Proper watering is absolutely critical to the survival of transplanted trees. Too much water or too little water means the difference between life and death.
One of the best ways to insure that adequate water gets to the root ball, is to build a berm about 3 to 4 inches high around the planting hole. This saucer of soil will aid in directing water to the root ball and prevent water runoff.
In an average loam soil, spring-planted trees should be soaked once a week, if there is less than 1 inch of rainfall. Water more often in sandy soils, and in heavy loam or clay soil, a good soaking every 10 to 14 days should be sufficient. Continue watering during the fall and into the winter until the soil freezes. Many fall-planted trees are lost because of inadequate moisture. So during winters of inadequate snow and rainfall, you should water when temperatures permit.
During the first few months after transplanting, a tree gets most of its moisture through the roots in the root ball, which can dry out in a day or two, while surrounding soil stays moist. So it is worthwhile to probe the root ball in order to check its moisture level. As the tree establishes itself in its first growing season, it is better able to sustain itself, but watering is still critical during the second growing season, especially in periods of low rainfall.
That said, in an effort to make sure the transplanted tree has sufficient water it is easy to overdo. Too much water is just as injurious as too little. It is very easy to drown a newly planted tree. Mulch will help keep an even moisture level in the rootball. Additionally, it is best to soak the planting area every one to two weeks (depending on weather and soil conditions), rather than to water frequently, which may provide so little water that the moisture doesn’t actually reach to the full depth of the roots, or so much water that you flood an already saturated soil and cut off all oxygen to the roots.
If you have any questions about how to properly water transplanted trees in your particular planting location, please call your County Extension Service.