Limited summer rainfall and water restrictions can deplete subsurface soil moisture. Upon digging, people may find little moisture at 8 to 12 inch depths where most tree roots are located. Paying special attention to fall watering is important for trees to mature buds and enter dormancy in a healthy condition. Consider tree watering in addition to whatever general landscape sprinkling local water restrictions allow in fall months.
Colorado horticulture experts recommend watering underneath the branches within the circle bounded by the drip line. Water to a depth of 12 inches. Trees should receive ten gallons per inch of trunk diameter measured at knee height. This amount can be reduced by that supplied by general lawn watering or if rain or snow is received. Water trees three times per month in September. Cut back to one or two times per month from October through March, two times monthly for young trees and for evergreens.
Mulch within a circle bounded by the drip line to a depth of 4 inches allowing 6 inches of space between the mulch and tree trunk. A mulch circle of any area will be beneficial whether it extends to the dripline or stops short of that.
Many water application methods can be used. Consider soaker hoses, soil needles, or soft spray nozzles. On hard or compacted soils, soak, wait and soak again to avoid water runoff. Be especially careful with soil needles, also known as deep root feeders. Some people insert these well below a 12 inch depth, placing water out of reach of tree roots. Soil needles should be inserted at an angle to a depth of 6 to 8 inches. Leave the needle in place for 3 to 5 minutes with water turned on low to moderate pressure. Water the area under the branches in at least twelve sites for a medium sized or larger tree. Disperse water sites evenly within the circle bounded by the dripline. For new trees, water all four sites at least 3 feet from the trunk (stem).
In dry years, established shrubs will need additional amounts of winter watering. Apply 5 gallons for a small shrub (less than 3 feet), and 18 gallons for a large shrub (more than 6 feet) on a monthly basis from October through March. Newly planted shrubs will require more winter water, twice monthly using these same amounts at each watering. Be sure to mulch shrubs to retain moisture.
For more information, see the following Colorado State University Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s).