We are extremely dry here in Colorado and even if you have established trees, you need to water them. Although plants are dormant, they still require water in the winter which they would usually get from natural snowfall.
A great rule of thumb for winter watering is a 2” tree requires 10 gallons of water a month in the winter. We recommend you water every two weeks if we have no measurable snowfall. It isn’t a lot of water really, just enough to keep the hair roots supple. Protect your investment and water this winter!