Many tree, grass, shrub, and perennial roots remain viable during cold weather. That means they still need water. In many regions snow fall and occasional rain are enough to provide adequate moisture but during and after prolonged bouts of dry weather or drought it's possible and likely that soil can dry out and the roots will be damaged. Any plant with a shallow root system is at threat and supplemental watering becomes a requirement if you want your plants to survive winter.
Newly-planted trees and shrubs are also susceptible to injury in the same weather conditions. They typically require more water than established plants until their root systems become strong enough to sustain them, a period that takes one to two years, and dry winter conditions can be deadly.
Perennials, particularly new transplants, that are exposed to winds and full sun can quickly dry out. Potted perennials are especially at risk.
Not every day is a good watering day, even if plants need it. You should only water when the outside air temperature is above 40F degrees (4C). The soil should not be frozen or covered with snow.
Also, try to water around noon. The air is above freezing and that will allow the water time to soak into the soil and avoid the possibility of freezing into an ice layer at night. Compacted soil, typical in many lawns, may need a second watering to ensure water soaks into the soil without running off.
Watering by hand is usually the best method. Soaker hoses and drip systems may be good during the warm season but in winter any residual water can freeze quickly, thaw slowly, and render them ineffective. A sprinkler on a hose works well for lawns, but be sure that the hose is completely drained after use or you'll encounter the same problem when you reach for it again.
Look closely at your landscape to identify areas that need supplemental winter water. Areas with south-facing walls can dry out quickly due to reflective heat. Snow can remain under the shade of a tree, but the roots can extend well beyond that into dry zones. High spots can receive more sun and wind and be the first to dry. Note where the snow melts first and that will probably be the same spot that needs extra water first. Mulched plants may need less water but when the soil is dry beneath the mulch watering is needed.
Generally, trees need about ten gallons (38 liters) for each inch (2.5 cm) diameter of the trunk. That means a four-inch thick (10 cm) tree requires forty gallons (150 liters) of water. Thankfully that amount only needs to be supplied once a month in winter; young trees may need two waterings. This amount ensures the water soaks in to the depth that the roots are growing in an area encompassing the drip zone. This is a total amount of water and can be reduced by the amount of snow or rain.
Small, established shrubs (less than three feet or one meter) require about five gallons (19 liters) of water per month. The amount increases to about 18 gallons (68 liters) to shrubs taller than six feet or two meters. Newly-planted shrubs need twice that amount.
Many established perennials can handle dry conditions for prolonged cold periods but when sustained warm winter days combine with dry weather, watering is advised. Potted plants will dry out faster and should be the first to receive supplemental water. The amount of water varies with the size of the plant so provide a good soaking once or twice a month. Plants that you put in the ground in fall will probably need at least twice that much.
I've talked with many people who find large patches of turf dead in the spring. Others wonder why so many of their flowers or shrubs don't survive the winter. All around town I see trees where only half is alive and growing. This can all be attributed to winter kill. Sure, I live in a region with harsh, dry winters, but the solution for me and so many others is supplemental watering.
It takes extra effort to pull the hose out on a cool day, spend the time to water each plant, thoroughly drain the hose afterward, and put it all away, but for the two or three times it may be necessary over the course of winter it can save countless hours later. Pruning unnecessarily dead and damaged branches, replacing flowers and sod, and removing dried-out bushes is work that few gardeners look forward to doing. A simple application of water when plants need it most, in winter, can be a life saver, literally.