During periods of dry weather it is important to keep a close eye on your trees and watch for signs of drought stress. Leaves may appear wilted, yellowed, curled, or they may show browning around the margin or between the veins.
Trees that have been transplanted within the past 2 years, as well as any established trees showing signs of moisture related stress, should be given supplemental irrigation equal to approximately 5-10 gallons of water per inch of trunk diameter. Ideally the soil should be moist, but not saturated, to a depth of 6-8 inches as this is the zone where most tree roots are located. Allow the soil to dry to a depth of 2-3 inches before the next irrigation cycle. This will encourage a deeper, more drought resistant root system, and reduce the chance of Phytopthora root rot and other diseases assoicated with overly wet soils.
It is important to apply the water slowly so that it has time to penetrate down into the soil rather than run along the surface. Two devices that do this well are slow-release irrigation bags and soaker hoses.
Slow-release irrigation bags are placed around the trunk of the tree and filled with water. Small holes in the base of the bag allow the water to trickle out over the course of several hours to several days. Since irrigation bags only supply water to the roots near the trunk of the tree they are best used only for recent transplants; less than 2-3 years old. After this initial establishment period watering this close to the trunk should be avoided since we want the tree roots to stretch outward and not form a dense mass right at the base of the tree.
Soaker hoses are composed of a porous material that allows water to seep through the walls of the hose and slowly wet the soil beneath it. Covering the hose with mulch helps minimize water loss to evaporation and also hides the hose, creating a more aesthetically pleasing look. As noted above, avoid watering too close to the tree trunk. For larger trees you can buy several lengths of soaker hose and connect them end to end. Then lay the hose down around the tree in a spiral pattern with the turns spaced about 1 foot apart, starting 3 feet from the trunk, and extending outward approximately 1 foot for every inch of tree diameter. So, for a 6" diameter tree, you might use 100' of soaker hose to create a spiral covering the area located between 3 feet and 6 feet from the trunk. I often use 4" landscape staples to keep the hose in place as I lay it out.
Avoid sprinklers that wet the tree foliage as this can encourage disease and may even cause leaves to scorch during very hot weather. If sprinklers must be used, time them to turn on shortly before dawn.