Colorado’s eastern plains are a semi-arid environment. All newly planted trees and shrubs will need supplemental water.
Apply as much water as the size of the pot or root ball each week. For balled and burlapped stock, apply 10 gallons of water for each trunk diameter inch. For example, give five gallons of water for a five gallon pot, and give 20 gallons of water for a two-inch diameter tree. Trees may be planted with soil mounded up around the edge of the root ball forming a dam, which will help the tree retain water and direct it to the root ball. To water these trees, fill the well with water each week.
Regular watering from lawn sprinklers is usually sufficient for newly planted shrubs. Too much water will kill a new plant as quickly as not enough water. Trees and shrubs native to semi-arid climates require less water to establish. To test if the soil is too dry, take a handful of dirt from around the plant and squeeze it in a fist. If it clods up, there is sufficient moisture. If it crumbles, the soil is too dry. Check the root ball moisture as well as the surrounding soil.
For more information, see the following Colorado State Extension fact sheet(s). http://www.ext.colostate.edu/ptlk/1719.html