Evaluating a Physiological-based, On-demand Irrigation System for Container-grown Woody Plants with Different Water Requirements

Two sensor-based irrigation scheduling systems were compared for water use and plant growth in container-grown Green Velvet boxwood (Buxus sempervirens L. x B. microphylla Siebold & Zucc. var. koreana ‘Green Velvet’) and slender deutzia (Deutzia gracilis Siebold & Zucc). These crops were chosen because they have different water requirements during production. The two sensor-based irrigation systems included a physiological-based on-demand (OD) irrigation system where the set point was derived from the relationship between substrate moisture and photosynthetic rate. The second system was a daily water use (DWU) method where the amount of water used by the crop was replaced each day. The objective of the study was to evaluate and compare water use and growth metrics using the OD and DWU irrigation scheduling regimes for two container-grown woody plants that differed in their water use demand.  Both plants transpired at a faster rate at 1.5 kPa than at 0.5 kPa.  Deutzia plants transpired at a higher rate than boxwood under both VPD conditions. There were no differences in root and shoot biomass or growth index due to the irrigation schedule employed for either boxwood or deutzia. For boxwood plants, OD irrigation reduced water consumption by 35.5% and increased WUE by 54.5% compared to DWU.  Total water use of deutzia in OD zones was decreased by 26.5% compared to DWU. One consideration explaining reduced water use for OD irrigation is that plants were irrigated on average every 3.4 days in boxwood and 1.6 days in deutzia. This contrasts to the daily irrigation event with DWU. Since each irrigation event is inefficient (less than 50% of irrigation volume entering the container substrate), the more frequent irrigation events associated with DWU treatments would lead to greater water losses thus increasing water use and decreasing water use efficiency. Both DWU and OD reduced water use by 41% and 68% compared to a traditional timer-based ¾ inch per day irrigation application in boxwood. For deutzia, there were similar trends; water use was reduced compared to traditional overhead irrigation applications by 61% and 70%, respectively, for DWU and OD treatments. DWU offers the labor scheduling advantage of a standard time of day when irrigation will operate, and OD offers the advantage of watering as needed, potentially reducing water stress as the season progresses and as the plant size and atmospheric demand increase.