The first week the goal is to dissolve the root-ball so that it tightly conforms to the receiving hole. There should be no air gap between the root-ball and the hole. Roots cannot bridge an air gap.
Thereafter the goal is to keep the soil moist, but not waterlogged. Root transpiration requires a gaseous interchange. So it is possible to drown a tree: to suffocate the tree with too much water, for too long a period of time, can occasionally kill a tree. For these reasons a single weekly saturation of water provides 4- 6 days for the tree to breathe.
It is also possible to rot the root-hairs: even with infrequent watering. If the tree is installed in a water tight clay soil, that clay can become a virtual clay pot. Drainage is important. It is prudent to examine the receiving hole before the tree goes in and remember the soil condition for your tree, and water accordingly.
The best practice may be to use a soaker hose to distribute the water evenly. Be sure that the original soil ball and the backfill soil,and the entire “donut” get water. Allowing the hose to dribble water slowly into the planting pit to saturate the entire donut area is acceptable, but the hose must be moved to ensure that water gets to all of the roots. It is a better practice to use the soaker hose.
The best time of day to water is morning. On occasion a plant may wilt slightly in the heat of a scorching summer afternoon. Misting the canopy on extremely hot days for first three years is a desirable practice. Watering in late afternoon , or all night long may supply more water than the plant can use. Root rot can develop
We have been experiencing spring drought. So for the first three years while the roots re-establish equilibrium with the canopy , supplemental early wagering is essential for good growth.
Proper watering is then a delicate balance between too much and too little. Most trees die or become stunted from under watering. If the ground is so dry and hard that you cannot thrust a large screwdriver into the dirt, imagine what problems the tender root-hairs are having in elongating. A sound is rule of thumb is do not rely on rains.If you cannot thrust a large screwdriver easily in the donut area of your tree it probably is thirsty.
If your tree cannot be reached by hose, try the bucket method. A five-gallon bucket each caliper inch of trunk is tied to each treeto avoid upset. Drill a single tiny hole (3/16 inch) in the bucket side one half inch above the bottom, Fill the bucket once or twice weekly. This is a minimum. Rain fall less than 1 inch doesnt count.