Drought-tolerant trees can withstand dry conditions once established. No native or commonly used landscape plant is drought-resistant until it becomes established in the site. All plants require a steady supply of moisture when first planted.
Trees can tolerate drought if they are deep-rooted. Deep roots can harvest soil moisture from past rainfalls. Trees planted in the landscape should be deeply watered for the first two or three years before they can become full drought resistant.
Roots follow moisture in the soil; deep watering will result in deep roots. Surface watering–the water you might give a lawn or perennial plants–will be insufficient to sustain mature trees. Avoid shallow-rooted trees and increase drought tolerance by watering trees deeply until they are well established.