In its native environment, poinsettia (Euphorbia pulcherrima) grows as a weak shrub or small tree with narrow bracts that can reach 3 to 4 m (Trejo et al. 2012). Commercial growers have been able to capitalize on years of plant breeding and introduction of refined cultivars with shorter, fuller, and more visually appealing plants possessing intense colors of orange, pink, light green, white, and the hallmark holiday reds in the bracts. These attributes have made poinsettia one of the most economically important potted plants sold in the United States, with wholesale sales of more than $157 million (US Department of Agriculture, National Agricultural Statistics Service 2021).
Poinsettia are greenhouse-grown under exacting horticultural practices to produce crops of uniform size and quality with height tolerances dictated by both marketing and shipping factors (Fisher et al. 1996). Stem height often can be controlled by pinching, cultural practices, or plant growth regulators (PGRs) applied as a foliar spray or a drench. PGRs are commonly used to prevent excessive poinsettia stem elongation (Ecke et al. 2004). Currey and Lopez (2011) demonstrated that when applied early in the season, PGRs such as triazoles can be effective in regulating shoot growth and height suppression of poinsettia when applied as a drench and absorbed through the root system.
Unfortunately, the application of PGRs is not exact and can result in plants that are too short (Latimer and Whipker 2012). To promote stem elongation in poinsettia, a combination of BA, a synthetic cytokinin, and GA4 + 7 can be useful for recovering height from overapplication of PGRs. However, excess stem elongation can cause a delay in flowering and formation of weak shoots on the plant (Bergmann et al. 2017).
Traditionally, poinsettia have been pinched to remove apical dominance and allow the lower nodes to branch out. This results in a wider and fuller plant, but mechanical pinching is labor-intensive and may not be economically feasible in high-density production (Latimer and Whipker 2012). Additionally, pinching in some cultivars can result in poor branching or failure to produce lateral shoots (Faust and Heins 1996), and it can negatively affect flowering, with inflorescences developing below the bract canopy when the pinch is not properly executed (Berghage et al. 1989). Cultural practices affecting light (Liu and Heins 2002; Mata and Botto 2009), temperature (Berghage and Heins 1991; Faust and Heins 1996), water deficit (Alem et al. 2015), and nutrition (Latimer and Whipker 2012; Scoggins and Mills 1998) can also be manipulated to alter vegetative growth rate and branching pattern.
Dikegulac sodium is a metabolic inhibitor used in foliar applications that chemically pinches the plant by causing meristematic death and the promotion of lateral branching. A secondary benefit is that treated plants may require less fertilizer and water than pinched plants as plant growth is halted until new growth begins (Latimer and Whipker 2012). Lateral shoot length, tertiary shoot number (shoots in the leaf axils of secondary shoots), visual appearance, and quality of poinsettia cultivars have exhibited varied responses to the use of dikegulac sodium (Dunn et al. 2021; Padhye et al. 2008).
The objectives of this research were to discern best practices of timing, concentration, and method of application using BA+GA(4+7) to increase plant height, evaluate the optimum concentration and number of applications needed when using dikegulac sodium to chemically pinch poinsettia, and assess the potential effects on flowering and vegetative growth of poinsettia when using these PGRs.
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