Typically, trees go into a period of hibernation during winter's cooler, shorter days, conserving energy by dropping leaves and slowing growth. Still, most trees in our North Phoenix yards need occasional watering during the winter.
Reasons to Water Trees in Winter
Trees less than one year old need more water than mature trees, regardless whether or not they’re xeric (or “water wise”) trees. An immature tree's root system is actively growing the first year and the roots and tree are more susceptible to injury from lack of water during the winter.
Desert trees grow roots in winter in preparation for spring and summer growth. The roots need water to grow, especially during winter’s drier conditions.
If you’re growing orange and other citrus trees they’ll need year-round watering to stay healthy, although the amount of water they require in winter is much lower than in other months. See this publication from the University of Arizona Cooperative Extension Service to calculate your orange tree's water requirements by month and canopy size.
If you have trees in your landscape that could suffer freeze damage, watering before the freeze hits can help protect the tree from severe dehydration as ice pulls moisture from leaves. This is more of a concern later in winter or early spring, when trees begin to leaf out or bloom from warm weather and then are hit by frost.
How Often to Water Trees in Winter
In winter, you should water your trees deeply but less often. If your trees are on an automatic irrigation system, adjust the frequency of watering but maintain the length of time each cycle runs.
Every two to four weeks typically is enough during periods of dry winter weather for native trees and plants adapted to drought. Some trees, such as mesquites, acacias and native Arizona Palo Verdes, are more drought tolerant and need even less water, about once every four weeks in winter.
There's no single schedule that works for every tree in the North Valley. Aside from drought tolerance, soil makeup influences a tree's water needs, as do periods of cold, wet or hot weather.
Winter Watering Tips
In the unlikely event the soil is frozen, wait until it thaws before you water. The rare times in which Phoenix-area temperatures dip below 32 degrees likely won't last more than a day or so.
If daytime temperatures hover below 40 degrees, you can delay your watering schedule a few days until it warms up.
Water evaporates more slowly on winter days, so it's important to avoid watering trees if you've had rain within the past two weeks to prevent rotting from roots that stay damp. You'll also save water!
When watering a tree, remember that the roots extend underground to or beyond the reach of the tree's branches. Water the tree on all sides with a hose or drip irrigation placed around the tree’s drip line (the outermost circumference of the tree’s canopy); don’t water at the base of the tree trunk as there are no roots there to absorb the water.
Give the tree a slower, deeper watering rather than a quick deluge of water; you want the water to penetrate down to where the roots are (about 2 to 3 feet below ground level).
Finally, you usually can go with a conservative approach in winter, which is healthier for desert trees in the long run than is overwatering. Too much water, especially for desert plants, can harm a plant's health and cause a tree to grow too rapidly. Just keep track of when you water with a handwritten or digital schedule and inspect your trees regularly – in winter and year-round – for signs of possible stress.