It is always prudent to deep water trees and shrubs in the fall. This year it may become critical. Southeast Idaho has received very little precipitation in the last 30 days. Yards typically rely on supplemental irrigation all summer long. Then around the first of September we usually begin receiving fall moisture. Traditionally, during September – October we average about 2-3 inches of rain. This year, most areas have received less than 1 inch. This means drier than normal soils going into winter which translates into stressing conditions for plants.
All trees should have one last deep watering in October. With no change in precipitation forecast, this has become very important. You have likely already drained your garden hoses and possibly even blown out underground sprinkler systems. Considering the dry conditions, I recommend dragging hoses out and giving trees and shrubs another soaking before the ground freezes. Evergreens continue to transpire during winter. It is especially important they have access to soil moisture.
To ensure soil moisture, you can let a hose run on a slow stream, periodically moving it around the dripline. Small trees need only about 40 minutes. Larger trees require more water. It may take 4-5 hours or more to adequately wet the soil around your tree. As a general survival rule, 10 gallons of water should be applied for each diameter inch of the tree. For example, a two-inch diameter tree should have 20 gallons of water applied. Plants receiving reflected heat from buildings, walls and fences are more subject to damage. The low angle of winter sun makes this more likely on south or west exposures. Windy sites result in faster drying of soils and will require additional water. A layer of mulch can combat soil moisture losses. If you choose to use a sprinkler that may wet branches, water only when air temperatures are above 40 degrees Fahrenheit.
Newly planted trees are more susceptible to winter drought injury. Trees generally take one year to establish for each inch of trunk diameter. For example, a two-inch diameter (caliper) tree will need about two years to establish, meaning the root structure is balanced with the above-ground portion. Fall is a great time to plant a tree, but extra care is needed to ensure the roots are maintained in moist soil throughout the winter.