The 2012 ASHS Annual Conference

Independently controlled irrigation plots were designed to test two container nursery irrigation regimes on one and three gallon oakleaf hydrangea (Hydrangea quercifolia ‘Alice’).  Plants were automatically irrigated by one of two soil moisture probe-based regimes: 1) a daily water use (DWU) system that delivered the exact amount of water that had been lost in the previous 24 hours and 2) an on-demand (OD) irrigation system based on the relationship between substrate moisture level and photosynthetic rate.  In this system, irrigation was applied when the substrate moisture level fell below 33% container capacity, which corresponded to 90% maximum predicted photosynthetic rate.  Both treatments delivered the exact amount of water required to return the containers to container capacity by overhead irrigation.  Photosynthesis rates, leaf water potential, transpiration, and stomatal conductance were measured at the driest points throughout the experiment.  Gas exchange was measured at the driest point prior to the next irrigation event.  Biomass and growth index were measured after approximately 35 days in production.  In plants produced in 3-gallon containers, photosynthesis, stomatal conductance, and transpiration were not different for the two irrigation regimes.  Leaf water potential, leaf temperature, and vapor pressure deficit were also not different for the two treatments. Also, total dry weight was not different, 161g and 121g for DWU and OD, respectively (P = 0.0623), however, growth index was greater for DWU than OD, 70.9 and 60.6, respectively.  Water use efficiency per irrigation zone was greater for OD than DWU, 5.9 and 3.8 g/L, respectively.  Season-long water use per zone was greater by more than 2-fold for DWU than OD, 351 and 164L, respectively.  For plants grown in 1-gallon containers, total dry weight (22.9 and 24.1g) and growth index (35.9 and 36.6) did not differ, for OD and DWU, respectively.