The following is taken from the KSU Horticulture Newsletter, by Ward Upham:

Outside Plants May Need to be Watered

As we mentioned in an earlier newsletter, waterlogged soils push oxygen out of the soil that roots need to survive. Every living cell in a plant must have oxygen or it dies. Numerous parts of Kansas have had such a wet winter and spring that plants are entering the summer with weakened root systems.

Therefore, even in areas with good subsoil moisture, plants may need additional water until the root system recovers to prevent leaf scorching and/or excessive leaf drop. This will be especially important for trees and shrubs that have been planted in the last few years. Newly transplanted trees need at least 10 gallons of water per week, and on sandy soils they will need that much applied twice a week. The secret is getting that water to soak deeply into the soil, so it evaporates more slowly and is available longer to the tree’s roots. One way to do this is to punch a small hole in the side of a 5-gallon bucket and fill it with water. Let the water dribble out slowly next to the tree. Refill the bucket once, and you have applied 10 gallons.

Very large transplanted trees and trees that were transplanted 2 to 3 years ago will require more water.
A perforated soaker hose is a great way to water larger trees, a newly established bed or a foundation planting.
In sun-baked soil, you may need to rough up the surface with a hoe or tiller to get the water to infiltrate easily. It may be helpful to set the kitchen oven timer, so you remember to move the hose or shut off the faucet. If you
are seeing surface runoff, reduce the flow, or build a berm with at least a 4-foot diameter
around the base of small trees to allow the water to percolate down through the soil, instead of spreading out.

Regardless of method used, soil should be wet at least 12 inches deep. Use a metal rod, wooden dowel, electric fence post or something similar to check depth. Dry soil is much harder to push through than wet. Water established trees once every two weeks if rainfall is insufficient.