Performance of Landscape Trees in the Semi-Arid Southwest under Three Irrigation Regimes

Nine species of landscape trees commonly planted in the semi-arid Southwest were irrigated with three different regimes to determine their growth response and aesthetic functionality. Irrigation treatments started in May 2010 and were based on applying 80% (wet), 60% (medium), and 40% (dry) of reference evapotranspiration (ETo) from May to October and half of that (40%, 30%, and 20% of ETo) from November through April. Soil moisture data was collected by time domain reflectometry from three species irrigated with the medium treatment. Soil moisture data were used to calculate crop coefficients (Kc) which ranged from 0.2 to 0.4 for  Prosopis hybrid (palo verde hybrid), 0.3 and 0.6 for Cupressus arizonica (Arizona cypress), and  0.3 to 0.7 for Fraxinus velutina ‘Rio Grande’ (Rio Grande ash). The three irrigation treatments resulted in different irrigation frequencies. Plants in the wet treatment received about twice the number of irrigations compared to plants in the dry treatment. In summer, irrigation was applied every five, six and seven days, and in winter the longest interval between irrigations was 77, 94, and 136 days for the wet, medium, and dry treatment, respectively. Fast growing species in descending order were palo verde hybrid, Prosopis velutina (mesquite), Chilopsis linearis (desert willow), and Pistachio x ‘Red Push (pistache) based on their growth index. Trunk area was largest for palo verde hybrid trees, followed by pistache, mesquite, desert willow, and Pinus elderica (Afghan pine). Smallest trees based on growth index and trunk area were Quercus virginiana (live oak) and Rio Grande ash. Although all species increased in height, growth index, and trunk surface area, no significant differences in growth of the same species receiving the different irrigation treatments were recorded by October 2012 with the exception of growth index for live oak. Symptoms of deficit irrigation started to develop on some species in spring and progress through the summer as marginal leaf burn, foliage dieback, terminal branch dieback, or loss of interior foliage. Overall quality of some trees such as Arizona cypress, Afghan pine, and Rio Grande ash started to decline due to reduced foliage cover. After 30 months of treatments trees of similar size can be grown with half the amount of water that is applied to a tree of the same species in the wet irrigation treatment without detrimental consequences for several species.