The last thing most of our clients want to do on a Sunday afternoon in winter is unpack their garden hose to water their trees. If you don’t, don’t be surprised when they are suffering next summer. Here is what the CSU website has to say. It’s informative if a bit "dry."

"Established large trees have a root spread equal to or greater than the height of the tree. Apply water to the most critical part of the root zone within the dripline. Dry air, low precipitation, little soil
moisture, and fluctuating temperatures are characteristics of fall and winter in many areas of Colorado. There often can be little or no snow cover to provide soil moisture, particularly from October through
March. Trees, shrubs, perennials and lawns can be damaged if they do not receive supplemental water.

The result of long, dry periods during fall and winter is injury or
death to parts of plant root systems. Affected plants may appear
perfectly normal and resume growth in the spring using stored food
energy. Plants may be weakened and all or parts may die in late spring
or summer when temperatures rise. Weakened plants also may be subject
to insect and disease problems.

Water trees, shrubs, lawns, and perennials during prolonged dry fall
and winter periods to prevent root damage that affects the health of
the entire plant.

Water only when air and soil temperatures are above 40 degrees F with
no snow cover. Established large trees have a root spread equal to or
greater than the height of the tree. Apply water to the most critical
part of the root zone within the dripline. "