Effect of Irrigation Frequency On Arid-Adapted Landscape Trees

This ongoing research project addresses irrigation management of landscape trees in the low desert of Arizona. The objectives of the project are to determine how nine species of commonly used landscape trees will perform in Maricopa, Arizona with different irrigation frequencies. Irrigation is activated for each treatment when the soil moisture in the root zone is depleted by 30%, 50% or 70% based on the reference evaporation at the site. Trees were planted in January 2007 and irrigation treatments started in September 2008. Tree species used were: Cupressus arizonica (Arizona cypress), Chilopsis linearis ‘Art’s Seedless’ (desert willow), Ebanopsis ebano (Texas ebony), Fraxinus velutina ‘Rio Grande’ (Rio Grande ash), Parkinsonia thornless hybrid (palo verde hybrid), Pinus eldarica (Afghan pine), Pistacia x ‘Red Push’ (Red Push pistachio), Prosopis velutina (velvet mesquite), and Quercus virginiana (Southern live oak). Plant height, canopy width, and caliper are measured twice a year and quality ratings are monitored on a monthly basis. Fastest growing plants were palo verde hybrids followed by mesquite with an intermediate growth rate and the slowest growing trees were Afghan pine and Texas ebony. Growth rates of the remaining trees ranged between the intermediate and slow growing ones. After 12 months of treatments no differences in height, caliper, or growth index were found when comparing irrigation treatments within each species except for desert willow and Afghan pine where trees receiving irrigation with the longest interval started to have smaller caliper than trees more frequently irrigated. However, trends are starting to emerge where some species receiving the least frequent amount of irrigation are starting to show signs of water stress. Leaf tip burns were observed through the summer and fall on desert willow and Arizona cypress. Overall quality of trees was good according to the monthly ratings with the average ranging from 3.4 to 4.8 for all tree species since irrigation treatments started and no differences between treatments. The overall quality rating of 3 = medium quality and minimum acceptable appearance, 4 = high quality, good appearance, and 5 = outstanding quality and appearance.  The number of irrigation events for the 30%, 50%, and 70% water deficit was 46, 30, and 20, respectively over a 12-month period. In summer, the shortest interval between irrigation events was 3, 4, and 8 days, and in winter the longest interval between irrigation events was 36, 60, and 68 days with increasing soil moisture deficit treatments.