Daily Water Requirements of Poinsettias as a Function of Plant Age and Environmental Conditions

Efficient irrigation in greenhouses is critical in ornamental plant production to assure plant quality and minimize leaching and runoff. However, information on water needs of potted plants is limited.  The goal of this study was to quantify the effects of plant age and environmental factors on water use. Two poinsettia cultivars (Euphorbia pulcherrima 'Prestige Red' and 'Strawberries 'N Cream') were grown in three different size pots (diameters = 12.5, 15, or 20 cm) for 16 weeks to quantify their daily water use. We used a soil moisture sensor-based automated irrigation system to maintain a substrate water content of 0.4 m³·m^-3 throughout the growing period. Daily water use and environmental factors such as daily light integral (DLI), temperature, relative humidity, and vapor pressure deficit (VPD) were recorded. Plants of both cultivars used little water (0-10 mL) on days that DLI was low (< 5 mol·m^-2·day^-1). The maximum daily water use of 'Prestige' was 106, 112, and 158 mL and that of 'Strawberries 'N Cream' was 57, 93, and 147 mL in 12.5-, 15-, and 20-cm pots, respectively. Poinsettias generally used the most water when DLI was high (> 25 mol·m^-2·day^-1). Cumulative water use for the 100-d growing period was 4.5, 4.7, 5.7 L for 'Prestige' and 2.2, 2.9, and 4.7 L for 'Strawberries 'N Cream' in 12.5-, 15-, and 20-cm pots, respectively. Throughout the growing period, DLI and VPD were the most important environmental factors affecting the daily water use of poinsettias. Models predicting daily water use based on plant age and environmental conditions may be useful in predicting irrigation requirements of poinsettias. This in turn would reduce water use and reduce leaching and runoff of nutrient-rich water from greenhouses.