculata (Kawabata and Criley, 1996)].
Plant response to PGRs varies under different environmental and cultural conditions; therefore, it is recommended to test the efficacy of PGRs on individual plant species (Currey and Erwin, 2012). The objective of this study was to evaluate branching, vegetative growth, and flowering of potted red firespike following pinching or PGR applications of DS or BA.
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