Comparison of Irrigation Scheduling Based on Daily Water Use or Plant Water Demand of Container Grown Woody Plants

Nursery irrigation scheduling based on two methods were compared (1) daily water use (DWU) and (2) a recently proposed plant demand-based irrigation system that assumes that growth would not be compromised when basing the irrigation set point on the substrate water content where photosynthesis begins to decline due to water stress. Buxus microphylla ‘Boxwood’ 4-inch liners were potted into 1 gallon containers with 85% pine bark : 15% peatmoss (vol:vol). Each irrigation zone was controlled by a 13DE04K solenoid valve (Rain Bird Corporation). Irrigation was applied through four overlapping Toro 570 Shrub Spray Sprinklers (The Toro Co., Riverside, CA) per irrigation zone. Emitters were mounted on 1.3-cm diameter risers at a height of 66 cm. DWU was calculated based on the average soil moisture readings of ECHO-5 probes (Decagon Devices, Pullman, WA) inserted into two containers per irrigation zone and irrigation was applied daily at 9 am. The demand-based irrigation system was designed to apply irrigation to return the moisture to container capacity (0.53 cm³) after substrate moisture set point (0.28 cm³) has been reached. Acquisition and control were monitored using a data logger (CR 1000, Campbell Scientific, Logan, UT). Gas exchange and pH and electrical conductivity of leachate were monitored during the experiment.  Plant biomass metrics were measured at the termination of the experiment. Plant water use efficiency (WUE) was estimated by dividing total dry weight at the time of harvest by total water volume applied (irrigation plus precipitation; L per container). Plant physiological parameters such as leaf water potential, photosynthetic rate, transpiration rate and stomatal conductance were not different among plants in both the treatments. The average growth index and average plant dry weight at the end of study were not different among plants grown in DWU and demand-based irrigation treatments. Total irrigation water applied was greater (35%) for the DWU-based treatment than the on-demand irrigation treatment. Plants under on-demand treatment had greater WUE (31%) than plants in the DWU treatment. In general, the DWU treatments were irrigated when the volumetric water content was about 23% above the plant demand treatment’s set point. The pH and electrical conductivity of leachate were similar between the treatments and were within the acceptable range. These results suggest that for woody plants with low water requirement, such as boxwood, irrigation based on plant physiological parameters can significantly reduce water use compared to DWU based irrigation methods.