Most trees go full or semi-dormant once winter arrives, but they still need some water. Moist soil is more resilient against temperature fluctuations, which leads to healthier roots. Many trees, like evergreens and young trees, only go semi-dormant and will still take in some water during dry periods of the winter months.
Apply a Fall Mulch
Mulch offers two types of protection for your tree roots during winter. First of all, it insulates the soil so that temperatures around the roots don't fluctuate as drastically or as quickly. This prevents temperature shock to the roots and helps stop problems like soil heave. Second, mulch conserves soil moisture by preventing evaporation and freeze-drying of the soil. The moister soil is also less prone to temperature fluctuations and it provides needed moisture to the tree.
Irrigate Regularly Until Dormancy
Trees may remain active for a bit longer than a lawn in fall, especially if you grow a warm-season grass. Yet, most landscape trees are provided water through the lawn irrigation system, so when you shut down the system for the season, the trees are no longer watered. Continue to water your trees by hand up until the ground begins to freeze. Water deeply, providing enough moisture to wet the root zone out to the edges of the tree canopy drip line. Newly planted trees may require watering every 7 to 10 days, while mature trees only need water every 2 to 4 weeks.
Water During Winter Dry Spells
If you have a long dry period in winter when temperatures are above 40 degrees F and there is little to no precipitation, then you may need to water. This is especially necessary if there is no snow on the ground to melt and provide moisture. There is no need to water as deeply as you would in the growing season. Provide enough water to just moisten the soil under the tree. Water in the morning so that moisture has time to seep in before any nighttime frost.
Protect Evergreens From Desiccation
Evergreen trees can suffer from drying out even if you keep the soil moist. Winter wind and cold can combine to suck moisture right from the tree's needles or leaves. Wrapping the trees in burlap can protect against desiccation. Leave the top of the wrap open to allow air circulation, and remove the wrapping as soon as temperatures are regularly above freezing.
Contact a residential tree service if you have more questions about your winter tree irrigation needs.