Answer: Your goal in watering is to keep the soil evenly moist but not sopping wet. You need to use your finger to dig down and check and see what is happening; sometimes the potting soil dries out faster than the surrounding soil. It is usually better to water deeply about once a week instead of the daily light sprinkling.
Since this particular plant is best suited to average or well drained soil (it is very drought tolerant once established), you would not want to overwater it and saturate the soil. At the same time, as with any newly planted shrub, you would still need to water to supplement natural rainfall during dry spells. Mid to late summer is a stressful time to transplant because it is so hot and dry. This means you do need to be careful about your watering.
Usually, drying leaves are a sign of underwatering, but they can also be a sign of overwatering. It is possible that the soil you added and/or the original native soil are not very well drained and that excess moisture from the daily watering is accumulating in the hole. It is also possible that, if your daily watering is just a little sprinkling, that it is not enough to keep the soil evenly moist. There is no exact rule I can give you, you should water, wait a few hours, then dig down and see how effective your watering was. Sometimes it is surprising.
Also, use several inches of organic mulch around the plant to cover the root zone but not up against the stem. This helps keep the soil evenly moist and also keeps out weeds.
Next, I would caution you on overfertilizing. This can "burn" plants and can cause foliage problems also. Ideally you would fertilize based on soil test results. (Your county extension shoudl be able to help you with the tests and interpreting the results.) Barberries are not very heavy feeders, so a once a year application of compost and/or a complete granular fertilizer such as 10-10-10 in the early spring per the label instructions should be adequate unless you have truly poor soil. It is always better to underfeed than to overfertilize, so if in doubt, err on the light side.
This is a very sturdy plant and should overcome transplant shock with careful watering and some time while it becomes adjusted, but if you feel you have been watering it correctly and have not overfertilized it, then you might also want to consult with the nursery where you purchased the plant to see if they have any suggestions. Good luck with your barberry.
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