Growth of Hibiscus acetosella ‘Panama Red' In Response to Sensor Controlled Irrigation In Two Outdoor Nursery Settings

Decreasing water availability is necessitating more efficient water use by horticultural operations.  Best management practices (BMPs) are often employed in efforts to conserve water; however, BMPs do not account for water requirements of plants.  Soil moisture sensors can be used to control irrigation by irrigating when substrate volumetric water content (θ) drops below a particular threshold. Sensor technology, along with automated irrigations systems, allow for more precise irrigation and improved water conservation compared to traditional BMPs.  Quantifying plant growth responses to varying θ thresholds can increase our understanding of plant water requirements.  The aim of this research was to measure growth of Hibiscus acetosella ‘Panama Red’ (PP#20,121) in response to various θ thresholds.  Experiments were performed on commercial-style nursery pads in Watkinsville (USDA hardiness zone 7b) and Tifton, GA (USDA hardiness zone 8a) to understand both plant and system responses to environmental conditions frequently encountered in outdoor environments. Soil moisture sensors were used to maintain θ above specific thresholds (0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, and 0.45 L∙L^-1).  A datalogger converted the soil dielectric readings from capacitance sensors to θ using a substrate-specific calibration.  These θ readings were compared to the programmed threshold for a given plot.  When the sensor reading was below the threshold, the datalogger opened a solenoid valve and irrigation was applied for 120s.  Sensor readings were taken and recorded every 20 minutes.  The daily irrigation volume for a plot was calculated from the number of irrigation events and the volume of water applied per event. Plant growth was greatly reduced at θ threshold below 0.35 L∙L^-1.  Plant height, shoot dry weight, and area of the uppermost fully expanded leaf all increased with increasing θ.  Growth of plants maintained at the lowest thresholds was stunted with mortality observed at the 0.10 L∙L^-1 threshold.  Maintenance of θ thresholds was disrupted by rain events, with recovery occurring within 10 days after most rain events. Growth of Hibiscus acetosella ‘Panama Red’ was correlated with θ thresholds in both locations despite the different environmental conditions.