Water use of many landscape plants is not only a factor of how much water a plant needs, but rather how much it receives. Previous research has found that desert-adapted trees such as live oak, once thought to be a low water user, will readily consume several times the quantity of water considered necessary for such a plant. Knowing the actual amount of water a particular tree species needs to survive or to grow to mature size will be helpful in applying irrigation water more judiciously. This information will help landscape managers and managers of water to increase the water use efficiency. In times of water shortage, the actual minimum, rather than the estimated minimum water requirements can be applied to maintain functionality of established landscape trees. Applying less irrigation water than required to achieve maximum growth can also be helpful in reducing plant maintenance such as pruning. The objectives of this project were to determine how nine species of commonly grown trees in southwestern semi-arid landscapes perform regarding growth and quality when irrigated to allow 30%, 50% or 70% depletion of available water in the soil. Results from this study will help to determine safe levels of soil water depletion to ensure growth and functionality. Data was collected on soil moisture and when combined with local weather data, resulted in the development of crop coefficients for three species of trees considered high, medium, or low water use.