Answer: Newly planted trees require regular watering until their roots become established. This means keeping the soil evenly moist whenever it is not frozen, with water being applied by rain and/or the garden hose. Watering would initially continue for a full year after planting and then be applied in times of dry weather for the next several years.
Watering is best done by a deep soaking rather than a daily sprinkling; deeper watering encourages roots to grow deep where the soil naturally stays moister longer. To judge when to water, dig into the soil a bit with your fingers. The goal is to taper back to watering about once a week or possibly slightly more often during extremely hot or dry weather. Roots will not grow into dry soil, so you need to water an area larger than the root ball. Using several inches of organic mulch can also help the soil stay cooler and moister.
Transplanting a tree is stressful in many ways, and it is certainly possible that the tree has been underwatered this spring due to the dry period we had. It is also possible that there is a pest problem, or, perhaps something else happened such as an accidental herbicide contact or it was overfertilized.
If the leaves were severely damaged they will drop and the tree will have to muster strength to releaf out. If the problem was dry soil, then watering is the best thing you can do to help it recover, fertilizing will not be helpful in this instance. Time will tell if it can manage to survive or not.
In the meantime, you might want to take a sample to your county extension (270-4391) and see if they can give you a more precise diagnosis of the problem and if there is anything besides lack of watering at work here. I'm sorry about your tree.
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