Scheduling Irrigation Based On Plant Daily Water Use Conserves Water without Decreasing Growth of Container-Grown Ornamental Shrubs

In many regions of the U.S. regulations are in place or impending that regulate water use by nurseries.  The primary strategy to meet these regulations is to modify irrigation practices to reduce irrigation inputs.  A study was conducted using 8 landscape shrub taxa potted in 10.2L (#3) containers in 85% pine bark: 15% (vol:vol)  peat substrate to examine the impact of three daily water use (DWU) irrigation treatments versus a conventional irrigation regime on plant growth.  Irrigation was applied through an overhead system at four levels: 1) control application of 19 mm-ha 2) irrigation applied to replace 100% DWU, 3) applications alternating 100% DWU with 75% DWU in a 2-day cycle, and 4) a 3-day application cycle replacing 100% DWU on the first day and 75% DWU on the second and third days.  Each treatment was replicated 3 times with 6 subreplications of each taxon per treatment.  An ML2x ThetaProbe was used to measure substrate volumetric moisture content (SVMC) at 30 minutes and 24 hours after irrigating to calculate DWU (DWU=SVMC_30min – SVMC_24hr).  Subsequent DWU calculations were made every 3 weeks throughout the growing season to adjust volume of water applied.  Considering all taxa and treatments, total irrigation applied by the three DWU treatments was reduced by 16% to 57% compared to the control.  Only DWU of Weigela florida ‘Alexandra’ exceeded the control by 12% during one 14-day interval between DWU measurements.  Growth index(GI) [(plant height + width A + width B perpendicular to A) / 3] taken the last week of the experiment  indicated that all taxa in DWU treatments were the same size or larger than control plants.  Electrical conductivity using the PourThru method was measured for all taxa and at no time exceeded the maximum recommended levels.  Foliar nitrogen concentration in Hydrangea paniculata ‘Limelight’, Itea virginica ‘Morton’, Physocarpus opulifolius ‘Seward’, Spiraea media ‘Darsnorm’, and Weigela florida ‘Alexandra’ did not differ (P ≥ 0.05) between treatments.  Foliar concentrations of P and K for the DWU treatments were the same as or greater than the control.