Red firespike is a vigorous ornamental shrub growing to ≈6 ft in height. It has an upright growth habit and is commonly cultivated as an ornamental plant in the garden due to its attractive tubular red flowers (Daniel, 1995). The plant has potential for introduction as a flowering potted plant because of its attractive red spikes of flowers and shiny green leaves. However, without height control, firespike grows too tall, exceeding the commercial requirements for potted plants. Thus, control of firespike stem elongation is necessary to provide an aesthetically balanced, desirable, and marketable product.
PGRs are commonly used as an effective technique to restrict stem elongation to achieve aesthetically and commercially pleasing plants. Most PGRs act by inhibiting different steps during the biosynthesis of gibberellins (GAs) responsible for stem elongation in plants (Rademacher, 2000). In addition to type of PGR, plant height can be influenced by concentration, time of application, number of applications, method of application, formulation, and growing substrate composition (Barrett and Bartuska, 1982; Gilbertz, 1992; Ruter, 1992). The most important issue when using PGRs is determining the most effective application method and concentration for consistent results.
Several PGRs effectively inhibit stem elongation of ornamental plants and influence the time of flowering. The triazole class of PGRs, including flurprimidol, uniconazole, and paclobutrazol, are one of the largest commercially used groups of PGRs. They have been shown to be effective for height control and producing compact plants (Krug et al., 2005; Warner and Erwin, 2003). The triazoles are often more effective than other PGRs in inhibiting GA synthesis, so high concentrations can result in excessive height control and stunted plants (Latimer and Whipker, 2012). Sanderson et al. (1988) compared five application methods of paclobutrazol and reported a substrate drench was the most effective method for controlling plant height. In a study on purple firespike (Odontonema callistachyum), plant height was 56% and 46% shorter than the control using drenches of paclobutrazol and flurprimidol at 3.35 and 1.77 mg/pot, respectively (Rezazadeh and Harkess, 2015). Flurprimidol is effective as a foliar spray or substrate drench for height control (Currey and Lopez, 2011; Krug et al., 2005). Daminozide is not a triazole but rather suppresses plant height by reducing the conversion of GA to active forms of GA and reducing translocation of GA to actively growing tissues (Rademacher, 1991). Whipker and Dasoju (1998) reported that spray application of daminozide at 4000 to 8000 ppm resulted in marketable size potted sunflower (Helianthus annuus) plants.
Specific PGR recommendations for red firespike have not been previously described. This study was conducted to determine the efficacy of flurprimidol, paclobutrazol, uniconazole, and daminozide PGRs on the growth and flowering of potted red firespike.
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