A Study of Irrigation Requirements of Southwestern Landscape Trees
Trees planted in urban landscapes need regular watering during establishment to develop a healthy root and shoot system. After establishment, tree species differ in how much supplemental irrigation they need to grow to their mature size and to remain healthy. Increasing the amount of irrigation water does not always result in more tree growth. Responses vary by how often and how much water a tree receives, the degree to which the soil dries between irrigations, and the amount of water a plant needs based on weather conditions, primarily solar radiation, temperature and wind. Recommendations for irrigation amounts and frequencies can be found in several publications (AMWUA, 2005; Costello and Jones, 2014) however, these values are generally based on expert agreements or anecdotal evidence and not on scientific experiments. Following are the results of a study conducted at the University of Arizona Maricopa Agricultural Center in Maricopa, Arizona to determine the irrigation needs of nine species of commonly planted landscape trees. After planting, trees were well watered for 1.5 years for establishment, allowing approximately 25% soil moisture depletion between irrigations during summer (June to August) and 35% depletion during the remainder of the year. In summer, this required weekly irrigation with 20 gallons of water per tree. After this establishment period, three irrigation frequency treatments were applied to trees. This study describes the subsequent four seasons of irrigation regimes after establishment and the irrigation frequency study. During the last year of the study, trees were grown without supplemental irrigation to simulate what happens when irrigation is suddenly turned off. Recommendations are given for irrigating the nine species of trees that were studied.