Perennials are an important component in north-central Texas landscapes, providing color and foraging for local pollinators. However, severe summer drought and a rapidly growing urban environment forces limitations on water availability for landscape irrigation. Therefore, we need to evaluate the impact of deficit irrigation on landscape plant performance. Salvia guaranitica ‘Black and Blue’ is a perennial known to perform well in north-central Texas summers, and it attracts numerous pollinators, including bumblebees, butterflies, and hummingbirds. Twelve beds were created in four blocks, with three beds per block and one plant planted per bed. Within each block, irrigation was applied at one of three levels, 60% of evapotranspiration (ET), 30% of ET and no irrigation. Soil moisture monitors [time domain reflectometry (TDR), 10HS, Onset Computer Corp., Bourne, MA] and a handheld moisture meter (TDR, Fieldscout 350, Spectrum Technologies, Aurora, IL) were used to measure water content by volume (Θv). Quantum efficiency (ΦPSII) and stomatal conductance (gs) were measured using a fluorometer/porometer (LI-600, LiCor Corp., Lincoln, NE), and stem water potential of each plant was measured using a pressure chamber (Model 615, PMS Instrument Company, Albany, OR) on a weekly basis. Surprisingly, soil moisture levels in the 30% treatment (0.10 m-3·m-3) were the lowest, with 60% at 0.25 m-3·m-3) and no irrigation at 0.14 m-3·m-3. Quantum efficiency was similarly lower in 30% (0.60), compared to 0.72 and 0.69 for the 60% and unirrigated plots, respectively. Stomatal conductance was similar across all treatments. Stem water potentials were highest in plants receiving 30% (800 kPa), compared to 380 kPa in 60% and 360 kPa with no irrigation. Across all treatments, plant performance was correlated to soil water content, (ΦPSII P=0.03, r=0.24; gs P=0.05, r=0.22; kPa P=0.003, r=-0.33). Based on this study, Salvia guaranitica ‘Black and Blue’ should be irrigated at 60% of ET for best plant performance during summer heat and drought.
