ASHS 2024 Conference

To develop landscape irrigation recommendations, 22 taxa of landscape plants were evaluated under three levels of deficit irrigation in a field trial. The taxa consisted of common horticultural perennials and shrubs including multiple cultivars of Hibiscus syriacus, Nandina domestica, Rosa spp., and Salvia spp. Based on anticipated plant size and cultural requirements, taxa were planted in one of three adjoining fields, with each field laid out in a grid pattern with either 2 or 3 meter spacing between rows and between individual plants in each row. All fields were covered with a 2-3” layer of chipped bark mulch. For each taxon, 24 individuals were planted with eight replicates per irrigation treatment in a randomized complete block layout with both taxa and irrigation treatment being randomly assigned. Plants were installed in March as bareroot inputs, with containerized plants installed from April to early June of 2021. Daily reference evapotranspiration (ETo) values from a local weather station were obtained from the California Irrigation Management Information System. The amount of plant available water (PAW) for the trial site was obtained from the US Geological Survey via the Soilweb application. In 2021, plants were irrigated regularly to fully establish their root systems. From April to October 2022, plants were irrigated with one of three deficit irrigation treatments equal to 80%, 50%, or 20% of ETo. The treatments functioned similarly to a crop coefficient to modify daily ETo values. The modified ETo values were used to estimate when 50% of PAW was depleted for each treatment. Irrigations occurred whenever this threshold was reached, with the volume of water applied being equal to 50% of PAW. The volume applied at each irrigation was constant across treatments with the frequency between irrigations varying by treatment. As a result, the 80% treatment was irrigated most frequently with the 20% treatment was irrigated thrice during the deficit period. Monthly during the deficit period, each plant was rated on six categories of aesthetic quality and length, width, and height measurements were collected to develop a plant growth index. Aesthetic performance between the treatments was compared using an ANOVA in conjunction with Tukey’s HSD post-hoc test in R. A majority of the taxa evaluated maintained acceptable aesthetic quality ratings on the lowest irrigation treatment. Two taxa evaluated, Lagerstroemia ‘SMNLCIBF’ Center Stage® Red and Rosa ‘ChewPatout’ Oso Easy® Urban Legend achieved high levels of aesthetic performance on the 20% treatment.