The decision to water depends on the plants. Plants adapted to dry winters, those well adapted to xeriscapes, will need no water. Some plants native to dry environments, such as the desert willow, prefer dry conditions during the winter. Plants native to moist environments will need supplemental irrigation during these dry periods.
For many of the trees and shrubs grown in our landscapes, the most critical time for irrigation is in the late winter as the buds begin to swell. This may be more than a month before growth begins, and this is the time when small absorbing roots form for many plants. These roots need moist soil to grow, survive, and do their job absorbing water and nutrients. If the soil is too dry at the time these new absorbing roots form, the roots may die and the plant will have a difficult time beginning growth in the spring. Other times in the winter, however, water is also necessary. Water is lost through twigs and stems, even when leaves are absent. This water must be replaced. Evergreens which have leaves through the winter lose even more water which must be replaced to keep the plant alive.
When the soil is frozen, it is difficult to successfully irrigate plants. Before the soil freezes and during warm periods when the soil thaws, it is possible to irrigate. Fortunately, when the soil is cold, the water is lost less rapidly and the plants need less water.