Control of Poinsettia Stem Elongation: Height Limits using Deficit Irrigation

Height regulation is crucial in poinsettia production for both aesthetics and transportation. Shorter plants are preferred by consumers and occupy less space during transport, allowing for more plants per truck.  Controlled water deficit, reducing substrate water content in a controlled fashion when plants are too tall, offers an alternative to plant growth regulators (PGRs) for poinsettia height regulation. We have previously shown that a controlled water deficit can be used to regulate poinsettia stem elongation. However, it is not clear what the limits are for height control using deficit irrigation and how this affects aesthetic qualities, such as bract size. Our objectives were to determine how much shoot elongation can be inhibited using controlled water deficits and to investigate possible adverse effects of on shoot morphology. Rooted cuttings of poinsettia (Euphorbia pulcherrima ‘Classic Red’) were transplanted into 6-inch pots filled with peat:perlite (80:20) substrate. The plants were fertigated through drip irrigation system with 200 mg∙L^-1 N of water soluble fertilizer (15N–2.2P–12.5K). Three target heights (43.2, 39.4, and 35.6 cm) were set at pinching and growth tracking curves were used to monitor plant height throughout the production cycle. Substrate water content (θ) was maintained at 0.40 m³∙m^-3 (approximately –5 kPa) during normal growth and reduced to 0.20 m³∙m^-3 (approximately –75 kPa) when plants were too tall, based on the tracking curves. When plant height was once again within the appropriate range, θ was increased again to 0.40 m³∙m^-3. Control plants were maintained at a θ of 0.40 m³∙m^-3 throughout the study. The θ levels were maintained using a soil moisture sensor-based automated irrigation system. Plant height in the 35.6 cm target height treatment remained above the upper limits of the tracking curve, despite being kept at a θ of 0.20 m³∙m^-3) for 70 days after pinching and the final plant height of these plants was 39.8 cm. However, we were able to achieve the target heights of 39.4 and 43.2 cm. Relative to control plants, bract area was reduced by 53%, 47%, and 31% in the 35.6, 39.4, and 43.2 cm target height treatments, respectively. Our results indicate that the minimum height that can be achieved using deficit irrigation is about 39–40 cm for ‘Classic Red’, but water deficit at that level may also decrease bract size.