Use of Controlled Water Deficit to Control Height of Poinsettias (Euphorbia pulcherrima ‘Classic Red')

Production of poinsettias involves intensive use of growth regulators to control height. Height control is necessary for visual appeal and post-harvest handling. Growth regulators are relatively expensive and do not always provide consistent height results. Since turgor potential drives cell elongation, and thus stem elongation, drought stress has potential for regulating plant height.  Using soil moisture sensors, the severity of the drought stress can be both monitored and controlled. The objective of our study was to compare poinsettia height control using growth regulators (spray, mixture of B-Nine and Cycocel at 1000 ppm and drench, 0.25 ppm Bonzi) to the use of controlled water deficit. Graphical tracking of plant height was used to determine when to apply growth regulators or controlled water deficit.  In the water deficit treatment, substrate water content was reduced from 0.4 to 0.2 m³/m³ when height exceeded the target height. Growth regulator applications (spray or drench) reduced poinsettia height below the final target level of 43.5 cm. Water deficit resulted in an average height of 44.5 cm, closest to the target height. As expected, control plants were significantly taller (49.4 cm). Most stem elongation occurred between 14 and 30 days after pinching. There was no effect of drenching or water deficit on bract size, while spraying growth regulators reduced bract size. Bract color was not affected by water deficit or growth regulators. There was no difference in shoot dry mass between growth regulator- and water deficit-treated plants. Lateral growth was reduced by growth regulators while water deficit had no effect on lateral growth. These results indicate that water deficit can control poinsettia height without compromising quality and that soil moisture sensors can be used to control this water deficit.